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apps2/!ChangeFSI/FSIinfo

This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

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Filename: apps2/!ChangeFSI/FSIinfo
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File size: D24A bytes
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Exec address: 0000
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File contents
     ChangeFSI: Colour Image Processing Using the 256 colour modes

Archimedes computers have full colour display capabilities built into them as
standard. However, compared with the hardware found in larger workstations, the
facilities provided have been carefully reduced (in order to make them cheap
enough to fit all the time). So the precision of the digital to analogue
converters is only 4 bits (instead of 8) and the colour palette only has 16
entries (instead of 256).

The VIDC 256 colour mode
=========================

Has 4 direct bits (top 2 green, top red and blue) plus 4 palette look up bits
which affect the remaining DAC bits. This therefore gives a choice of 256
colours from 4096, but the choice is not a free one in that the 4 direct bits
are always connected: this leaves only 256 selections available from the 4096
colour range. To repeat: there are 256 simultaneous colours in a selection out
of 4096 colours, where there are only 256 "legal" selections. The new VIDC20
allows an arbitrary selection of colours, but the default situation is
identical.

There are "optimal" selections. An optimal selection is one which covers the
entire R, G, B colour cube (i.e. 0-fullscale for each of the colours) and can
thus be dithered to from any picture (if one considers picture's colour range
as an arbitrary volume in the colour cube, then you can dither/display a
picture accurately if and only if the volume covered by the available colours
encloses the picture's). The optimal selection used for the RISC OS 256 colour
mode palette contains 2 most significant bits of Red, 2 most significant bits
of Green and 2 most significant bits of Blue, with the last 2 bits controlling
the 2 least significant bits in parallel ("adds white" to a colour). Thus peak
R, G, B is obtained (without side effects if varied simultaneously, with a side
effect if peak R (say) alone is required). A correctly designed error diffusion
dither algorithm can display pictures with very high fidelity. [note that
dithering all pictures with a single palette permits their display
simultaneously on the screen, also note that dithering to a fixed palette is
much quicker than selecting a palette first!]

One can also program the system for a large number of n bits per colour modes,
with the n bits starting at the most significant end of the colour range. For
example 4 bits of green, 3 bits of red and 1 bit of blue. The constraints are:
     2 <= nGreen <= 4
     1 <= nRed   <= 4
     1 <= nBlue  <= 4
     (and nRed+nBlue+nGreen=8)
Modes like these are often useful for real time solid modelling (where the
expense of dithering with error diffusion (circa 100 cycles per pixel) cannot
be afforded). These selections are non-optimal: the colour cube is not
completely spanned.

Displaying A Picture
====================

Since a colour in the 256 colour mode can be such a complicated thing, with
varying capability of spanning the colour cube and an unknown accuracy, there
is a RISC OS standard way of specifying a colour you wish to draw with using 8
bits for R, G and B components. A call to the operating system converts this to
the nearest colour and new objects can be drawn.

Where the object to be drawn is a previously generated picture, the modules
"SpriteExtend" (in the ROM) and "ColourTrans" (in the !System directory) in
RISC OS provide some capabilites for displaying pictures in different screen
modes. ColourTrans can look at the palette which a sprite uses and return a
list of the equivalent 'closest' colours in the current screen mode.
SpriteExtend can paint sprites using the list of equivalent colours and can
change the size of the image into the bargain. This is all very well for simple
pictures or pictures created directly for a particular mode, but what happens
for more detailed information or pictures from other sources with resolution
not matched by the Archimedes hardware?

Also the above facilities are rather basic: ColourTrans makes no effort to use
dithering (the practise of putting patterns of different coloured pixels
together to represent other colours) and SpriteExtend simply discards
additional information if reducing the picture in size (and together they make
no attempt to enhance the picture when making it larger). The reason for these
shortcomings is quite simple: speed. The algorithm used in ChangeFSI uses 105
ARM instructions per pixel - and that's apart from any instructions used to
read the image in, change its size and write the result out. For comparison,
the normal sprite plot takes only one instruction per pixel (4 instructions for
four 8 bit pixels). Processing a MODE 15 picture thus uses up too many seconds
of ARM cpu time to imagine it as part of the interactive desktop.

So what does ChangeFSI do to make the situation better? And why does it take so
long? One can immediately draw up some desirable properties for any solution to
the image changing problem:

(1) Maximising the volume of the colour cube. Colour represented in the Red,
Green and Blue computer graphics system can be thought of as a point in a 3D
cube whose axes are the red, green and blue values. Whatever clever
approximations, dithering or error diffusion techniques are used, the colour
volume spanned by the r, g and b axes in the target should be large enough to
contain the source volume. If not, then the picture will appear faded in some
way compared to the original ("the red doesn't seem quite the same"). For a
single picture a colour volume only as large as the input could be chosen;
alternatively for an arbitrary set of input pictures, the colour volume on the
output system has to be made as large as possible. Where animation is required
the colour volume has to be consistent for all the pictures. When generating
the largest volume, it is important to use the system hardware to the limit:
for example, if one has two bits of control over the Archimedes 4 bit D to A
converters, the largest range is covered by values 0, 5, 10 and 15 (rather than
0, 4, 8 and 12).

(2) Giving hue consistency at different saturations. For example, with a
palette with different numbers of bits of r, g and b it may be impossible to
have a consistent set of colours representing derived colours at different
levels of intensity. Shades of white and secondary colours (cyan, magenta and
yellow) are particularly difficult.

(3) Working with the multi-tasking desktop. Acorn are asking for all new
software to be built on the multitasking system. This implies that the palette
should not be changed (or that the agreement of all programs currently running
is obtained first).

The desktop (default) palette in the 256 colour modes (e.g. modes 13, 15, 21,
24 and 28) is quite well designed. It does have the properties of (1) and (2)
(it even has 16 shades of grey) and automatically satisfies (3). It is used by
many Acorn programs (Draw, Paint) and by external programs (Euclid, ProArtisan,
Atelier, ..) to great effect and is a safe choice for ChangeFSI. There are two
ways ChangeFSI can choose a colour:

(a) a method which derives values for r, g, b and tint algorithmically (no
suffix) (suitable only for the default RISC OS 256 colour palette)

(b) search the palette for the closest colour (r suffix). The precise method by
which this is done is the subject of another document.

ASIDE: the closest colour is chosen such that the distance between the true
colour and the choice is minimised in the colour cube space. Since the human
eye is more sensitive to green, then red and lastly blue, the distance function
is weighted 10, 3 and 1 in green's favour.

With the 16 colour modes (e.g. modes 9, 12, 16, 20 and 27) the default palette
does not satisfy (1) and (2) unless the output is purely shades of grey -
ChangeFSI can use either 8 greys (for the desktop) or 16 greys (out of the
desktop or with the desktop palette altered to provide 16 greys). For colour
representation, ChangeFSI can either:

(a) use 1 bit of red, 1 bit of green and 1 bit of blue (i.e. the standard 8
digital colours Black, Red, Green, Blue, Yellow, Magenta, Cyan and White). This
spans the colour cube and is hue consistent, but doesn't use the full range of
bits provided in the format. ColourTrans can map this to the desktop with the
standard palette with fair success, except for the lack of magenta (full
red+full blue) in the palette. A better result can be achieved by setting 6 of
the colours in the palette to red, green, yellow, blue, magenta and cyan. Since
ColourTrans will map the colours anyway, it doesn't matter which 6, but the
most consistent values are 8=blue, 9=yellow, 10=green, 11=red, 13=cyan,
14=magenta (leaving 0-7, 12 and 15 unchanged). This option is selected by a "d"
(digital RGB) suffix to the program's mode argument.

or (b) search the palette for the closest colour (r suffix) [ChangeFSI needs to
be in the 16 colour mode in order to read the palette]

With the 4 colour modes (e.g. modes 8, 11, 19 and 26) ChangeFSI is really
pushed. Shades of grey output is done with the 0, 5, 10, 15 level palette. But
how can colour be done? For example, each pixel can display only one of (say)
black, red, green or blue. This fails to span the colour cube (it does only
half of it), however, so black, cyan, magenta and yellow are used instead. The
rest has to be left to luck: there is no way it can approximate to pure shades
of red, green or blue. 4 bit colour pictures can be seen on the desktop in 16
colour modes with the above palette or in 256 colour modes. This option is by a
"c" suffix. If you can provide a better palette, then ChangeFSI can use it with
the "r" suffix search method.

