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Script/PT2

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.

Tape/disk: Home » Personal collection » Acorn ADFS disks » Archimedes » Files.ADF
Filename: Script/PT2
Read OK: ✔
File size: 28A3 bytes
Load address: FFFFFF45
Exec address: ACEA4319
Duplicates

There is 1 duplicate copy of this file in the archive:

File contents
PROGRAMMING TECHNIQUE
by Dominic Ford
Part 2: Graphics

     The most important decision which you must make when you are 
writing the graphics routines for a program is which graphics mode 
to use. There is seven different display types available on the 
Electron, and an eighth is available on the BBC. Each display type 
has its own characteristics, and you must choose which is most suitable 
for your particular program. Some graphics modes, for example, allow 
you to have many different colours on screen, but poor detail, while 
others allow you to have very great detail but few colours. You select 
graphics modes by using the MODE x command. Details of the seven 
different available modes are given below:

x Colours Graphics? Res/G   Res/T  Memory
0   2      Yes      640x256 80x32   20K
1   4      Yes      320x256 40x32   20K
2   16     Yes      160x256 20x32   20K
3   2      No       N/A     80x25   16K
4   2      Yes      320x256 40x32   10K
5   4      Yes      160x256 20x32   10K
6   2      No       N/A     40x25   8K
7   16     No       N/A     40x25   1K <- BBC ONLY!

KEY:

x: MODE number.

Colours: The maximum number of different colours which may be 
used on screen at any one time.

Graphics?: Does this graphics mode allow the use of the graphics 
commands (ie. MOVE, DRAW, etc.)? Modes 3 & 6 do not allow these 
commands, as there are thin horizontal stripes in between the rows 
of characters, in which nothing can be displayed. Thus, any graphics 
drawn in these modes would have black horizontal stripes across them, 
and would look rather silly! To see what I mean, turn on your computer 
and type: MODE 6:COLOUR 129:COLOUR 0:CLS
The background should be entirely white, but there are horizontal 
stripes where the screen can be no colour except black.

Res/G: The resolution of the screen in this mode (ie. the number of 
pixels across and down the screen. Generally, the higher the value, 
the smoother your graphics will look. Note that this is not the same 
as the parameters taken by the MOVE and DRAW commands - these always 
scale the number of pixels across the screen to 1280 and down the screen 
to 1024, so that (1280,1024) will always be the top-right corner of the 
screen, and (0,0) the bottom left.

Res/T: The number of characters which fit across the screen in this 
mode when using text.

Memory: The amount of the computer's 32K of memory which is used by 
the graphics mode. If you are writing a long program, this is an important 
consideration, as you may not have room for the program after 
subtracting 20K from the computer's 32K RAM for a MODE 2 screen. If this 
is the case, you must use a mode which uses less memory, such as mode 5.

     My advice in choosing which mode to use would be to try, if at 
all possible, to use one of the 20K modes. The detail of MODE 1, and 
the colours of MODE 2, are both far superior to the quality of MODE 5. 
If you are an efficient programmer, you should find that it is perfectly 
possible to fit a reasonably lengthed program into memory at the same 
time as 20K of screen. Shipwrecked, for example, used MODE 2 for its 
graphics, as does Shipwrecked II on this issue.
     Fitting a program into memory as well as a large screen memory is 
fairly easy when you are using a tape or DFS filing system, but this 
can prove difficult with ADFS, because 4K of the memory normally used 
to store programs is taken away for the ADFS to use, leaving you only a 
mere 4.5K free for programs. Assuming that once you have loaded the 
program you do not intend to use the disk again, there is a way around 
this. If you switch over to a tape filing system, the ADFS memory becomes 
redundant and can be reclaimed for BASIC use. Once you have done this, of 
course, you cannot access disks without resetting the computer, so load 
and save options in your program are restricted to tape only. This 
technique will be explained in greater depth in a later article on 
memory management.

