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FEMSdoc
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 » Archimedes archive » Acorn User » AU 1994-01.adf » !FEMS_App_FEMS_App |
| Filename: | FEMSdoc |
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| File size: | 4D86 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
This file describes, in a more complete way than the magazine article, the
syntax of script files for FEMS version 2.21. In the interests of clarity,
some parts of the article are also included in this file.
DESIGNING YOUR OWN SCENES
To give maximum flexibility in the setting up of simulations, and
because there are so many possibilities, the scene and parameters are
described in a text file "script" which tells the program what to do. Even
if you don't feel up to creating your own scenes, fiddling with the provided
definition files will allow you to change things like the screen resolution,
rendering style (whether the picture is drawn with dots or lines) and the
viewpoint used. Since the simulation setup files are just text files, they
can be created and modified in Edit or any other text editor. The magazine
explains the general idea of script files and some hints for the successful
creation of animations.
SCRIPT FILES: GENERAL
1 The first line of a script must be "FEMS2". This is so the program can
recognize valid definition files, and knows which version they are.
2 The program doesn't care whether the letters in the script are upper or
lower case, as it converts them all to lower case when it reads the file.
3 Lines can be concatenated by replacing a newline with a colon, and
spaces can be used instead of commas if you wish.
4 Comments are anything between a /* and a */ (like in C), and are
ignored. Comments can be nested.
5 A "block" consists of a block name followed by several commands
enclosed in curly brackets. The brackets must have a separator to either
side of them ie. use "rendering { type dots }" not "rendering{ type dots}"
6 A script consists of some global parameters, a "rendering" block, and
any number of "solid" and "background" blocks.
GLOBAL PARAMETERS
There are three global parameter commands:
frame_aim <number>
gravity <number>,<number>,<number>
time_div <number>
The number after frame_aim is the number of frames you want to aim for.
All this does is cause the program to pause after that many frames, so you
don't overshoot if the computer is being left unattended. It also sets the
number of frames used to estimate the finishing time. If the program has
continued past the aim point, or there is not enough space to store the
number of frames aimed for, the finishing time is based on the estimated
maximum number of frames.
"Gravity" sets the direction and speed with which things fall. There
are three numbers for the amounts in the x, y and z directions. Normally
these will be something like 0, 0 and -600 so things fall downwards. The
script "UpStairs" uses a sideways gravity to make the cube appear to climb
the stairs.
The "time_div" command sets the number of times the finite element
simulation procedures are called between frames, or in other words how
finely divided the time steps are. The number must be a power of 2, eg.
1,2,4,8 etc. The main purpose of this is to allow objects to be stiffer
without exploding, so the spring constants in the objects are automatically
increased when this value is increased.
THE RENDERING BLOCK
The other set of options affects the appearance of the picture on the
screen. This is called the "rendering block" and looks like this:
rendering {
type <render_type>
resolution <number>,<number>
view <number>,<number>
background <back_type>
}
<render_type> is one of "dots", "lines", "all_dots" or "wireframe".
"Dots" draws the points at the corners of the surface facets, and "lines"
draws the edges of the surface facets. The other two options do the same
but without removing hidden surfaces. The "dots" option uses the least
memory for frame storage, and "all_dots" is the quickest to draw.
The "resolution" command lets you choose the screen resolution you want
to use, by specifying the number of horizontal and vertical pixels. Useful
values are 320,256 (low resolution) 640,256 (high resolution) and 640,512
(multisync monitors), but you can use any resolution for which there is a
screen mode. The program will give an error of there is no mode which fits.
When choosing a resolution, bear in mind that higher resolutions will take
more frame storage space.
"View" specifies the angle from which you look at the scene. The
numbers are angles in degrees, the first being the direction in the x-y
plane. This is the angle through which the scene is rotated about the
vertical axis. If this is 0, the x direction is horizontally across your
screen (+x on the right, -y on the left) and the +y direction is out of the
screen towards you. The second number is the "elevation": an value of zero
will put the viewpoint on the x-y plane (looking horizontally, +z axis
vertically upwards), and an angle of 90 would have you looking directly down
from above.