With 2 colour modes (e.g. modes 0, 18, 23 and 25) pixels are either ON or OFF,
ChangeFSI will only do shades of grey output.... However, the method used can
be varied (see later).

So that's the canvas ChangeFSI has to work on. How does it display the input
range of colours on these outputs? The answer is, as stated above, dithering:
the process of approximating intensity variations with patterned areas. There
are two basic types of dithering technique: that used to print colour magazines
and newspaper photographs "clustered dot dither" - the size of the dot of ink
gives the intensity; and that used on dot matrix displays, for example the grey
level patterns used by 1 bit per pixel mode on the desktop "dispersed dot
dither" where the average number of dots in the area gives the intensity.

Why does the world need two different techniques? Well, different devices
reproduce pixels differently. The "ideal square" pixels produced by an LCD are
perfect for dispersed dot dither; a CRT produces a gaussian spot (a circle or
ellipse which decays in intensity towards the edges) instead of a square -
dispersed dot dither works well; a dot matrix impact printer or a "write black"
laser printer produce large dots (diameter at least SQR(2) over unit pixel
square) and thus make pictures too dark if dispersed dot dither is used; a
"write white" laser printer prints negative "white dots" on a black field and
thus "caves in" the sides of pixels making pictures too white with dispersed
dot dither. Clustered dot dither reduces the effects of mis-sized pixels by
ensuring they are adjacent (and thus the error is confined to the periphery of
the shape) but this does have a cost in poorer resolution.

Since ChangeFSI's colour or greyscale output is to CRTs (or LCDs in some
distant future) it automatically uses a dispersed dot dither. Monochrome output
could be for the screen or for a printer: the user can select between dispersed
and clustered methods by adding a various suffices ("c", "d" or "t") to the
mode number for clustered output. For "c" the algorithm (due to Robert L Gard,
1976) approximates tones in the range 0..16 (17 levels) by using 0..16 "on"
pixels in a 4 by 4 cell. For "d" the algorithm (developed by me, but similar to
Gard's) approximates tones in the range 0..4 (5 levels) by using 0..4 "on"
pixels in a 2 by 2 cell. For "t" the algorithm (ditto as "d") approximates
tones in the range 0..9 (10 levels) by using 0..9 "on" pixels in a 3 by 3 cell.
The "on" pixels are carefully chosen to provide: (a) symmetry of white/black
and black/white patterns (b) diagonally oriented dot grid. The diagonal angle
reduces the eye's ability to see the rectangular patterns produced. The various
sizes of the halftone cell allow some trade off between representing the tonal
values correctly and loss of resolution. Start with "c" and progress to "t" and
then "d" if the resolution is not acceptable.

ChangeFSI can generate reasonable pictures on "write black" printers with a
dispersed dot dither by using the "-black" parameter; however images produced
like this do not photocopy at all well (the photocopier makes the dots blacker
again!). The parameter to -black (default value 32, range 0..128) tells
ChangeFSI how much larger than the ideal square pixel the dots produced by the
printer are - 128 corresponds to an inked dot covering four times the area of
the ideal black square pixel (and this will generally mean ChangeFSI produces
an entirely white output!). Values above 64 give rather poor results on some
pictures but may be worth experimenting with. The default is 32, a dot covering
50% more area than the square pixel (the minimum circle enclosing the pixel
square). Some printers produce dots of (pseudo-randomly) varying sizes and
black correction may not help with these (the picture looks "noisy") in which
case use one of the clustered output methods. -black correction makes pictures
on the screen look very pale, since, if anything, black pixels on screen are
smaller than the ideal square.

[aside: to print these pictures, you need a method which copies the pixels
ChangeFSI has made exactly to those of the output device. Using !Paint, scale X
and Y by 90:300 for a 300dpi printer.]

In all cases, an approximation to a colour will produce an error (which might
possibly be zero if ChangeFSI is very lucky). ChangeFSI tracks these errors and
ensures that over wide areas there is no overall error using a technique called
"error diffusion", first devised by R W Floyd and L Steinberg in 1975. In this
technique the approximation is made and the error distributed to nearby pixels
in the following ratios:

                current pixel   7/16 of error
3/16 of error   5/16 of error   1/16 of error

ChangeFSI gets some of its name from Floyd and Steinberg. Additionally,
ChangeFSI scans through the picture in a serpentine fashion, doing a row of
pixels left to right followed by the next row right to left. This reduces the
probability of regular patterns which the eye is sensitive to. The final I of
the name is for Integer: ChangeFSI does all its work in 32 bit fixed point
integer arithmetic (with the point at bit 28) instead of floating point.
Dithering can be turned off using using "Disable Dithering" on the Processing
dialogue ("-nodither" on the command line) which at least allows you to see how
well it does!

The conversion from one colour range to another is made at the same time as a
change in size of the image. Size is changed by ratios of areas between the
input and output: the total weight of r, g, and b in the source area is
calculated using the fixed point arithmetic and this result is then
approximated to the output using the error diffusion to preserve information
(for example, consider halving in size an image with adjacent pixels of
intensities 1 and 2; the output pixel needs to be value 1.5, so the 0.5 error
is sent to adjacent pixels to keep the overall colour the same). The size
change is expressed as a ratio of output to input (e.g. 40:20) which ChangeFSI
reduces to the smallest possible terms (2:1 in this case) so that you are free
to give values like 256:512 if that's what you mean! Size change is either both
x and y (if one ratio is given) or separate x and y (if two ratios are given).
Note that the size of a RISC OS pixel is known, so changing from (say) mode 21
to (say) mode 13 does not need any additional specified scaling. The full size
output for mode 21 is the reference, so scale to 640 by 512. A common scaling
is to fill the output sprite with the input and an "=" parameter to x and/or y
ratios will do this without ever having to know the size of the input. Simply
omitting the fractional part causes ChangeFSI to put in the source size, thus
giving output which is the specified number of pixels in size.

Conversion from colour to monochrome shades of grey is done with the CIE
luminance weights for r, g and b (0.299, 0.587, 0.114) which is the standard
conversion for broadcast television. These values can be altered using the
-red, -green and -blue commands and allow different weightings for conversion
to monochrome to be tried, this also allows selection of an image composed of
only some of the colours (e.g. -red1 -green0 -blue0) in making a black and
white image.

ChangeFSI can take any RISC OS sprites (2, 4, 16 or 256 colours). If there is a
palette it will be used, otherwise the standard 2, 4, 16 and 256 colour desktop
palettes are used. ChangeFSI can also read the 16bpp and 24bpp sprites which
have no palettes.

ChangeFSI is both a command line driven program and a desktop application. The
file !Help describes how to use the application from the desktop, if you need
to use the program as a command line driven program you will need to have it in
the library (or on run$path) and press f12 to get out of the desktop. The file
"FSIuse" describes the parameters concisely. The program selects mode 0 when
run from the command line when doing any work in order to speed up processing
(this can be disabled with -nomode). It may also be a good idea to RMFaster
BASIC. An hourglass %age processed figure is displayed (this may run through
twice for some of the more expensive options). The program reads the source
image from disc and builds the output in memory, having claimed some from the
system.

