I’m pleased to announce the first (to my knowledge) public mirror of the world-famous MPlayerHQ multimedia samples archive, hosted right here at multimedia.cx (can you think of a better place?):

http://samples.multimedia.cx/

Here’s hoping the load doesn’t crush my simple hosting plan (which actually claims quite a bit of bandwidth per month, but we’ll see).

Readers know I have a peculiar interest in taking apart video games and that I would rather study a game’s inner workings than actually play it. I take an interest on others’ efforts in this same area. It’s still in my backlog to take a closer look at Clone2727′s body of work. But I wanted to highlight my friend’s work on re-implementing a game called Tele-Arena.

Back In The Day
As some of you are likely aware, there was a dark age of online communication that predated the era of widespread internet access. This was known as “The BBS Age”. People dialed into these BBSes using modems that operated at abysmal transfer speeds and would communicate with other users, upload and download files, and play an occasional game.

BBS software evolved and perhaps the ultimate (and final) evolution was Galacticomm’s MajorBBS (MBBS). There were assorted games that plugged into the MBBS, all rendered in glorious color ANSI graphics. One of the most famous of these games was Tele-Arena (TA). TA was a multiplayer fantasy-themed text adventure game. Perhaps you could think of it as World of Warcraft, only rendered as interactive fiction instead of a rich 3D landscape. (Disclaimer: I might not be qualified to make that comparison since I have never experienced WoW firsthand, though I did play TA on and off about 17 years ago).

TA was often compared to multi-user dungeons — or MUDs — that were played by telneting into internet servers hosting games. Such comparisons were usually unfavorable as people who had experience with both TA and MUDs were sniffy elitists with internet access who thought they were sooooo much better than those filthy, BBS-dialing serfs.

Sorry, didn’t mean to open old wounds.

Modern Retelling of A Classic Tale
Anyway, my friend Ron Kinney is perhaps the world’s biggest fan of TA. So much so that he has re-implemented the engine in Java under the project name Ether. He’s in a similar situation as the ScummVM project in that, while the independent, open source engine is fair game for redistribution, it would be questionable to redistribute the original data files. That’s why he created an AreaBuilder application that generates independent game data files.

Ironically, you can also telnet into a server on which Ron hosts an instance of Tele-Arena (ironic in the sense that the internet/BBS conflict gets a little blurry).

I hope that one day Ron will regale us with the strangest tales from the classic TA days. My personal favorite was “Wrath of a Sysop.”

I wanted to write down some notes about compiling Linux on Dreamcast (which I have yet to follow through to success). But before I do, allow me to follow up on my last post where I got Google’s libvpx library decoding VP8 video on the DC. Remember when I said the graphics hardware could only process variations of RGB color formats? I was mistaken. Reading over some old documentation, I noticed that the DC’s PowerVR hardware can also handle packed YUV textures (UYVY, specifically):

The video looks pretty sharp in the small photo. Up close, less so, due to the low resolution and high quantization of the test vector combined with the naive chroma upscaling. For the curious, the grey box surrounding the image highlights the 256-square texture that the video frame gets plotted on. Texture dimensions have to be powers of 2.

Notes on Linux for Dreamcast
I’ve occasionally dabbled with Linux on my Dreamcast. There’s an ancient (circa 2001) distro based around a build of kernel 2.4.5 out there. But I wanted to try to get something more current compiled. Thus far, I have figured out how to cross compile kernels pretty handily but have been unsuccessful in making them run.

Here are notes are the compilation portion:
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I got Google’s libvpx VP8 codec library to compile and run on the Sega Dreamcast with its Hitachi/Renesas SH-4 200 MHz CPU. So give Google/On2 their due credit for writing portable software. I’m not sure how best to illustrate this so please accept this still photo depicting my testbench Dreamcast console driving video to my monitor:

Why? Because I wanted to try my hand at porting some existing software to this console and because I tend to be most comfortable working with assorted multimedia software components. This seemed like it would be a good exercise.

You may have observed that the video is blue. Shortest, simplest answer: Pure laziness. Short, technical answer: Path of least resistance for getting through this exercise. Longer answer follows.

Update: I did eventually realize that the Dreamcast can work with YUV textures. Read more in my followup post.

Process and Pitfalls
libvpx comes with a number of little utilities including decode_to_md5.c. The first order of business was porting over enough source files to make the VP8 decoder compile along with the MD5 testbench utility.

Again, I used the KallistiOS (KOS) console RTOS (aside: I’m still working to get modern Linux kernels compiled for the Dreamcast). I started by configuring and compiling libvpx on a regular desktop Linux system. From there, I was able to modify a number of configuration options to make the build more amenable to the embedded RTOS.
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I haven’t given up on the Sega Dreamcast programming. I was able to compile a bunch of homebrew code for the DC many years ago and I can’t make it work anymore. Again, I was working with a purpose-built, open source RTOS named KallistiOS (or KOS). I can make the programs compile but not run. I had ELF files left over from years ago which still executed. But when I tried to build new ELF files, no luck– the programs crashed before even reaching my main() function.

I found the problem: ELF files are comprised of a number of sections and 2 of these sections are named ‘.ctors’ and ‘.dtors’ which stand for constructors and destructors. The KOS RTOS performs a manual traversal of .ctors section during program initialization and this is where things go bad. The traversal code doesn’t seem to account for a .ctors section that only contains a single entry. I commented out the function that does the traversal and programs started to work, at least until it was time to exit the program and return control to the program loader. That’s when the counterpart .dtors section traversal code ran and demonstrated the same problem. I’ll exhibit the problematic code at the end of this post.

So I’m finally tinkering with Sega Dreamcast programming once again and with a slightly better grasp of software engineering than the first time I did this.

Portable and Compatible C?
If nothing else, this low-level embedded stuff exposes you to some serious toolchain arcana, the likes of which you will likely never see working strictly in the desktop arena.

Still, this exercise makes me wonder why C code from a decade ago doesn’t compile reliably now. Part of it is because gcc has gotten stricter about the syntax it will accept. In the case of this specific crashing problem, I suspect it comes down to a difference in the way the linker generates the final ELF file. I’ve written a list of items I have had to modify in the KOS codebase in order to get it to compile on more recent gcc versions. I wonder if it would be worth publishing the specifics, or if anyone would ever find the information useful? Oh, who am I kidding? Of course I’ll write it up, perhaps publish a new version of the code, if only because that’s the best chance I have of finding my own work again some years down the road.

Problematic C Code
See if this code makes any sense to you. It somehow traverse a list of 32-bit function pointers (in different directions, depending on constructors or destructors), executing each in turn. However, it appears to fall over if the list of pointers consists of a single entry.
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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.

I’m still haunted by Sega Dreamcast programming, specifically the fact that I used to be able to execute custom programs on the thing (roughly 8-10 years ago) and now I cannot. I’m going to compose a post to describe my current adventures on this front. There are 3 approaches I have been using: Raw, Kallistios, and the almighty Linux.

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