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• Playing Video on a Sega Dreamcast

  9 March 2011, by Multimedia Mike — Sega Dreamcast

  Here’s an honest engineering question: If you were tasked to make compressed video play back on a Sega Dreamcast video game console, what video format would you choose? Personally, I would choose RoQ, the format invented for The 11th Hour computer game and later used in Quake III and other games derived from the same engine. This post explains my reasoning.

  Video Background
  One of the things I wanted to do when I procured a used Sega Dreamcast back in 2001 was turn it into a set-top video playback unit. This is something that a lot of people tried to do, apparently, to varying degrees of success. Interest would wane in a few years as it became easier and easier to crack an Xbox and install XBMC. The Xbox was much better suited to playing codecs that were getting big at the time, most notably MPEG-4 part 2 video (DivX/XviD).

  The Dreamcast, while quite capable when it was released in 1999, was not very well-equipped to deal with an MPEG-type codec. I have recently learned that there are other hackers out there on the internet who are still trying to get the most out of this system. I was contacted for advice about how to make Theora perform better on the Dreamcast.
  
  Interesting thing about consoles and codecs: Since you are necessarily distributing code along with your data, you have far more freedom to use whatever codecs you want for your audio and video data. This is why Vorbis and even Theora have seen quite a bit of use in video games, "internet standards" be darned. Thus, when I realized this application had no hard and fast requirement to use Theora, and that it could use any codec that fit the platform, my mind started churning. When I was programming the DC 10 years ago, I didn’t have access to the same wealth of multimedia knowledge that is currently available.

  Requirements Gathering
  What do we need here?

  • Codec needs to run on the Sega Dreamcast; this eliminates codecs for which only binary decoder implementations are available
  • Must decode 320x240 video at 30 fps; higher resolutions up to 640x480 would be desirable
  • Must deliver decent quality at 12X optical read speeds (DC drive speed)
  • There must be some decent, preferably free, encoder readily available; speed of encoding, however, is not important; i.e., "take as long as you need, encoder"

  Theora was the go-to codec because it’s just commonly known as "the free, open source video codec". But clearly it’s not suitable for, well... any purpose, really (sorry, easy target; OW! stop throwing things!). VP8/WebM — Theora’s heir apparent — would not qualify either, as my prior experiments have already demonstrated.

  Candidates
  What did the big boys use for video on the Dreamcast? A lot of games relied on CRI’s Sofdec middleware which was MPEG-1 video and a custom ADPCM format. I don’t know if I have ever seen DC games that used MPEG-1 video at a higher resolution than 320x240 (though I have not searched exhaustively). The fact that CRI used a custom ADPCM format for this application may indicate that there wasn’t enough CPU power left over to decode a perceptual, transform-based audio codec alongside the 320x240 video.

  A few other DC games used 4X Technologies’ 4XM format. The most notable licensee was Alone in the Dark: The New Nightmare (DC version only; PC version used Bink). This codec was DCT-based but incorporated 16-bit RGB colorspace into its design, presumably to optimize for applications like game consoles that couldn’t directly handle planar YUV. AITD:TNN’s videos were 640x360, a marked improvement over the typical Sofdec fare. I was about to write off 4XM as a contender due to lack of encoder, but the encoding tools are preserved on our samples site. A few other issues, though: The FFmpeg decoder doesn’t seem to work correctly as of this writing (and nobody has noticed yet, even though it’s tested via FATE).

  What ideas do I have? Right off the bat, I’m thinking vector quantizer (VQ). Vector quantizers are notoriously slow to compress but are blazingly fast to decompress which is why they were popular in the early days of video compression. First, there’s Cinepak. I fear that might be too simple for this application. Plus, I don’t know if existing (binary-only) compressors are very decent. It seems that they only ever had to handle small videos and I’ve heard that they can really fall over if anything more is demanded of them.

  Sorenson Video 1 is another contender. FFmpeg has an encoder (which some allege is better than Sorenson’s original compressor). However, I fear that the wonky algorithm and colorspace might not mesh well with the Dreamcast.

  My thinking quickly converged on RoQ. This was designed to run fullscreen (640x480) video on i486-class hardware. While RoQ fundamentally operates in a YUV colorspace, it’s trivial to convert it to any other colorspace during decoding and the image will be rendered in that colorspace. Plus, there are open source encoders available for the format (namely, several versions of Eric Lasota’s Switchblade encoder, one of which lives natively in FFmpeg), as well as the original proprietary encoder.

  Which Library?
  There are several code choices here: FFmpeg (LGPL), Switchblade (GPL), and the original Quake 3 source code (GPL). There is one more option that I think might be easiest, which is the decoder Dr. Tim created when he reverse engineered the format in the first place. That has a very liberal "do whatever you like, but be nice and give me credit" license (probably qualifies as BSD).

  This code is no longer at its original home but the Wayback Machine still had a copy, which I have now mirrored (idroq.tar.gz).

  Adaptation
  Dr. Tim’s code still compiles and runs great on Linux (64-bit!) with SDL output. I would like to get it ported to the Dreamcast using the same SDL output, which KallistiOS supports. Then, there is the matter of fixing the longstanding chroma bug in the original sample decoder (described here). The decoder also needs to be modified to natively render RGB565 data, as that will work best with the DC’s graphics hardware.