In addition to strictly converting the source image, ChangeFSI can also process
it. A trivial option is "-info": ChangeFSI will tell you what it is doing (a
good way of finding out what the source image actually is, and how the scaling
turns out!). Similarly trivial are -vflip and -hflip - the picture can be
produce upside down and left/right reversed. Not so trivial is -rotate, which
will rotate the image by 90 degrees (that is, an anti- clockwise quarter turn).
-rotate- rotates by -90 degrees (clockwise quarter turn).

Sometimes useful is "-nosize": ChangeFSI can read pixel size information from
the source image (if present) or guess it (e.g. if there are less than 320 by
256 pixels in a source image, ChangeFSI assumes the pixels are the same size as
MODE 13's [45 per inch]) or derive it from other information and scale the
output image (composed of RISC OS square or 1:2 sized pixels) appropriately.
-nosize stops this from happening. [this seems particularly relevant on formats
like TIFF or IFF where the pixel size is specified *but may be wrong*] Using
-info will display the pixel ratios which ChangeFSI is using - a combination of
the size guess, the output pixel size and any deliberate scaling. "-noscale"
turns off the output scaling as well.

The simplest image processing option is "-range" which will expand the dynamic
range of the input picture so that it fills the available area (not a good
option for animated sequences unless all source images possess the same range).
Since r, g, b are processed by the same amount, -range preserves the colour of
pixels in coloured pictures.

A useful option for output intended for the screen only is Gamma correction.
CRT displays do not have a simple linear relationship between brightness of the
spot and input voltage: instead of brightness = const * voltage, the response
is brightness = const * voltage ^ (1/gamma). In the TV industry, gamma has been
standardised as 2.2, but there is wide variation in the computer industry: the
monitor's response will vary depending on what make it is and how it is
adjusted. Low values of gamma (0 to 1) make colours darker and high values
(above 1) make colours lighter with the effects being particularly noticeable
on dim colours. "-gamma" uses the default of 2.2, and it can be set by
"-gamma1.5" say. Gamma correction is most relevant when its known that the
input has a linear intensity (input from a scanner or from another computer
with more bits per colour component than the Archimedes).

A more complex primitive is "-equal" which performs "histogram equalisation" on
the image. This makes the histogram of level versus number of points at that
level as flat as possible giving full use of the number of colours available.
-equal is a very harsh primitive, even on monochrome inputs it can produce an
inferior picture; while on colour pictures it can distort the colour of each
point (since r, g, b are computed separately). But -equal can often recover a
picture that is otherwise completely useless or expose information locked in a
small part of the input scale.

Image colours may be inverted with the -invert option. Which may be fun in
colour but is rarely useful! However, monochrome images frequently need
inverting, especially one bit per pixel sources.

A more powerful option is "-sharpen" which enhances the edges of objects in the
picture: this is very useful since the dithering process inevitably smears
information over the display. Pre-sharpening the image can result in a more
acceptable output. Default values of sharpening can be overridden for special
effects: -sharpen8 does edge detection. Lower amounts of sharpening are
obtained with larger -sharpen numbers. In general the more input and output
pixels and the greater the output range in a pixel in the images, the less
sharpening is required. Sharpening a picture which is already dithered can give
a very bad result....

The sharpening is achieved with the following matrix:

   -1    -1    -1
   -1 sharpen  -1
   -1    -1    -1

ChangeFSI computes the sum of nine pixels with the above weights for each input
pixel and renormalises the result (divides by sharpen-8).

Similar is "-smooth" which averages the pixels to eliminate noise. It uses the
following matrix:

    1     1     1
    1  smooth   1
    1     1     1

ChangeFSI computes the sum of nine pixels with the above weights for each input
pixel and renormalises the result (divides by smooth+8). Only one of smoothing
or sharpening may be performed (whichever is last on the command line).

An option for testing is "-nodither" which disables the Floyd Steinberg error
diffusion step completely. Versions after 6th June 1990 are more accurate at
doing this.

The various processes described above are applied in the following order for
pictures with more output pixels than input:
   (a) range or histogram equalisation,
   (b) sharpen or smooth,
   (c) scale,
   (d) error diffusion.
This order is chosen so that:
   (1) since the input pixels (rather than the output pixels) are sharpened,
       there is no false edge introduced when pixels are replicated.

The various processes described above are applied in the following order for
pictures with fewer output pixels than input:
   (a) scale,
   (b) range or histogram equalisation,
   (c) sharpen or smooth,
   (d) error diffusion.
This order is chosen so that:
   (1) ranging can enhance down-sized images. Say a black/white dithered image
       is reduced in size, so that the output from the scaling process is (some
       approximation to) the original grey levels; then ranging can expand this
       if possible.
   (2) sharpening can be used to offset the blurring effects of scaling.
   (3) since the output pixels (rather than the input pixels) are sharpened,
       the effect of -sharpenN is consistent over differently scaled images.

There may be times when this automatic selection of order gets it wrong,
particularly when changing the aspect ratio, it is possible to trigger some
undesirable sharpening artifacts. Use the program twice with sharpening (say)
disabled and enabled in the order required.

Examples of use:

Make "standard palette" versions of existing 256 colour images (for example
the Watford digitiser plus colour board doesn't use the standard palette).
  (e.g. "changefsi in out 13p" for a coloured Watford picture)

Or convert a colour picture to something you can see on your high res mono
monitor.
  (e.g. "changefsi in out 18 -sharpen -info")

Or convert a large monochrome picture (from a scanner, say) to something
which can be seen.
  (e.g. "changefsi in out 20t 1:4 -gamma")

Or convert a colour picture to grey scale for the desktop.
  (e.g. "changefsi in out 20")

Or change a 512 by 480 image to standard format and aspect ratio
  (e.g. "changefsi in out 15 640:512 512:480")
  (or "changefsi in out = -range")

Or change to digital r, g, b
  (e.g. "changefsi in out 20d -sharpen16")

Or convert a picture for printing on a 300dpi laser.
  (e.g. "changefsi in out 18c 300:90 -sharpen")

And many more uses such as making miniatures of pictures, animated sequences of
pictures changing size (or sharpness!), converting Artisan pictures to the
desktop (try looking at the Artisan Garden with the desktop in 256 colours and
ChangeFSI's version - the stripes on the lawn vanish with the standard
version). Obviously you will need to have captured input as a sprite first!
Using more pixels per inch can improve the quality of the result, particularly
converting to multisync modes.

Note that, for all work with ChangeFSI, it is a good idea to start from the
unprocessed input each time and do everything in one pass (rather than using
the program several times with simple arguments). It is not a good idea to
sharpen a dithered image (unless it has been shrunk). It can be very useful
to use a small amount of smoothing on a dithered image (e.g. -smooth16).

*** It is a very good idea to keep the highest resolution master that you can
afford the disc space for....

The ChangeFSI program can also convert from non RISC OS sprite formats while
doing all of the above processing. ChangeFSI will work out what the format is
automatically. For native RISC OS formats, it uses the filetype and there can
be no confusion. For alien formats, it first tries looking for particular
identifying information inside the file and then (if this fails to have
identified a format) tries using the file name itself. See the details below:

The RISC OS sprite format
 Contains 1, 2, 4 or 8 bits per pixel with a modification palette of up to
 16 entries; any number of pixels wide and high uncompressed. Details are in
 Acorn's Programmer's Reference Manual.
 + Recognised by file type FF9 (a paint brush and house icon)
 - Details of how many bits per pixel and the pixel size are obtained from
   the operating system (rather than being in the file) from the "MODE"
   number. Mode extensions can result in a source image containing a MODE
   number which your computer does not understand.
 - Mask information ignored
 - Lefthand wastage ignored

The new RISC OS sprite format
 Contains 1, 2, 4, 8, 16 or 32 bits per pixel.
 + Recognised by file type FF9
 + no problem with mode extensions: information is coded into the format
 - Mask information ignored

Kodak PhotoCD
 24 bit per pixel images at various resolutions
 + Recognised by file type BE8
 - Set ChangeFSI$PCDIndex to read resolutions other than base (=3)

The ArVis format [of ArVis Multimedia]
 Contains 5 bits of red, green and blue information encoded into two RISC OS
 sprites "HIP.<filename>" and "LOP.<filename>" comprising a 640 pixel wide
 and 256 line tall image with pixels sized 1:2. Details from ArVis Multimedia
 - Recognised by providing the HIP.<filename> (which must be of file type FF9)
 - Uncompressed (indeed represents 16 bits instead of 15).