The "background" command is optional: if you include "background back"
in the rendering block, the program assumes that the background object(s)
are always behind the solid object(s), and does not redraw the background
every frame. This is useful, for example, when the background is a set of
stairs, as it reduces the frame drawing time. The default setting is
"background mix" which lets the background object(s) be partly in front of
and partly behind the solid(s).
The order in which any of the above options are arranged does not
matter, as long as the rendering options are kept inside the rendering
block.
CREATING OBJECTS: BACKGROUND OBJECTS
Background objects are the fixed pieces of scenery which the moving
objects interact with. You can have as many background objects as you want:
they are all created at the origin but they can be moved to any position.
The script to create a background object is as follows:
background {
create {
type <object_type>
points <points_spec>
size <size_spec>
}
translate <x>,<y>,<z>
}
The indentation is optional: it was added for clarity. The first thing
in a background block must be a create block, and there must be only one of
these. The translate command is optional, and there may be any number of
these. The three parameters are the amounts by which to move the object in
the x, y and z directions. All distances should be followed by a unit, eg
3.3in (with no separating space). Units recognized are in, ft, cm and mm.
The default unit is 1/640 of a printer's point (a very small distance), so
if the translate command has no visible effect, check the units.
The first word in the create block must be "type", because the effect
of the "points" and "size" commands are dependant on the object type. For a
background object, <object_type> must be one of "sheet", "sphere" or
"stairs".
THE SPHERE OBJECT
This has one "points" parameter and one "size" parameter, i.e. the word
"points" must be followed by one number, and "size" must be followed by a
distance. The previous remarks about units for distances also apply to all
size parameters. The size parameter for a sphere sets its radius. The
points parameter sets the number of points used to draw the sphere: if this
parameter is P, the number of points used is 2 x P x P. There are P rings
of points, each containing 2P points. 12 is an adequate value for this
parameter.
You should avoid creating objects which extend into a sphere, because
these will be ejected outwards violently.
THE SHEET OBJECT
This has one "points" parameter and one "size" parameter. Sheets are
always square: the points parameter sets the number of points along one
side, and the size parameter sets the length of one side. For example, if
you use "points 21" and "size 5in" the program will create a sheet with 21 x
21 = 441 points, so if you use "lines" as the rendering type the sheet will
be divided into 20 x 20 squares, each 0.25in x 0.25in. The sheet is created
horizontal (ie. on the x-y plane) and centred on the origin, so in the above
example the sheet would extend from -5in to +5in in the x direction, from
-5in to +5in in the y direction, and have z=0 for all points.
When the sheet is created as a background object, it defines a "floor".
No solid will be allowed to pass below the height of the sheet. You should
avoid creating objects which extend below the floor, because these will be
ejected upwards violently. Note that the floor effect does not depend on
the size of the sheet, only on its vertical position.: the effective floor
is an infinite plane at the same height as the sheet. If you wanted an
invisible floor, therefore, you could create a small sheet with points set
to 2, and move it out of sight in the x or y direction.
THE STAIRS OBJECT
Being a more complex object, this takes three points parameters and
three size parameters. Just to confuse things, the points parameters do not
correspond to the same features as the size parameters, unlike the other
objects. Let's take the size parameters first; these are:
size <step_width>,<step_height>,<step_depth>
These are quite obvious: <step_width> is the width of the steps along
the x axis (see diagram below), <step_height> is the amount you would have
to lift your feet (z direction) and <step_depth> is the amount of room you
have for your feet on each step (y direction).
The points parameters are:
points <pts_width>,<steps>,<pts_depth>
The first and third of these just set the number of points used to draw
the horizontal part of the steps: the second is unusual in that it changes
the size of the object, since it sets the number of steps.