  After making the code work, I want to profile it and test whether it can handle full-frame 640x480 playback at 30 frames/second. I will need to contrive a sample to achieve this.

  Unfortunately, things went off the rails pretty quickly when I tried to get the RoQ decoder ported to DC/KOS. It looks like there’s a bug in KallistiOS’s minimalistic standard C library, or at least a discrepancy with my desktop Linux system. When you read to the end of a file and then seek backwards to someplace that isn’t the end, is the file still in EOF state?

  According to my Linux desktop:

  open file;          feof() = 0
  seek to end;        feof() = 0
  read one more byte; feof() = 1
  seek back to start; feof() = 0
  

  According to KallistiOS:

  open file;          feof() = 0
  seek to end;        feof() = 0
  read one more byte; feof() = 1
  seek back to start; feof() = 1
  

  Here’s the seek-test.c program I used to test this issue:

  PLAIN TEXT

  C:

  1. #include <stdio .h>
  2.  
  3. int main()
  4. {
  5.   FILE *f;
  6.   unsigned char byte;
  7.  
  8.   f = fopen("seek_test.c", "r");
  9.   printf("open file;     feof() = %d\n", feof(f));
  10.   fseek(f, 0, SEEK_END);
  11.   printf("seek to end;    feof() = %d\n", feof(f));
  12.   fread(&byte, 1, 1, f);
  13.   printf("read one more byte; feof() = %d\n", feof(f));
  14.   fseek(f, 0, SEEK_SET);
  15.   printf("seek back to start; feof() = %d\n", feof(f));
  16.   fclose(f);
  17.  
  18.   return 0;
  19. }

  EOF
  Speaking of EOF, I’m about done for this evening.

  What codec would you select for this task, given the requirements involved?

• Demo of WebM Running on TI OMAP 4 Processor

  15 October 2010, by noreply@blogger.com (John Luther)

  Texas Instruments has made a video of HD-resolution (1080p) VP8 (WebM) video playing on their new TI OMAP™ 4 processor, in both Android and Ubuntu.

  (If you have a WebM-enabled browser and are enrolled in the YouTube HTML5 beta the video will play in WebM HTML5, otherwise it will play in Flash Player.)

  For more info about the OMAP 4 and the IVA 3 video accelerator that enables low-power HD playback of VP8 on the chip, see the TI web site.

  

• Google’s YouTube Uses FFmpeg

  9 February 2011, by Multimedia Mike — General

  Controversy arose last week when Google accused Microsoft of stealing search engine results for their Bing search engine. It was a pretty novel sting operation and Google did a good job of visually illustrating their side of the story on their official blog.

  This reminds me of the fact that Google’s YouTube video hosting site uses FFmpeg for converting videos. Not that this is in the same league as the search engine shenanigans (it’s perfectly legit to use FFmpeg in this capacity, but to my knowledge, Google/YouTube has never confirmed FFmpeg usage), but I thought I would revisit this item and illustrate it with screenshots. This is not new information— I first empirically tested this fact 4 years ago. However, a lot of people wonder how exactly I can identify FFmpeg on the backend when I claim that I’ve written code that helps power YouTube.

  Short Answer
  How do I know YouTube uses FFmpeg to convert multimedia? Because:

  1. FFmpeg can decode a number of impossibly obscure multimedia formats using code I wrote
  2. YouTube can transcode many of the same formats
  3. I screwed up when I wrote the code to support some of these weird formats
  4. My mistakes are still present when YouTube transcodes certain fringe formats

  Longer Answer (With Pictures!)
  Let’s take a video format named RoQ, developed by noted game designer Graeme Devine. Originated for use in the FMV-heavy game The 11th Hour, the format eventually found its way into the Quake 3 engine as well as many games derived from the same technology.

  Dr. Tim Ferguson reverse engineered the format (though it would later be open sourced along with the rest of the Q3 engine). I wrote a RoQ playback system for FFmpeg, and I messed up in doing so. I believe my coding error helps demonstrate the case I’m trying to make here.

  Observe what happened when I pushed the jk02.roq sample through YouTube in my original experiment 4 years ago:

  
  
  

  Do you see how the canyon walls bleed into the sky? That’s not supposed to happen. FFmpeg doesn’t do that anymore but I was able to go back into the source code history to find when it did do that:

  
  
  

  Academic Answer
  FFmpeg fixed this bug in June of 2007 (thanks to Eric Lasota). The problem had to do with premature colorspace conversion in my original decoder.

  Leftovers
  I tried uploading the video again to see if the problem persists in YouTube’s transcoder. First bit of trivia: YouTube detects when you have uploaded the same video twice and rejects the subsequent attempts. So I created a double concatenation of the video and uploaded it. The problem is gone, illustrating that the backend is actually using a newer version of FFmpeg. This surprises me for somewhat esoteric reasons.

  Here’s another interesting bit of trivia for those who don’t do a lot of YouTube uploading— YouTube reports format details when you upload a video:

  
  
  

  So, yep, RoQ format. And you can wager that this will prompt me to go back through the litany of unusual formats that FFmpeg supports to see how YouTube responds.