Category Archives: General

Basic Video Palette Conversion

How do you take a 24-bit RGB image and convert it to an 8-bit paletted image for the purpose of compression using a codec that requires 8-bit input images? Seems simple enough and that’s what I’m tackling in this post.

Ask FFmpeg/Libav To Do It
Ideally, FFmpeg / Libav should be able to handle this automatically. Indeed, FFmpeg used to be able to, at least at the time I wrote this post about ZMBV and was unhappy with FFmpeg’s default results. Somewhere along the line, FFmpeg and Libav lost the ability to do this. I suspect it got removed during some swscale refactoring.

Still, there’s no telling if the old system would have computed palettes correctly for QuickTime files.

Distance Approach
When I started writing my SMC video encoder, I needed to convert RGB (from PNG files) to PAL8 colorspace. The path of least resistance was to match the pixels in the input image to the default 256-color palette that QuickTime assumes (and is hardcoded into FFmpeg/Libav).

How to perform the matching? Find the palette entry that is closest to a given input pixel, where “closest” is the minimum distance as computed by the usual distance formula (square root of the sum of the squares of the diffs of all the components).



That means for each pixel in an image, check the pixel against 256 palette entries (early termination is possible if an acceptable threshold is met). As you might imagine, this can be a bit time-consuming. I wondered about a faster approach…

Lookup Table
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Ode to the Gravis Ultrasound

WARNING: This post is a bunch of nostalgia. Feel free to follow along if you recall the DOS days of the early-mid 1990s.

Also, if you’re interested in the Gravis Ultrasound Experience CD-ROM, you can download the compressed ISO image here (137 megabytes). This only contains the data track as a .ISO file. However, the Internet Archive hosts the disc in CUE/BIN format which includes all the data tracks.

I finally let go of my Gravis Ultrasound MAX sound card a little while ago. It felt like the end of an era for me, even though I had scarcely used the card in recent memory.



The Beginning
What is the Gravis Ultrasound? Only the finest PC sound card from the classic DOS days. Back in the day (very early 1990s), most consumer PC sound cards were Yamaha OPL FM synthesizers paired with a basic digital to analog converter (DAC). Gravis, a company known for game controllers, dared to break with the dominant paradigm of Sound Blaster clones and create a sound card that had 32 digital channels.
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PC Video Conferencing in the Year 1999

Remember Intel’s custom flavor of H.263 cleverly named I.263? I think I have finally found an application that used it thanks to a recent thrift shop raid— Intel Video Phone:



The root directory of the disc has 2 copies of an intro.avi video. One copy uses Intel Indeo 3 video and PCM audio. The other uses I.263 video and an undetermined (presumably Intel-proprietary) audio codec — RIFF id 0x0402 at a bitrate of 88 kbits/sec for stereo, 22 kHz audio. The latter video looks awful but is significantly smaller (like 4 MB vs. 25 MB).

This is the disc marked as “Send it to a friend…”. Here’s the way this concept was supposed to operate:

  • You buy an Intel Video Phone Camera Pack (forgotten page courtesy of the Internet Archive) which includes a camera and 2 CDs.
  • You install the camera and video phone software on your computer.
  • You send the other CD to the person whom you want to be able to see your face when you’re teleconferencing with them.
  • The other party installs the software.
  • The 2 of you may make an internet phone call presumably using commodity PC microphones for the voice component; the person who doesn’t have a camera is able to see the person who does have a camera.
  • In a cunning viral/network marketing strategy, Intel encourages the other party to buy the physical hardware as well so that they may broadcast their own visage back to the other person.

If you need further explanation, the intro lady does a great job:



I suspect I.263 was the video codec driving this since Indeo 3 would probably be inappropriate for real time video applications due to its vector quantizing algorithm.

Music Video Idiosyncrasies

So I’m watching a fairly recent music video for a song named “XXXO” from an artist named M.I.A. when I’m suddenly assaulted by this imagery:



… and I enter nervous convulsions. You see, while this might seem to be an odd video effect to the casual viewer, to a multimedia hacker, it appears to be deliberately antagonistic. To anyone who has written a video codec, this scene looks like an entire casserole of video bugs, combining creeping plane offsets errors, chroma problems, and interlacing havoc. The craziest part is to realize that this is probably some kind of standard video effect / filter type. Upon a repeat viewing, I realized that the entire video sort of looks like an amateur video editor’s first week using video software.

Elsewhere in the video, a YouTube-style video frame vortex highlights the proceedings. I guess I need to come to terms with the fact that the ubiquitous player frame is just part of the digital Zeitgeist now:



Vintage Video Strangeness
I’m a long-time music video junkie but I have a tendency of examining them entirely too closely. I first saw Paula Abdul’s video for “Cold-Hearted” when I was just starting to understand multimedia technology and how it interacted with emerging home computers. Imagine how confused I was when I tried to make sense of the actions performed by our eMaestro “Chuck” whom Paula has instructed to “hit it”. First, he hits a key followed by 3 quick strikes on a second key:



Then, the “start music” action is apparently bound to a particular key on the electronic keyboard:
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