The Pineapple colour digitiser format (of Pineapple Software)
 Contains 6 bits of green, 5 bits of red and blue information filed as a
 single data file starting "FSIfile" comprising a 512 pixel wide and 256
 line tall image with pixels sized 1:2.
 - Recognised by "FSIfile" in the file (which can be any type)
 - Uncompressed

The Watford Video Digitiser "picture" format
 Contains a run length encoded 64 grey level 512 pixel wide, 256 line image,
 with pixels sized 1:2. Details from Watford Electronics Ltd.
 + Recognised by file type DFA (a small grey picture of Stevie Nicks)
 - Warning: don't forget to save the pictures in *un-dithered* state!
 When used with a colour converter, ChangeFSI can read three colour
 separations stored in a directory of separate "red", "green" and "blue"
 files. Beware of auto-gain/contrast when using such information!

The ProArtisan compressed picture file format
 Contains a (mildly) compressed MODE 15 (640 by 256 by 256 colours) picture.
 Details from Clares.
 + Recognised by file type DE2 (a beige monitor in a grey surround)

The TimeStep satellite image format
 This gives an uncompressed 800 by 800 with 256 grey levels.
 + Recognised by file type 7A0 (icon never seen)
 - No resolution specified, equal to mode 27 assumed
 - Documentation never seen. An extra header file <name>! is not understood
   at all.....

CCIR601 4:2:2 images
 Contain 720 pixels by 288 rows (PAL) [243 rows NTSC] YUV coded with
 pixels sized 1:2.
 + Recognised by file type 601
 - one field only

Another TimeStep satellite image format
 Giving an uncompressed 128 pixels wide by 256 lines with 256 grey levels,
 pixels sized 2:1. Used by TimeStep's !ImProcess application.
 + Recognised by file type 300 ("i" in a document frame)
 - Documentation never seen.
 - A bit coarse
 The grey levels represent intensity in various different wave bands. By
 naming files "red", "green" and "blue" and handing ChangeFSI the directory,
 you can produce a false colour image. Hours of experimentation can produce
 an almost reasonable picture...

The AIM and Wild Vision V10 format
 This provides an uncompressed 256 by 256 image with 256 grey levels. Details
 from Delft University of Technology and ECD Computers Delft B.V.
 + Recognised by file type 004 (a picture of 'Trui' with "TU" in blue)
 + ChangeFSI can write to this format (specify "aim" in the mode position)
   and specify = in the sizing to get a 256 by 256 output (otherwise it will
   be the same size as the source image)
 - No resolution specified, equal to mode 27 assumed
 - The associated file <name>+ (file type 010) which contains additional
   information is ignored by ChangeFSI.
 The grey levels may represent intensity in various different wave bands. By
 naming files "red", "green" and "blue" and handing ChangeFSI the directory,
 you can produce a false colour image. Hours of experimentation can produce
 an almost reasonable picture...

The Wild Vision V12 format
 This provides an uncompressed 512 by 512 image with 256 grey levels.
 + Recognised by file type 006
 - No resolution specified, equal to mode 27 assumed

The Wild Vision V9 and SnapShot formats
 This provides an uncompressed 512 by 256 image with 4 bits of red, green
 and blue, pixels sized 1:2.
 + Recognised by "MercSoft" or "SnapShot" header
 + No file type needed

The !Translator Clear format
 This provides 1, 2, 4, 8 and 24 bits per pixel.
 + Recognised by file type 690
 - uncompressed, especially for <8 bits per pixel which is recorded at one
   byte per pixel.
 - No resolution specified, equal to mode 27 assumed

The Atari ST "Degas" format
 1, 2 or 4 bits per pixel, clear or run length encoded. RISC OS file type by
 John Kortink of !Translator.
 + Recognised by file type 691
 - No documentation and only 5 images read
 - No resolution specified, equal to mode 27 assumed

>> All the file formats so far have been designed on RISC OS (or Arthur) and
recognition of the format is defined by file type, the files arrive on RISC
OS floppy discs, and the originators live in the same time zone. For the
following formats many of these things are not true... Which makes it more
difficult to guarantee that ChangeFSI will be able to read the file.

The Millipede Prisma format (used by CadSoft also)
 The Millipede Prisma 3 is a 768 pixels wide by 574 line interlaced display
 board for the Archimedes and BBC Microcomputer. It allows use of 256
 colours from 2^24. Files may be stored uncompressed (432KBytes!) or run
 length encoded. More details from CadSoft or Millipede Electronic Graphics.
 + Recognised by "MILLIPEDE" at offset 16 in the file
 - No resolution specified, equal to mode 27 assumed
 - slowish to read the compressed formats
 - scaling the picture using = can be poor since 574 lines don't relate very
   well to 512 or 256. Use -info to display the scale ratios.

The Aldus/MicroSoft TIFF format (also filetype FF0 is assigned to TIFF)
 Frequently used by scanners, TIFF is a common interchange format for
 graphics images on the Mac and PC (particularly DTP and Windows). It
 provides an arbitrary number of bits per pixel, size etc using a numeric tag
 scheme allowing new information to be introduced without upsetting existing
 programs. The format may be compressed (in 5 different ways at present).
 Details from Aldus UK, MicroSoft, Hewlett Packard...
 + Recognised by file type FF0
 + Recognised by "II" plus the 16 bit number 42 at the start of the file for
   little endian machines (ix86, ARM, VAX...)
 + Recognised by "MM" plus the 16 bit number 42 at the start of the file for
   big endian machines (M680x0...)
 + Can read PackBits images and LZW compressed forms (at least the 3 its
   seen so far...)
 - Cannot read predictor compressed LZW images.
 - So far ChangeFSI has only done 1, 4, 8 and 24 bit images.
   It can't read "planar" images or CCITT compressed forms.
 - Some TIFF images have incorrect pixel sizes: use -nosize if this is
   true. If no pixel size is specified, ChangeFSI assumes mode 27 size.
 + TIFF is a trademark of Aldus Corporation

The CompuServe GIF format
 Used for wide area network transfer of images. It provides an LZW
 compressed up to 256 colours from 2^24 arbitrary sized image. Details from
 CompuServe (USA) [or usenet]
 + Recognised by "GIF8" at the start of the file
 + LZW compression is efficient even for dithered images
 + ChangeFSI can decompress LZW quickly
 - No resolution specified, equal to mode 27 assumed
 - many low quality images around from PCs
 - no processing of GIF89a masks

The Electronic Arts IFF ILBM format
 Provides an arbitrary sized image with up to 4096 colours sometimes run
 length encoded. ChangeFSI understands the generic format and also the
 special Amiga format for "HAM" and "Half-bright" images.
 + Recognised by "FORM" as the first four characters and "ILBM" as
   characters 8 to 11
 - Only pixel aspect ratio specified, not its size (and then sometimes
   incorrectly): use -nosize *and* a size change (a:b) if its wrong
 - images often small
 - doesn't understand Amiga "hires"
 - can't convert HAM pictures directly to monochrome

The MicroSoft Windows 3 .BMP format
 Used by MicroSoft in Windows 3 and PaintBrush for Windows 3. Provides an
 arbitrary sized image and 1, 4 or 8 bits per pixel. Uncompressed.
 + Recognised by "BM" as the first two characters
 - No resolution specified, equal to mode 27 assumed
 - documentation never seen - 7 files examined and intelligent guesses made!