+
|\
+---+ \ Axes:
|\ \ \ z
+---+ \ \ + |
|\ \ \ \| |
+---+ \ \ +---+ *-----y
\ \ \ \| \
O \ +---+ \
\ \| x
+---+ O is the position of the origin
The above diagram shows the orientation and initial position of the
stairs. You will normally want to translate the stairs in the -y,-z
direction after creating them. Note that due to the orientation, the first
view parameter will need to be between 90 and 270 for the viewpoint to be on
the right side of the stairs.
Like the sheet, the extent of the stairs is in fact infinite: solids do
not fall off the sides of the stairs, and the stairs continue above and
below those which are visible.
CREATING OBJECTS: SOLID OBJECTS
solid {
create {
type <object_type>
points <points_spec>
size <size_spec>
}
scale <xs>,<ys>,<zs>
nearbonds {
spring_const <lambda>
range <range_val>
}
damping <damp_val>
<other commands>
}
The block to create a solid starts with a "create" block, like
backgrounds. The only difference here is the choice of objects you can
create: a sheet, a cuboid, a cube, a tube or a compound object. These will
be described individually later.
The scale command is optional: it lets you scale an object in the x, y
and z directions. This command is mainly present to allow you to change the
size of a compound object, which would otherwise be very tedious. I would
advise that you keep the x, y and z scalings similar, as objects which have
different point spacings in different directions tend to be unstable. The
scale command goes before nearbonds because otherwise would get stretched:
you may want to use this effect, but keep the scale factors close to 1 to
prevent explosions.
The "nearbonds" block is what makes the solid a solid. It needs to be
put after the "create" block, because it creates all the little springs
which hold the points in the object together. It is called "nearbonds"
because it only bonds together points which are fairly close. The number
after "spring_const" sets the stiffness of the springs, and "range" sets how
near two points have to be for a spring to connect them. For each object,
the program works out an average "nearest-neighbour" distance (call this D),
and the "range" value is in terms of D. Points with a separation of between
D/2 and D are linked with a spring of strength <lambda>, and points with a
separation between D and D*<range_val> are linked with a spring of strength
which decreases smoothly down to 0 for D*<range_val>. Points with a
separation of less than D/2 are not connected.
The "damping" command should be followed by a single integer: 3 or 4
are typical values. Solid objects need have the motion of their points
"damped", otherwise the springs would make them vibrate out of control.
Unfortunately, the damping also has the effect of stopping the whole object
moving or rotating very quickly, so things sometimes look as if they are
underwater. Generally, you will want the smallest value of damping which is
sufficient to stop the thing exploding.
OTHER COMMANDS
There are two commands you can use to place the object where you want
it:
translate <x>,<y>,<z>
rotate <xr>,<yr>,<zr>
The translate command has been described above. The rotate command
rotates the object ABOUT THE ORIGIN. It rotates by <xr>, <yr> and <zr>
degrees about the x, y, and z axes respectively. You can have any number of
translate and rotate commands without fear of slowing down the program,
because the commands only affect the initial setting up. For example, if
you want to rotate by 15 degrees about the point (1in,1in,2in), around the x
axis, you could use:
translate -1in,-1in,-2in
rotate 15,0,0
translate 1in,1in,2in
and then translate the solid to where you want it.
There are another two commands which can go in a solid block, but these
should only be given once in each solid. They are:
velocity <xv>,<yv>,<zv>
pull <first>,<last>,<step>,<xv>,<yv>,<zv>
The "velocity" command just gives the object an initial velocity. Use
trial and error to find the best value, but the size of numbers will need to
be around a few thousands (a setting of 1843 is 1 inch per second). Note
that high velocities will quickly be damped down.
The "pull" command is rather low-level: it needs to refer to the
numbering of the points in an object. The first three parameters are like a
FOR-NEXT loop which defines a set of points, and these points are given a
fixed velocity given by the last three parameters. Look at the simulation
script "ClothPull" for an example of the use of this command.
THE CUBOID OBJECT
To create a cuboid, use the following parameters in the "create" block:
type cuboid
size <x_size>,<y_size>,<z_size>
points <x_pts>,<y_pts>,<z_pts>
The meaning of the parameters is hopefully obvious. It is a good idea
to keep the separation of the points the same in all three directions, ie.
