Movie Making from tcl

Making a movie on the computer amounts to taking a series of snapshots, and then running a movie encoder. I discuss here how to make an mpeg-encoded movie, using the graphics generated by a tcl program.

The basic ideas are more generally useful: a program can take snapshots of itself, for example, using basically the same steps.

The idea is to run a script, giving commands to the tcl program, and then taking a snapshot. Start the tcl program in background, for example,

~sethna/run_Jupiter 
^Z
bg

and set up the parameters until just before the first frame (i.e., set masses and time intervals).

Then start up wish. For the time being, version 4.0 is available via a shell script:

~sethna/run_wish

Wish can be used to try out tcl commands. We're using it in a different way: to send commands to other tcl programs! (This doesn't work on some systems, depending on how the x-authority is worked out... It works in ruph, though.)

Wish acts like a tcl interpreter, which is very much like shell scripts. (That is, some of this can also be done without tcl, so what you're learning is more general than it sounds.) Type in the following commands:

set xmin 2
set ymin 46
set width 441
set height 217
set window_name Jupiter
set max_frames 5

modified suitably for your movie. (An easy way to find the size of the clipped region is to grab one frame using xwd, load it into xv, clip it, and then use the Image Info window to find out what you've clipped out.) After setting up the shell-script xwd_it (see below), the following command will loop over frames, clip them, convert them to gif files, and store them as movie.XXX.gif in the current directory:

for {set frame 0} {$frame < $max_frames} {incr frame} {
     send Jupiter {.run invoke}
     if {$frame < 10} {
	xwd_it 00$frame $window_name $xmin $ymin $width $height
     } elseif {$frame < 100} {
	xwd_it 0$frame $window_name $xmin $ymin $width $height
     } else {
	xwd_it $frame $window_name $xmin $ymin $width $height
     }
}

The guts of this is done by the shell-script xwd_it.

#!/bin/csh

xwd -name $2 | xwdtopnm | pnmcut $3 $4 $5 $6 | ppmtogif > movie.$1.gif
exit 0

If tcl was a better shell, we could have just typed the line in: but tcl seems to interpret the vertical bars | as instructions to pipe both stdout and stderr into the following stdin... Anyhow, this makes the file movie.$frame.gif from whatever window is named $window_name. It does an x-window-dump, converts it into a pnm file, cuts out a rectangle starting at (xmin, ymin) pixels from the upper left of (width, height), and then stores it as a gif file. It should be easy to modify this for your own purposes...

Finally, you'll want to encode the frames into a movie. Copy this file

# parameter file with good default values
#
# use this as a guideline for any parameters you don't really understand
# or don't care about
#

PATTERN		IBBPBBPBBPBBPBB
OUTPUT		output.mpg

BASE_FILE_FORMAT	PNM
GOP_SIZE	30
SLICES_PER_FRAME	1

PIXEL		HALF
RANGE		10
PSEARCH_ALG	LOGARITHMIC
BSEARCH_ALG	CROSS2
IQSCALE		8
PQSCALE		10
BQSCALE		25

REFERENCE_FRAME	ORIGINAL

#
# you really need to understand the following
#
YUV_SIZE	352x240
INPUT_CONVERT	giftopnm *

INPUT_DIR	.

INPUT
movie.*.gif	[000-005]
END_INPUT

as gif2mpg.param, and modify it to your taste. (In particular, you're likely to have more frames than five!) YUV size is not important, since we're using PNM as the file format. Then, type

~scarlet/bin/mpeg_encode gif2mpg.param

wait, and

mpeg_play output.mpg

Movie Making from tcl

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