A1: High Throughput Ray-Tracing with Condor

A ray-tracing tool creates very precise computer images from a detailed geometric specification. Unlike other forms of computer graphics, ray-tracing is very slow, because it computes exactly how a single beam of light propagates through a scene to arrive at each pixel on the screen. Ray-tracing is too slow to be used in real-time for, say, a computer game. Instead, it is used when quality is more important than speed, such as when rendering a high quality movie.

Rending a single frame of a Hollywood-quality movie may take minutes to hours even on a very fast computer. Rendering a movie measured in hours would take decades on a single computer. Fortunately, this is a perfect application of high-throughput computing. Each frame of a movie can be rendered on a different computer simultaneously, and then assembled at the end to produce a complete movie.

In this assignment, you will learn the basics of POVRay, an open source ray-tracing tool, and then use HTCondor to perform high-throughput generation of simple movies.

Objectives

At the conclusion of this assignment, students will be able to:
  • Operate an open-source ray-tracer at a basic level.
  • Divide a computational workload into pieces appropriate for high-throughput computing.
  • Construct, submit, and manage large numbers of batch jobs using HTCondor.
  • Analyze and explain the vital statistics of a high-throughput workload.

    • Quick Introduction to POVRay

    POVRay is a widely-used open source ray-tracer that can be run in batch mode on the command line or with a graphical display. You will also need ffmpeg, a video processing tool. Both are already installed in AFS, so just set your PATH like this: 
    setenv PATH /afs/nd.edu/user37/ccl/software/external/povray/bin:$PATH
setenv PATH /afs/nd.edu/user37/ccl/software/external/ffmpeg/bin:$PATH
    Then, download the example files rubiks.pov and WRC_RubiksCube.inc. (You can try out other examples from the POVRay collections site). To render a single image of a Rubik's cube: 
    povray +Irubiks.pov +Orubiks.png
    And then display the output file: 
    display rubiks.png

    Now, open up rubiks.pov in a text editor. The POVRay language describes exactly how to render the image, and the elements are fairly self explanatory. Scroll to the bottom, and look for the invocation of the cube: 

    object
{
WRC_RubiksRevenge("F'f' Rrd2R'r' U'u'Rr d2 U ld'l' U' ldl' R'r'Uu Ff")
translate < -6.6/2, 0, -6.6/2 >
rotate y*45
}
    WRC_RubiksRevenge is just a function defined in WRC_RubiksCube.inc, and the funny string following it simply defines how to set up the cubes and the colors. translate just moves the cube in the x, y, and z, directions, and rotate spins it around the Y axis by 45 degrees. (The cube is 6.5 centimeters on each edge, so the translate is just moving the rotation axis to the middle of the cube.)

    Add a few more cubes to the scene by copying the object clause three or four times. For each one, tweak the rotation and add a second translate after the rotate to move the new cubes around the scene. Adjust the settings until you have a couple of cubes in the view.

    Now, how do we make a movie? POVRay uses the built-in clock variable to represent the passage of time. clock is a floating point that varies only from zero to one through the course of an animation. You can set the clock value to, say, 0.5 by using the option +K.5.

    For example, change one of the rotation statements to: 

    rotate y*(45+clock*360)
    Render three slightly different frames like this: 
    povray +Irubiks.pov +Oframe000.png +K.0
povray +Irubiks.pov +Oframe001.png +K.1
povray +Irubiks.pov +Oframe002.png +K.2
    And then join them into a movie like this: 
    ffmpeg -r 10 -i frame%03d.png -r ntsc movie.mpg
    You can play the (very short) movie like this: 
    ffplay movie.mpg

    Quick Introduction to HTCondor

    We discussed HTCondor extensively in class. If you need more information, check out these pages:
  • HTCondor at Notre Dame
  • HTCondor Quick Start
  • HTCondor Complete Manual

    • The Assignment

    Create a tool called condor_povray that renders a long movie quickly by submitting each frame to be rendered as a separate Condor job. You may use any language that you are comfortable with, but I would recommend either Perl or Python. condor_povray should generate an appropriate condor submit file for each job and call condor_submit as needed. (Make sure to have each job specify the same user log file with log = something.log.) Then, use condor_wait to wait for all to finish and join them together with ffmpeg into a single finished .mpg movie that runs at 10 frames per second. The tool should be invoked like this: 
    condor_povray inputfile.pov outputfile.mpg numberofframes xsize ysize
    where the first argument is the input file to POVRay, the second argument is the output movie, the third is the number of frames to be created, and the fourth and fifth of the desired resolution. The user of condor_povray should not have to know anything about Condor; it should just work.

    Note: Make a point of deleting all of your .png files when each movie is completed, otherwise you will quickly run out of disk space!

    Exercises

    1. Using POVray directly, time how long it takes to render a single video frame in the following resolutions: 320x400, 640x480, 1280x720, and 1920x1080. From that measurement, figure out how long it would take to render one minute of video at 10 frames per second on a single machine.
    2. Using condor_povray, render a one minute video at each of the resolutions above. Keep both the output movie and the log file for each run. Submit each log file to the Condor Log Analyzer. Study the analysis critically, and describe what happened during the run. How many jobs ran at once? Did the work proceed smoothly? Did anything unusual happen?

    What to Turn In

    Your dropbox directory is: 
    /afs/nd.edu/coursefa.18/cse/cse40822.01/dropbox/YOURNAME/a1
    Into that directory, submit:
  • Source code to condor_povray.
  • Your modified rubiks.pov.
  • For each resolution, a movie file and a condor log file named e.g. rubik320.mpg and rubik320.log
  • A lab report titled answers.html containing answers to the questions above, along with links to the corresponding files and Condor Log Analyzer pages.
    • This assignment is due by 11:59PM on Friday, September 14th. Late assignments are not accepted.

    Extra Credit

    For (up to) 10 percent extra credit, pick a different image from the POVRay Object Collection, and render (one) new movie with some interesting movement or activity going on. I'll show the best examples in class.

    (Don't do this until your main results are working!)