The Digital Research GEM .IMG format
 Used by GEM (and GEM Paint) on both PC and Atari ST. Provides an arbitrary
 sized image and number of bits per pixel (though its only usually used with
 up to 5 bits per pixel) and pixel size. Compressed by run length encoding
 and line repeats.
 - Recognised by 00 01 00 08 as the first four bytes ("version 1, header
   length 8") or by "IMG" as the name of the directory the file is in.
 - Planar file format slow to read
 - Palette not specified: ChangeFSI assumes linear shades of grey. You might
   need to use -invert if the image is a negative
 - The size of a pixel is specified in microns. The standard RISC OS 90
   pixels per inch translates to (1/90*2.54*10,000) microns - 282. (use
   -info to get information about the scaling, -nosize to stop it)
 - Documentation from the "ST World" magazine's clinic.

An unknown PC .PIC format
 Used for some VGA demos, the format provides an uncompressed 320 by 200 by
 256 colour (from 2^24) VGA screen dump.
 + Recognised by "AV_VO" as the first five bytes in the file
 - Documentation never seen
 - Pixel size assumed the same as MODE 13 (45 pixels per inch).

The MacPaint format
 Provides a 576 by 720 bitmap, run length coded by the Mac's "PackBits"
 algorithm (repeated bytes).
 - Recognised by "PNTG" at position 65 in the file. This may only relate to
   files which have been through a program called "MacFix". Data assumed to
   start at position 128 in the file. Header ignored.
 - No resolution specified, equal to mode 25 assumed

The Mac PICT2 format (PICT extended version 2)
 Provides an arbitary size packed picture, run length coded by the Mac's
 "PackBits" algorithm (repeated bytes).
 + Recognised by looking for the PICT header at position 514 in the file.
   The header is 00 11 02 FF 0C 00 FF FE.
 - Documentation available but mostly useless (it assumes you are about to
   call QuickDraw routines - "Note: for maximum safety and convenience, let
   QuickDraw generate and interpret your pictures." Apple fail to describe
   the PackBits algorithm in Inside Mac...)
 - PICT is the loose equivalent of RISC OS Draw file format, but without
   the explicit sizing of objects - for PICT, if there is an object that
   you don't understand, there's no way of skipping it (whereas for Draw,
   the size of the object is specified in an object indep. way). Apple
   present a large table of the sizes of the objects (0000 - FFFF) which is
   not in ChangeFSI. Consequently, if ChangeFSI finds an object it doesn't
   know about before the picture, it stops...
 - Only PackBitsRect (0098) (8bpp) and DirectBitsRect (009A) (24/32 bpp) are
   allowed.
 + ChangeFSI tries to skip a Clip (0001) region if it encounters it, since it
   is hard to get the Mac OS to make PICT files without a clip opcode in them.
 + size of pixel specified.
 - Only 3 pictures read - since the format is very complicated (e.g. the size
   of things change if >250 rows in the picture!) this is no guarentee that
   anything else will be right.

The ZSoft .PCX format
 Used by PC Paintbrush. Usually a 4 bit per pixel image (CGA, EGA, VGA) but
 can be 1-8 bits per pixel.
 - Recognised by 10,[0,2,3,4,5],1 at start of file. Or by name PCX.<foo>. Or
   by type &697.
 + Intelligent check for resolution of the DACs on the PC card. Use -info to
   find out what ChangeFSI thinks it is.
 + Pixel size information read. You may need to stop the scaling with
   -nosize, since its often wrong.

The RIX Softworks ColoRIX format
 Used by VGA Paint, image extension .SCE .. .SCX. 4 or 8 bits per pixel.
 - Recognised by "RIX3" at start of file
 - Documentation never seen
 - Only 5 files read: ChangeFSI checks to see if the header is the same
   as these files and complains otherwise.
 - No resolution specified, equal to mode 27 assumed

The Sun "pixrect" format
 Used under SunOS and also under X11. Commonly 1 or 8 bits per pixel, but up
 to 24 bits per pixel can be found (see the Sun NeWS release tape!).
 + Recognised by &59A55A95 at the start of the file.
 - No resolution specified, equal to mode 27 assumed
 - Sun didn't specify the pixel ordering of 24 bit per pixel images: many
   assume RGB when it was actually BGR. Change byte &17 in the Sun
   rasterfile between 1 and 3 if you have a problem.

The "pbm" 'portable bitmap' file format
 Used by the "PBM-PLUS" toolkit (mainly Unix) by Jef Poskanzer. 1-24 bits
 per pixel (more can be specified in the format but ChangeFSI cannot read
 them). ChangeFSI can only read the "RAWBITS" formats (it can write all of
 the pbm formats).
 + Recognised by P4/P5/P6 at start of file.
 + Writable by ChangeFSI
 - No resolution specified, equal to mode 27 assumed

The UNIX "rle" format
 Used by a program called "svfb" to save the contents of large frame
 buffers, checked with four images at 24 bits per pixel.
 + Recognised by &CC52 at the start of the file.
 - No resolution specified, equal to mode 27 assumed

The TrueVision Targa format
 Used by TrueVision for Vista boards. Up to 32 bits per pixel.
 - Recognised by suffix _TGA or _VDA on the name. Or by type &69d.
 - Only uncompressed formats readable.
 - Only 8 and 16 bit per pixel formats tested.

The "Flexible Image Transport System" (FITS) format
 Used for astronomical data. Up to 16 bits per pixel component.
 + Recognised by "SIMPLE" at the start of the file.
 - Only 8 and 16 bit per pixel component formats tested.
 - Only two files ever seen!
 - No resolution specified, equal to mode 27 assumed

The Irlam instruments colour scanner format
 24 bit per pixel from colour scanner.
 + Recognised by Irlam 24 at the start of the file.
 + Writable by ChangeFSI
 - No resolution specified, equal to mode 27 assumed
 - may need gamma correction

The Irlam instruments YUV 411 colour video format
 7 bits Y, up to 7 bits U and V.
 + Recognised by Irlam YUV 411 at the start of the file
 - no resolution specified, equal to mode 27 assumed

The JPEG 'JFIF' File Interchange Format
 Up to 24 bits R,G,B, compressed by the JPEG compression system.
 + Recognised by JFIF at position 6 in the file (you may have to
   cut headers off files, particularly if they are from Macs).
 + Often highly compressed
 + If ChangeFSI$Cache is large enough (bigger than the file), then high
   speed routines are used to decompress. Otherwise:
 - ChangeFSI calls a djpeg routine. This can be on the Run$Path, implemented
   as an Alias (in which case it *must* use WimpTask to start) or found
   inside ChangeFSI$Dir if all else fails. cjpeg (the compressor) is also
   available inside ChangeFSI$Dir - you will need to convert the image to
   P6 format before it can be compressed. Djpeg and cjpeg are by the
   PD JPEG group.
 - ... and therefore slow. Needs a lot of space in <Wimp$Scrap>.
 - no resolution specified, equal to mode 27 assumed

The Ronald Alpiar 36 and 48 bit per pixel formats
 12 and 16 bits for each of R,G,B, uncompressed
 + Recognised by ALPIAR12 or ALPIAR16 at start of file
 + followed by little endian word of X size and word of Y size
 + followed by r,g,b as little endian half words
 - no resolution specified, equal to mode 27 assumed

The btpc file format
 Up to 24 bits R,G,B, compressed by the btpc compression system
 + Recognised by btpc at start of the file
 + Highly compressed (sometimes more than JPEG)
 - ChangeFSI calls a (slow) dbtpc routine
 - no resolution specified, equal to mode 27 assumed

>> The following file formats are only recognised by their names.