     After you have decided which mode to use, and so you know how 
many colours are available, you must decide what colours these are 
going to be. In MODE 2, where you have 16 colours, you have access 
to all of the colours available, and so this choice does not affect 
you. In 4 colour modes, on the other hand, you only initially have 
access to black, red, yellow and white. These are a warm selection 
of colours, but you might want a colder selection in your program, 
or just simply a set of colours which go well with your screen design. 
To do this you use the command:

VDU 19,logical colour number,actual colour number,0,0,0

     The logical colour number is the number that you use with the 
colour command to select that particular colour. In a four colour 
mode this must be between 0 and 3, and in a two colour mode this 
must be either 0 or 1. The actual colour number is the number of 
the colour which you want to assign to that logical colour. The 
actual colour numbers are given below:

0-Black               8-Flashing black/white
1-Red                 9-Flashing red/cyan
2-Green              10-Flashing grenn/magenta
3-Yellow             11-Flashing yellow/blue
4-Blue               12-Flashing blue/yellow
5-Magenta (purple)   13-Flashing magenta/green
6-Cyan (light blue)  14-Flashing cyan/red
7-White              15-Flashing white/black

     Looking at the table above, you might wonder what the difference 
is between flashing black/white and flashing white/black. If you look 
at them both seperately, there is little difference, but if you look 
at them together, you see that when one is black the other is white, 
and vice versa. So, for example, if you put a logo on the screen 
with the logo in colour 8 and the background in colour 15, the 
screen flashes between white on black and black on white. To 
demonstrate this, turn on your Electron and type:

MODE 6:VDU 19,1,8,0,0,0:VDU 19,0,15,0,0,0

     You can take this further. Four is the maximum number of colours 
allowed on screen at any one time. Therefore for different screens 
you can have different selections of colours. The main menu screen 
might be in warm colours, and then subsequent menu screens in colder 
colours. The changing of these logical colours is called "Palette 
Switching". You can go even further and create simply animation 
using palette switching; an effect which is called "Colour Cycling". 
If, for example, you put a row of colour 3 above a row of colour 2, 
with a row of colour 1 below that, and colour 0 at the base. 
Initially, you set all logical colours to black. Then, if you 
set logical colour 0 to white, the base appears. Now set logical 
colour 1 to white as well, the bar moves up the screen. Repeat 
this with colours 2 and 3, and you have a simple animation. This 
is even more powerful in MODE 2, where you can go through 16 rows 
up. This is demonstrated at the end of the tutorial.
     Having decided which graphics mode you are going to use, and 
which colours to have on the screen, you 
must decide how you are going to get your graphics onto the screen. 
There are many methods for producing graphics, and which you 
choose depends upon what you want the graphics for, and what you 
want them to do (and, of course, your programming ability). The 
most obvious is the PRINT command. This is a quick, easy way of 
putting text onto the screen. When used with the COLOUR command 
and the TAB(x,y), many interesting effects can be achieved. 
These are, however, greatly limited by the fact that only letters 
can be displayed on the screen. This limitation may be overcome using 
user defined characters (see the Graphics chapter of the User Manual 
for details). Even using these characters, however, there are serious 
limitations. They can only use one colour per character, and so even 
if you make different characters different colours, your screen will 
still appear rather dull. Also, they are difficult to use when 
dealing with large areas of the screen. 
     This is where MOVE, DRAW and PLOT are useful. Using these 
commands, lines can be drawn across the screen, and, using PLOT, 
triangles can be drawn. These commands are covered in the 
Graphics chapter of the user manual. The other major advantage of 
these graphics commands is that they can be used not just to set 
the colour of an area, but also to invert it. The GCOL command, 
which selects the colour in which graphics are displayed, takes 
two parameters:

GCOL x,c

c is the new colour in which graphics should be drawn.
x selects how this colour should be applyed:

x=0 means to set every pixel to that colour, regardless of its 
previous colour.
x=1 means to mix the light from the pixel already on the screen 
with that of the new colour. For example, if a red pixel is set 
to blue, the red and blue mix to form purple.
x=2 means that the light from the pixels is filtered with the new 
colour. If a red pixel is set to blue now, it is as if a blue 
filter has been placed in front of the red light, and the 
resultant colour is black (red light contains no blue component).
x=3 means to invert the old colour with the new colour.
x=4 means to set the pixels with the inverse of the new colour 
(regardless of the old colour).

