Animation

The question often comes up about how to make an animation in BRL-CAD. As BRL-CAD is primarily a solid modeling system for engineering and analysis purposes, there is (presently) only limited support for generating animations but it is possible with the assistance of some third-party tools. The basic method is to make your model, decide what keyframes and sequencing you want to have in your movie, render those keyframes as images, and then composite them into an animation. BRL-CAD can help will all but the last compositing step.

So basically you need to render frames for your animation and then composite them together into a movie.

Rendering image frames for an animation

There are a variety of rendering and image processing tools that support a wide range of visualization techniques. Tools that may be of interest include rt, rtedge, rtxray, pixmatte, and pixblend as well as a variety of other rendering and image processing tools.

If the animation you want is very simple like having an animation of something spinning, the relatively simple process is described in the #Fly-around animation section. If, however, what you want to animate is considerably more complex, there are a variety of tools that become important for setting up animation paths, managing keyframes, interpolating between frames, and previewing the animation. There are animation tools in MGED (see the animmate command for starters) as well as a variety of animation table processing tools outside of MGED.

Detailed arbitrary path animation

See Animation Techniques in BRL-CAD.

Fly-around animation

It can be really easy to make a simple fly-around animation of your geometry. The rt command has options for azimuth and elevation that will let you render animation frames around a given object. The first step is to render your images. You can create a shell script like this:

#!/bin/sh
for i in `loop 000 359 10` ; do 
	rt -a $i -e 35 -w 640 -n 480 -o image$i.pix database.g top_level_object 
	pix-png -w 640 -n 480 image$i.pix > image$i.png
	rm -f image$i.pix
done

This renders 36 images of the object top_level_object from a geometry database that was named database.g, creating images every 10 degrees (azimuth) at an elevation of 35 degrees. The images are named image000.pix through image350.pix.

Walking through the shell script, the second line (for i in `loop 000 359 10` ; do) just sets up a loop to run with variable i set to the next number each pass through the for loop. You can see the expansion by running the loop command directly:

[sean@bz (Fri Feb 13 11:23:07) ~]$ loop 000 259 10
000
010
020
...
350

The third line calls rt to render an image. The -a option sets the azimuth value and the -e option sets the elevation value. The -w and -n options set the width and height of the image respectively. The -o option specifies the filename for the output image that is generated. Finally, there is the name of the database to read from and the name of the object in that database that we want to render. The object is automatically centered in the view.

The fourth line converts the .pix file to a .png file. This is often useful/necessary because .pix files are raw first quadrant image files that aren't as prevalently supported as the PNG file format.

The fifth line deletes the pix file (since we have the png file now anyway).

Compositing an animation

Once you run the script and have generated your image frames, you're done with BRL-CAD. The next step is to take those image frames that were rendered and provide them as input to any number of tools that will generate a movie for you.

With ImageMagick

Assuming all of your images are numbered sequentially with padded zeros, this should do the trick for a simple animation using ImageMagick:

convert *.png movie.mpg

With MJPEG

For more control of the animation, the MJPEG project is another toolset that may help. See this tutorial for example steps.

With ffmpeg

To be written.

With Photoshop

To be written.

On a Mac OS X using iPhoto

Open iPhoto and import the images (File->Import).

To create the movie, select all of the images, and then choose File->Export. Since they are numbered sequentially with padded zeros, they should already be in order.

Select the "QuickTime" tab. You may wish to un-check the "Add currently selected music to movie" button. Now click the Export button.

Type a filename and click "Save".

That's it. At this point you should have a quicktime movie of your geometry.

With Mencoder

mencoder mf://*.png -mf w=800:h=600:fps=25:type=png -ovc lavc -lavcopts vcodec=mpeg4:mbd=2:trell -oac copy -o movie.avi

If you modify in the 'Fly-aroundAnimation' shell script the line:

for i in `loop 000 359 10` ; do

to

for i in `loop 000 359 2` ; do

then you get a smooth animation of your geometry.