An unknown PC .DSP format
 Experimentally determined that it holds a 640 by 350 EGA picture at 4 bits
 per pixel. ChangeFSI "knows" the default EGA palette (from IBM literature).
 - Recognised by being in a directory "DSP"
 - Documentation never seen
 - No resolution specified, equal to mode 27 assumed
 - Only two examples read(!)

The QRT .raw format
 Output from Steve Koren's public domain ray tracer: arbitrary size in 2^24
 colours, uncompressed. Documentation part of QRT.
 - Recognised by the name ending ".Raw". Or by type &698.
 - No resolution specified, equal to mode 27 assumed

The MTV pic. format
 Output from Mark Terrence VandeWettering public domain ray tracer arbitrary
 size in 2^24 colours, uncompressed. Documentation part of MTV. Also used by
 "RayShade".
 - Recognised by being in a directory "Pic.". Or by type &699.
 - No resolution specified, equal to mode 27 assumed

The RT image. format
 Output from "RT", a private ray tracer by Brian D Watt(?) arbitrary size in
 2^24 colours, run length encoded.
 - Recognised by being in a directory "Image."
 - No resolution specified, equal to mode 27 assumed

For many of these formats, especially the uncompressed ones, setting the
variable "ChangeFSI$Cache" to the number of bytes of temporary memory you
wish the program to use will speed up reading the information. The default
is 49152 bytes. For example "*set ChangeFSI$Cache 80k". For those worried
about speed, the program will go faster if:

 - you use -noscale and keep 1:1 ratios (don't specify them)
   (although -nosize and 1:1 or 1:2 ratios are quick, too)
 - you don't need sharpening/smoothing, histogram equalisation, range expansion
 - you use monochrome (e.g. 25, 26, 27 and 27t) modes

additionally you can disable dithering (-nodither) especially when reading
monochrome source.

You can avoid the program changing mode by putting -nomode on the command line.

Very large output files can be built strips at a time and sent directly to disk
using the parameter "-max" e.g:

  ChangeFSI <in> <out> 28 -max512K

will convert the file with a maximum of 512KBytes held in memory. Options 25t,
-rotate and -vflip don't work when this is being done.

Work will be done on other formats when and if both image and documentation
arrive at Acorn. Adding a new format is a matter of (a) putting in automatic
recognition of the format at the start of the program (b) reading the size of
the image and the colour palette mapping [in the next section of the program]
(c) writing a "read pixel row" element inside PROCiprow (d) providing an entry
in PROCrewind. If you write a new format interface, please send it to me so
that it will be included in later versions. ChangeFSI has already got code for
dealing with images with 1, 2, 4 and 8 bits per pixel packed into bytes and for
images with up to 8 bit planes. There is support code for LZW decompression and
LZW decoding.

As a special feature, ChangeFSI will also write out 256 grey level files (alter
rwt, gwt and bwt to get 256 level colour separations) in AIM format. This is
file type 4 with bytes representing the grey level. Use the = operation to
restrict the picture to 256x256y, otherwise it will be the same size as the
input. Specify "aim" as the mode string to do this.

For portability to other machines, ChangeFSI can write out files in "pbm"
format. There are 3 types of format "black/white" (pbm), "grey" (pgm) and
"colour" (ppm) with pure ASCII and binary encodings of each:

ASCII                                   Binary
p1        "black/white" (like mode 18)  p4 (big endian bytes!)
p2        "grey" (like mode 20t)        p5
p3        "true colour"                 p6

Files can be read with Jef Poskanzer's portable toolkit on other machines. The
p2 format is easily convertible by programmers; it consists of:

P2
# comment line
xsize ysize
maximum_value
pixel_value <whitespace> pixel_value

where all the numbers are in decimal (ASCII). 0 means black, maximum_value (=
15) means white. The p3 format is similar with r, g, b triples for each pixel
value. The number of bits per component (default 8) can be set from the command
line with, for example, "p3,4" for 4 bits per component.

As a special packed output format, "P15" has been implemented. Output is binary
pixel values, 5 bits per component, little endian RGB order packed into 16
bits. Header information as the other p formats.

Files can be written as new RISC OS sprites using S16 and S32 as the formats.
The x and y pixels per inch are given as a comma seperated list - a 16 bit mode
12 equivalent is S16,90,45 while a 32 bit mode 28 equivalent is S32,90,90.

Files can be written as JPEG images using JPEG (colour) or JPEGMONO. A quality
in the range 1 to 100 must be added to the end (e.g. JPEG75 or JPEGMONO50).

ChangeFSI can write Irlam format images (somewhat slower than p6 images...).

                              Output summary
                              ==============

# cols   Colour Cube  Bits used  Consistent Hue  Desktop Palette  Mode&Suffix

2^24            Y         24           Y             -            S32, p3 or p6 or Irlam or JPEG
2^21            Y         21           Y             -            p6,7
2^18            Y         18           Y             -            p6,6
2^15            Y         15           Y             -            S16, p15 or p6,5
2^12            Y         12           Y             -            p6,4
512             Y          9           Y             -            p6,3
256 grey        n          8           Y             -            aim or JPEGMONO
256 grey        n          8           Y             -            28d
256             Y          8           Y             Y            28
256             Y          8           Y             Y            28r
16 grey         n          4           Y             n            27t
16 grey         n          4           Y             n            p2 or p5
8 grey          n          3           Y             Y            27
16              ?          4           ?             Y            27r
16              Y          3           Y           can be         27d
4 grey          n          2           Y             Y            26
4               n          2           n             n            26c
4               n          2           n             n            26r
2 (black/white) n          1           Y             Y            25
2 (black/white) n          1           Y             Y            25c/d/t
2 (black/white) n          1           Y             Y            p1 or p4


ChangeFSI can be used from both the desktop front end and from the command
line. It can also be used as a BASIC library function:
LIBRARY"<ChangeFSI$Dir>.ChangeFSI" gives a new function, FNChangeFSI to the
BASIC programmer. This function takes:
  A$: a command line argument
  spritearea%: an address or -1 for the spritearea built by ChangeFSI
  workspace%: an address or -1 for ChangeFSI's other workspace
  worklimit%: an address (unused if workspace%<0)
  fast%: TRUE for MODE 0 selection
It produces an error (to whatever error handler is present) if it fails. If
spritearea% is <0 it will save the sprite to the file given in the command
string, otherwise it won't. The function FNChangeFSIVersion returns the version
string of the library (in the info box of the desktop application).

                           Command Summary
                           ===============

-info     give details

-hist     don't process image, but display histogram of it instead

-hflip    horizontally flip the image
-vflip    vertically flip the image
-rotate   rotate the image by +90 degrees (i.e. anti-clockwise)
-rotate-  rotate the image by -90 degrees (i.e. clockwise)
-max      set the maximum size of the image before disc is used

-noscale  disable automatic pixel size correction
-nosize   disable automatic pixel size correction for input pixels only
-nodither disable dithering
-nomode   don't change to mode 0 while processing the picture
-invert   invert colour range of image

-range    expand input's dynamic range to full scale
-equal    histogram equalisation
-sharpen  digitally sharpen the picture
-smooth   digitally smooth the picture

-gamma    set gamma correction
-black    correct for black ink spot size
-red      change red weight for deriving monochrome luminance
-green    change green weight for deriving monochrome luminance
-blue     change blue weight for deriving monochrome luminance

-brighten horrendous hack to slightly brighten images: may convert hues
          close to white to white. (It causes the output range 0..15 to
          be considered as 0..(15/16) instead of 0..(15/15)).