     At first sight, it may seem that user defined characters serve 
no purpose once a programmer has learnt about the more flexible 
graphics commands, but this is not the case. If you try to produce 
detailed graphics using MOVE and DRAW, you will find that your 
program crawls along at an unacceptable rate. User defined characters, 
on the other hand, can produce detail quickly, especially if the 
pattern repeats, so that one character definition can be used 
repeatedly in several areas of the screen. This is demonstrated 
by the program which is available at the end of the tutorial.
     So, it seems that programmers are faced with a difficult 
problem. They can use graphics commands and get colour, or they 
can use user defined characters and get detail, but what if they 
want both at the same time? There must be a solution - how else 
would games like Citadel manage to produce such detailed 
colourful graphics, and at such a speed?
     There are, in fact, two possible solutions, both of which 
involve dealing with an area of memory known as screen memory. 
This is, however, very complicated, and will be discussed further 
in the next article.
00000000  50 52 4f 47 52 41 4d 4d  49 4e 47 20 54 45 43 48  |PROGRAMMING TECH|
00000010  4e 49 51 55 45 0a 62 79  20 44 6f 6d 69 6e 69 63  |NIQUE.by Dominic|
00000020  20 46 6f 72 64 0a 50 61  72 74 20 32 3a 20 47 72  | Ford.Part 2: Gr|
00000030  61 70 68 69 63 73 0a 0a  20 20 20 20 20 54 68 65  |aphics..     The|
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00000080  6e 67 20 74 68 65 20 67  72 61 70 68 69 63 73 20  |ng the graphics |
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00002740  6d 65 3f 20 54 68 65 72  65 20 6d 75 73 74 20 62  |me? There must b|
00002750  65 20 61 20 73 6f 6c 75  74 69 6f 6e 20 2d 20 68  |e a solution - h|
00002760  6f 77 20 65 6c 73 65 20  0a 77 6f 75 6c 64 20 67  |ow else .would g|
00002770  61 6d 65 73 20 6c 69 6b  65 20 43 69 74 61 64 65  |ames like Citade|
00002780  6c 20 6d 61 6e 61 67 65  20 74 6f 20 70 72 6f 64  |l manage to prod|
00002790  75 63 65 20 73 75 63 68  20 64 65 74 61 69 6c 65  |uce such detaile|
000027a0  64 20 0a 63 6f 6c 6f 75  72 66 75 6c 20 67 72 61  |d .colourful gra|
000027b0  70 68 69 63 73 2c 20 61  6e 64 20 61 74 20 73 75  |phics, and at su|
000027c0  63 68 20 61 20 73 70 65  65 64 3f 0a 20 20 20 20  |ch a speed?.    |
000027d0  20 54 68 65 72 65 20 61  72 65 2c 20 69 6e 20 66  | There are, in f|
000027e0  61 63 74 2c 20 74 77 6f  20 70 6f 73 73 69 62 6c  |act, two possibl|
000027f0  65 20 73 6f 6c 75 74 69  6f 6e 73 2c 20 62 6f 74  |e solutions, bot|
00002800  68 20 6f 66 20 77 68 69  63 68 20 0a 69 6e 76 6f  |h of which .invo|
00002810  6c 76 65 20 64 65 61 6c  69 6e 67 20 77 69 74 68  |lve dealing with|
00002820  20 61 6e 20 61 72 65 61  20 6f 66 20 6d 65 6d 6f  | an area of memo|
00002830  72 79 20 6b 6e 6f 77 6e  20 61 73 20 73 63 72 65  |ry known as scre|
00002840  65 6e 20 6d 65 6d 6f 72  79 2e 20 0a 54 68 69 73  |en memory. .This|
00002850  20 69 73 2c 20 68 6f 77  65 76 65 72 2c 20 76 65  | is, however, ve|
00002860  72 79 20 63 6f 6d 70 6c  69 63 61 74 65 64 2c 20  |ry complicated, |
00002870  61 6e 64 20 77 69 6c 6c  20 62 65 20 64 69 73 63  |and will be disc|
00002880  75 73 73 65 64 20 66 75  72 74 68 65 72 20 0a 69  |ussed further .i|
00002890  6e 20 74 68 65 20 6e 65  78 74 20 61 72 74 69 63  |n the next artic|
000028a0  6c 65 2e                                          |le.|
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