<size>/<pts> should be similar for x, y and z.
THE CUBE OBJECT
The setup for a cube is:
type cube
size <size>
points <pts>
This is basically the same as a cuboid, except the parameters for the
three directions are all set to be equal.
THE TUBE OBJECT
type tube
size <length>,<radius>,<thickness>
points <length_pts>,<circ_points>,<thick_points>
The <length> parameter sets the length of the tube. <Radius> sets the
distance from the axis of the tube to the centre of its walls, and
<thickness> sets the wall thickness.
The three points parameters set the number of points along the length,
around the circumference and across the wall respectively.
As I mentioned earlier, it is a good idea to keep the
"nearest-neighbour" distance fairly constant, so as a rough guide, try to
keep:
<length_pts> <circ_points> <thick_points>
------------ = ------------- = --------------
<length> 2*PI*<radius> <thickness>
and
<radius> > 2*<thickness>
These are only approximate rules, so feel free to try things out and
see what happens.
COMPOUND OBJECTS
Compound objects let you stick together any number of other solids to
create a more complex object. Here is an example of how a compound object
might look:
create {
type compound
part {
type cuboid
points 7,3,3
size 3in,1in,1in
offset 1in,0,0
}
part { type cuboid:points 3,3,11:size 1in,1in,5in }
part { type cuboid:points 3,7,3:size 1in,3in,1in:offset 0,1in,0 }
}
The first line in the "create" block is "type compound", then there is
a list of "part" blocks. Each of these is much like a "create" block (it
has type, size and points commands), but with the following differences: 1
You can add an "offset" command at the end 2 You cannot have a compound
type object as a part!
The "offset" command is exactly equivalent to a "translate" command,
but only applies to the particular part whose block it is in.
In the example above, the first "part" block is written out with one
command per line, but the rest are contracted onto one line each by
replacing newlines with colons. Remember that the curly brackets must be
separated from the commands, else you will get an error. There is no limit
on the number of parts a compound object can have. Compound solids work by
letting you create several objects before you use the "nearbonds" command.
When you do use this command, it connects the parts together. This method
means that parts will not get connected if they are separated by more than
the range of the "nearbonds" command. You should also not let parts pass
through each other, as the resulting compound object is unstable. Since the
nearbonds routine is working on all the parts in one go, you should also
keep the nearest-neighbour distances for all the parts similar.
GETTING SCREENSHOTS
There is no built-in facility for obtaining screenshots, but it is
simple to do this by adding a line to PROCframe_actions in the program.
This procedure is called after each frame has been drawn, with the frame
number passed in frame%. To save a picture of frame 30, add a line at the
beginning of PROCframe_actions which says:
IF frame%=30 THEN *ScreenSave FEMSpic30
You could also make the ScreenSave conditional on the Alt key being
held down, for example.
FACET OUTPUT
I have included in the program a facility to output the coordinates of
all the triangular "facets" which make up the surfaces of the objects. You
could use this data to create an input file for a ray tracer or other 3D
rendering program. As for screenshots, you just add a line to
PROCframe_actions like:
IF frame%=30 THEN PROCoutput_facets("$.Temp.facets30")
which will create a data file when the program reaches the 30th frame.
The data file consists of a list of numbers which can the read by a
INPUT#chan%,A%
type BASIC statement. You can read the file with a program which looks
something like this:
REPEAT
INPUT#chan%,A%
IF A%<>1 THEN ERROR 1,"Bad file"
INPUT#chan%,N%
FOR i%=1 TO N%
INPUT#chan%,X1%,Y1%,Z1%
INPUT#chan%,X2%,Y2%,Z2%
INPUT#chan%,X3%,Y3%,Z3%
REM do something with the data here
NEXT
UNTIL EOF#chan%
The coordinates of the three corners of each facet are given in units
of 1/640th of a point, which is 1/&B400 of an inch.
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*
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.