                           Mode Suffixes
                           =============

c
  1 bit modes: use clustered dithering with 4x4 cell
  2 bit modes: use the four colours "black", "cyan", "magenta" and "yellow"
d
  1 bit modes: use clustered dithering with 2x2 cell
  4 bit modes: use 1 bit red, 1 bit green, 1 bit blue
  8 bit modes: use 256 level grey scale
t
  1 bit modes: use clustered dithering with 3x3 cell
  4 bit modes: 16 level grey scale
r
  2 bit modes: Devious colour matching algorithm
  4 bit modes: Devious colour matching algorithm
  8 bit modes: Devious colour matching algorithm
-no suffix-
  1 bit modes: b/w dispersed dot dithering
  2 bit modes: 4 level grey scale
  4 bit modes: 8 level grey scale
  8 bit modes: John Bowler's colour matching algorithm
               (defaults to 'r' mode if not VIDC1 compatible)

Colour matching is done in r, g, b space: the minimum distance between colours
is the square root of:
   delta_r^2*rmatchwt+delta_g^2*gmatchwt+delta_b^2*bmatchwt
ChangeFSI has hard wired weights of 3, 10, 1 respectively for its colour
matching algorithms. John Bowler's algorithm (default for RISC OS 8 bit modes)
although somewhat slower, is fast enough to have replaced all the previous
methods [ASIDE it gives pictures at least as good as "p" mode in pre Aug 91
ChangeFSI's]. The Devious colour matching algorithm is recommended, and is
used automatically if the 8 bit per pixel mode does not have the default RISC
OS colour set.

[NOTE: thanks are due to Steve Green and the BBC and Irlam Instruments for
the intermediate systems that led to John Bowler's and the Devious
algorithms: until Aug 91 ChangeFSI used their work for its "Precise
Matching" algorithm. It no longer uses any of their code, but owes a debt
for overall approach. A small thank you to Spencer Thomas for putting me on
the track which led to the Devious colour matching system's algorithm.]

Further information on dithering can be found in "Digital Halftoning" by Robert
Ulichney published by the MIT Press, ISBN 0-262-21009-6. A book which would have
been useful when starting writing the program, rather than after it was nearly
finished! Histogram equalisation is in "Algorithms for Graphics and Image
Processing" by Theo Pavlidis published by Computer Science Press, ISBN
0-914894-65-X.

