Kostya's Boring Codec World

Okay, I looked into G2M4 closer, here’s the output:

As with the previous beast, there are two types of images combined in single tile — so-called synthetic layer and natural layer. What you see if the first layer decoded.

Here’s the general tile structure:

• Compression subtype (top bits from the first byte).
• Transparency colour (three bytes)
• Number of palette entries minus one (one byte)
• Palette entries (byte triplets)
• Synthetic layer (16-bit BE chunk size plus deflated data, may be not present)
• Natural layer (probably headerless JPEG data, too lazy to verify)

Synthetic layer image is (after decompression) contains packed bitmap that uses palette from above, each row is coded as 8-bit flag [packed row data]. If the flag is zero then row data is present (that’s my guess, it always seems to be zero). Row data is just palette indices stored as 1/2/4 or 8 bits per index depending on palette size. Sample output you can see above.

Feel free to complete RE.

Just to clarify status: I’m not working on this anymore so anyone can pick it up and finish.

And here are some possible questions that might be asked but more probably won’t.

Q: who cares about this codec anyway?

A: Not me. VLC does.

Q: so why don’t you do it?

A: there are several reasons. First, now I have idea how it works and it’s not that interesting anymore. Second, it would require some debugging and I cannot run that decoder under MPlayer2 (and I don’t use Windows at all).

Q: but wait, there’s G2M4!

A: right, and it uses completely different coding. I might look at it but no promises either.

Q: so, how does it work?

A: the idea is simple. Every frame is divided into tiles and some tiles can be updated from the previous frame or not; some additional information (i.e. mouse cursor shape and position) is also stored in the frame.
G2M2/G2M3 use the technology licensed from Accusoft that combines JPEG and ELS-coded image.

Q: how do they do it?

A: the approach (I call it JPEG-Binary Koder or J-BK for short) is quite simple. Every tile has ELS-coded picture with possible transparency. Transparent areas should be replaced with headerless JPEG data (i.e. only scan data without any markers but with escapes).

Q: sounds easy, where’s the catch?

A: I’m too lazy to catch bugs in my quick JPEG decoder reimplementation and ELS-coded image requires some debugging which I can’t do.

Q: okay, I want to do it, where shall I start?

A: is it the first of April? No? Hmm… Okay, here’s what I would do: grab a copy of g2m.dll (there are enough of them around, in various sizes too), disassemble it.
Find the ELS thresholds table (referenced values are 0x10000, 0x12A00, 0x15C00, 0x19600, 0x1DA00, 0x22800, 0x28500, ...) — the function referencing it is the one used to update ELS coder state, go up from there. Feel free to look at the wiki entry about G2M. Bonne chance!

There is one codec unrivaled in its monstrosity. I’m talking about GoToMeetingAndNeverReturn.

First of all, look at its size. The oldest version I could find (it decodes only G2M2 and G2M3) occupies 6196600 bytes. Newer one with G2M4 support (and that’s an additional compressor too) is 8247160 bytes, Current version (with seemingly the same functionality) is 15665528 bytes.

What does it contain? Everything. I kid you not, All (maybe except one) GoToUnpleasantPlace utilities refer this small file and actually export their real main() function out from it. And their size is about forty kilobytes each.

What does it contain from code point of view? Some libraries for networking, cryptography, speech codecs library, libjpeg, zlib, some internal code and tons of C++-induced crap. When you see wrapper calling wrapper calling wrapper calling something trivial (and the wrappers just pass arguments as is and do nothing else), or when you see that 90% of any function is used for exception handling, then we’re talking about enterprise-grade C++ indeed. And of course absolute bloatedness (including making indirect calls where unneccessary) and inability to run on Linux (not with Wine or MPlayer2 loader) greatly help debugging.

Now to the codec details: every frame consists of chunks which hold different kind of data — frame information, mouse cursor shape, mouse cursor position, image data, etc. Frame is divided into tiles (usually 8×8 tiles in frame) and each tile is coded separately.

There are two compression methods, known by their numbers stored in frame header. For G2M2 and G2M3 it’s compression method 2, for G2M4 it’s compression method 3. Both are horrible.

Method 2 seems to have some completely unrelated submethods.

Here’s the call graph from method 2 decompression function.

Looks like there are several possibilities when coding with this method.

There is a possibility to use JPEG compression but I don’t know under what circumstances it’s used.
Also I’ve discovered that MSA1(MSS3) and MTS2(MSS4) actually use standard recommended quantisation tables from ITU T.81 Appendix K.1 (they are not used by libavcodec JPEG decoder though) and quality to quantisation mapping from libjpeg6.

Another coding method employs ELS codes I’ve described in the previous post and uses it to code some bit plane and RGB triplet in dependent form (i.e. with prediction from two previous pixels and using R-G, G, B-G components).

For method 3 so far I know only a few facts. First byte contains image type and depending on it decoding may vary a bit. For example, for image type 0 and 3 three following bytes contain some RGB value, other image types don’t have it. And internally it’s called MPCCoder or SMPCCoder.

In conclusion I want to say that this codec is too large and horrible to use only static reverse-engineering (because it’s very hard even to determine what is the next function called indirectly) and debugging it requires Windows, so don’t expect me to RE it. But if you do you’ll get many thanks and some money from these guys. Good luck!