00000000  20 20 20 20 20 43 68 61  6e 67 65 46 53 49 3a 20  |     ChangeFSI: |
00000010  43 6f 6c 6f 75 72 20 49  6d 61 67 65 20 50 72 6f  |Colour Image Pro|
00000020  63 65 73 73 69 6e 67 20  55 73 69 6e 67 20 74 68  |cessing Using th|
00000030  65 20 32 35 36 20 63 6f  6c 6f 75 72 20 6d 6f 64  |e 256 colour mod|
00000040  65 73 0a 0a 41 72 63 68  69 6d 65 64 65 73 20 63  |es..Archimedes c|
00000050  6f 6d 70 75 74 65 72 73  20 68 61 76 65 20 66 75  |omputers have fu|
00000060  6c 6c 20 63 6f 6c 6f 75  72 20 64 69 73 70 6c 61  |ll colour displa|
00000070  79 20 63 61 70 61 62 69  6c 69 74 69 65 73 20 62  |y capabilities b|
00000080  75 69 6c 74 20 69 6e 74  6f 20 74 68 65 6d 20 61  |uilt into them a|
00000090  73 0a 73 74 61 6e 64 61  72 64 2e 20 48 6f 77 65  |s.standard. Howe|
000000a0  76 65 72 2c 20 63 6f 6d  70 61 72 65 64 20 77 69  |ver, compared wi|
000000b0  74 68 20 74 68 65 20 68  61 72 64 77 61 72 65 20  |th the hardware |
000000c0  66 6f 75 6e 64 20 69 6e  20 6c 61 72 67 65 72 20  |found in larger |
000000d0  77 6f 72 6b 73 74 61 74  69 6f 6e 73 2c 20 74 68  |workstations, th|
000000e0  65 0a 66 61 63 69 6c 69  74 69 65 73 20 70 72 6f  |e.facilities pro|
000000f0  76 69 64 65 64 20 68 61  76 65 20 62 65 65 6e 20  |vided have been |
00000100  63 61 72 65 66 75 6c 6c  79 20 72 65 64 75 63 65  |carefully reduce|
00000110  64 20 28 69 6e 20 6f 72  64 65 72 20 74 6f 20 6d  |d (in order to m|
00000120  61 6b 65 20 74 68 65 6d  20 63 68 65 61 70 0a 65  |ake them cheap.e|
00000130  6e 6f 75 67 68 20 74 6f  20 66 69 74 20 61 6c 6c  |nough to fit all|
00000140  20 74 68 65 20 74 69 6d  65 29 2e 20 53 6f 20 74  | the time). So t|
00000150  68 65 20 70 72 65 63 69  73 69 6f 6e 20 6f 66 20  |he precision of |
00000160  74 68 65 20 64 69 67 69  74 61 6c 20 74 6f 20 61  |the digital to a|
00000170  6e 61 6c 6f 67 75 65 0a  63 6f 6e 76 65 72 74 65  |nalogue.converte|
00000180  72 73 20 69 73 20 6f 6e  6c 79 20 34 20 62 69 74  |rs is only 4 bit|
00000190  73 20 28 69 6e 73 74 65  61 64 20 6f 66 20 38 29  |s (instead of 8)|
000001a0  20 61 6e 64 20 74 68 65  20 63 6f 6c 6f 75 72 20  | and the colour |
000001b0  70 61 6c 65 74 74 65 20  6f 6e 6c 79 20 68 61 73  |palette only has|
000001c0  20 31 36 0a 65 6e 74 72  69 65 73 20 28 69 6e 73  | 16.entries (ins|
000001d0  74 65 61 64 20 6f 66 20  32 35 36 29 2e 0a 0a 54  |tead of 256)...T|
000001e0  68 65 20 56 49 44 43 20  32 35 36 20 63 6f 6c 6f  |he VIDC 256 colo|
000001f0  75 72 20 6d 6f 64 65 0a  3d 3d 3d 3d 3d 3d 3d 3d  |ur mode.========|
00000200  3d 3d 3d 3d 3d 3d 3d 3d  3d 3d 3d 3d 3d 3d 3d 3d  |================|
00000210  3d 0a 0a 48 61 73 20 34  20 64 69 72 65 63 74 20  |=..Has 4 direct |
00000220  62 69 74 73 20 28 74 6f  70 20 32 20 67 72 65 65  |bits (top 2 gree|
00000230  6e 2c 20 74 6f 70 20 72  65 64 20 61 6e 64 20 62  |n, top red and b|
00000240  6c 75 65 29 20 70 6c 75  73 20 34 20 70 61 6c 65  |lue) plus 4 pale|
00000250  74 74 65 20 6c 6f 6f 6b  20 75 70 20 62 69 74 73  |tte look up bits|
00000260  0a 77 68 69 63 68 20 61  66 66 65 63 74 20 74 68  |.which affect th|
00000270  65 20 72 65 6d 61 69 6e  69 6e 67 20 44 41 43 20  |e remaining DAC |
00000280  62 69 74 73 2e 20 54 68  69 73 20 74 68 65 72 65  |bits. This there|
00000290  66 6f 72 65 20 67 69 76  65 73 20 61 20 63 68 6f  |fore gives a cho|
000002a0  69 63 65 20 6f 66 20 32  35 36 0a 63 6f 6c 6f 75  |ice of 256.colou|
000002b0  72 73 20 66 72 6f 6d 20  34 30 39 36 2c 20 62 75  |rs from 4096, bu|
000002c0  74 20 74 68 65 20 63 68  6f 69 63 65 20 69 73 20  |t the choice is |
000002d0  6e 6f 74 20 61 20 66 72  65 65 20 6f 6e 65 20 69  |not a free one i|
000002e0  6e 20 74 68 61 74 20 74  68 65 20 34 20 64 69 72  |n that the 4 dir|
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0000ba90  20 20 20 20 20 20 20 20  20 3d 3d 3d 3d 3d 3d 3d  |         =======|
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0000bb80  59 20 20 20 20 20 20 20  20 20 20 20 20 20 2d 20  |Y             - |
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0000bbc0  20 20 20 20 20 20 20 59  20 20 20 20 20 20 20 20  |       Y        |
0000bbd0  20 20 20 20 20 2d 20 20  20 20 20 20 20 20 20 20  |     -          |
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0000bc40  20 20 20 20 20 20 20 20  20 20 59 20 20 20 20 20  |          Y     |
0000bc50  20 20 20 20 31 32 20 20  20 20 20 20 20 20 20 20  |    12          |
0000bc60  20 59 20 20 20 20 20 20  20 20 20 20 20 20 20 2d  | Y             -|
0000bc70  20 20 20 20 20 20 20 20  20 20 20 20 70 36 2c 34  |            p6,4|
0000bc80  0a 35 31 32 20 20 20 20  20 20 20 20 20 20 20 20  |.512            |
0000bc90  20 59 20 20 20 20 20 20  20 20 20 20 39 20 20 20  | Y          9   |
0000bca0  20 20 20 20 20 20 20 20  59 20 20 20 20 20 20 20  |        Y       |
0000bcb0  20 20 20 20 20 20 2d 20  20 20 20 20 20 20 20 20  |      -         |
0000bcc0  20 20 20 70 36 2c 33 0a  32 35 36 20 67 72 65 79  |   p6,3.256 grey|
0000bcd0  20 20 20 20 20 20 20 20  6e 20 20 20 20 20 20 20  |        n       |
0000bce0  20 20 20 38 20 20 20 20  20 20 20 20 20 20 20 59  |   8           Y|
0000bcf0  20 20 20 20 20 20 20 20  20 20 20 20 20 2d 20 20  |             -  |
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0000bd20  65 79 20 20 20 20 20 20  20 20 6e 20 20 20 20 20  |ey        n     |
0000bd30  20 20 20 20 20 38 20 20  20 20 20 20 20 20 20 20  |     8          |
0000bd40  20 59 20 20 20 20 20 20  20 20 20 20 20 20 20 2d  | Y             -|
0000bd50  20 20 20 20 20 20 20 20  20 20 20 20 32 38 64 0a  |            28d.|
0000bd60  32 35 36 20 20 20 20 20  20 20 20 20 20 20 20 20  |256             |
0000bd70  59 20 20 20 20 20 20 20  20 20 20 38 20 20 20 20  |Y          8    |
0000bd80  20 20 20 20 20 20 20 59  20 20 20 20 20 20 20 20  |       Y        |
0000bd90  20 20 20 20 20 59 20 20  20 20 20 20 20 20 20 20  |     Y          |
0000bda0  20 20 32 38 0a 32 35 36  20 20 20 20 20 20 20 20  |  28.256        |
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0000bdd0  20 20 20 20 20 20 20 20  20 20 59 20 20 20 20 20  |          Y     |
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0000be00  20 20 20 20 20 20 34 20  20 20 20 20 20 20 20 20  |      4         |
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0000be60  20 20 20 20 20 20 6e 20  20 20 20 20 20 20 20 20  |      n         |
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0000beb0  20 59 20 20 20 20 20 20  20 20 20 20 20 20 32 37  | Y            27|
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0000bed0  20 3f 20 20 20 20 20 20  20 20 20 20 34 20 20 20  | ?          4   |
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0000bef0  20 20 20 20 20 20 59 20  20 20 20 20 20 20 20 20  |      Y         |
0000bf00  20 20 20 32 37 72 0a 31  36 20 20 20 20 20 20 20  |   27r.16       |
0000bf10  20 20 20 20 20 20 20 59  20 20 20 20 20 20 20 20  |       Y        |
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0000c050  20 20 20 59 20 20 20 20  20 20 20 20 20 20 20 20  |   Y            |
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0000d080  20 6c 65 64 20 74 6f 20  74 68 65 20 44 65 76 69  | led to the Devi|
0000d090  6f 75 73 20 63 6f 6c 6f  75 72 20 6d 61 74 63 68  |ous colour match|
0000d0a0  69 6e 67 20 73 79 73 74  65 6d 27 73 20 61 6c 67  |ing system's alg|
0000d0b0  6f 72 69 74 68 6d 2e 5d  0a 0a 46 75 72 74 68 65  |orithm.]..Furthe|
0000d0c0  72 20 69 6e 66 6f 72 6d  61 74 69 6f 6e 20 6f 6e  |r information on|
0000d0d0  20 64 69 74 68 65 72 69  6e 67 20 63 61 6e 20 62  | dithering can b|
0000d0e0  65 20 66 6f 75 6e 64 20  69 6e 20 22 44 69 67 69  |e found in "Digi|
0000d0f0  74 61 6c 20 48 61 6c 66  74 6f 6e 69 6e 67 22 20  |tal Halftoning" |
0000d100  62 79 20 52 6f 62 65 72  74 0a 55 6c 69 63 68 6e  |by Robert.Ulichn|
0000d110  65 79 20 70 75 62 6c 69  73 68 65 64 20 62 79 20  |ey published by |
0000d120  74 68 65 20 4d 49 54 20  50 72 65 73 73 2c 20 49  |the MIT Press, I|
0000d130  53 42 4e 20 30 2d 32 36  32 2d 32 31 30 30 39 2d  |SBN 0-262-21009-|
0000d140  36 2e 20 41 20 62 6f 6f  6b 20 77 68 69 63 68 20  |6. A book which |
0000d150  77 6f 75 6c 64 20 68 61  76 65 0a 62 65 65 6e 20  |would have.been |
0000d160  75 73 65 66 75 6c 20 77  68 65 6e 20 73 74 61 72  |useful when star|
0000d170  74 69 6e 67 20 77 72 69  74 69 6e 67 20 74 68 65  |ting writing the|
0000d180  20 70 72 6f 67 72 61 6d  2c 20 72 61 74 68 65 72  | program, rather|
0000d190  20 74 68 61 6e 20 61 66  74 65 72 20 69 74 20 77  | than after it w|
0000d1a0  61 73 20 6e 65 61 72 6c  79 0a 66 69 6e 69 73 68  |as nearly.finish|
0000d1b0  65 64 21 20 48 69 73 74  6f 67 72 61 6d 20 65 71  |ed! Histogram eq|
0000d1c0  75 61 6c 69 73 61 74 69  6f 6e 20 69 73 20 69 6e  |ualisation is in|
0000d1d0  20 22 41 6c 67 6f 72 69  74 68 6d 73 20 66 6f 72  | "Algorithms for|
0000d1e0  20 47 72 61 70 68 69 63  73 20 61 6e 64 20 49 6d  | Graphics and Im|
0000d1f0  61 67 65 0a 50 72 6f 63  65 73 73 69 6e 67 22 20  |age.Processing" |
0000d200  62 79 20 54 68 65 6f 20  50 61 76 6c 69 64 69 73  |by Theo Pavlidis|
0000d210  20 70 75 62 6c 69 73 68  65 64 20 62 79 20 43 6f  | published by Co|
0000d220  6d 70 75 74 65 72 20 53  63 69 65 6e 63 65 20 50  |mputer Science P|
0000d230  72 65 73 73 2c 20 49 53  42 4e 0a 30 2d 39 31 34  |ress, ISBN.0-914|
0000d240  38 39 34 2d 36 35 2d 58  2e 0a                    |894-65-X..|
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