Kostya's Boring Codec World

It is really a coincidence that about a week after I looked at their Pyramid codec I got reminded that there’s another codec of theirs exists, probably related to the WNV1 codec I REd back in 2005.

So apparently the codec codes YUY2 in 8×8 blocks. Each block is prefixed with a bit telling whether it’s a coded or skipped block. Coded block have additional 4-bit mode that seems to determine which quantisation they’ll use. The data is packed as deltas to the previously decoded value (per-component) using static codebook with values in -7..7 range (plus scaling by shifting left). There’s also an escape value in case raw value should be read instead. Overall it feels like Winnov Video 1 coding.

In other words, nothing remarkable but still a bit more advanced than usual DPCM-based intermediate codecs.

LGT 5/3 wavelet transform is the second simplest lossless wavelet transform (the first one is Haar transform of course) so it’s used in a variety of image formats (most famously in JPEG-2000) and intermediate video codecs. Recently I helped one guy with implementing it and while explaining things about it I understood it myself, so here I’m going to write it down for posterity.
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So I’ve spent an hour or so to look at IMM4.

What do you know, it’s a very simple IDCT codec with interframes. Intraframes have only DCT with usual run-level VLC coding, interframes have skip flag to tell whether this macroblock should be skipped or there’s a difference to the previous frame coded or intra block. See, no motion vectors, quantisation is single value per block (except for DC in intra block), there seems to be no zigzagging either. You cannot get much simpler than that.

A friend of mine Mario asked me to look at DNxHD 444. It turned out to be quite easy to support in libavcodec decoder (at least for CID 1256 for which I has sample) after I looked at the binary decoder. And I was curious what formats were there.

Here is the list of internal IDs supported by Avid decoder with a family they belong to, image parameters (width x height @ bitdepth) and other properties.

• 1233: Avid_HD (1920×1080@10) interlaced (marked as debug format)
• 1234: Avid_HD (1920×1080@10) interlaced (marked as debug format)
• 1235: Avid_HD (1920×1080@10) progressive
• 1236: Avid_HD (1920×1080@10) progressive (marked as debug format)
• 1237: Avid_HD (1920×1080@8) progressive
• 1238: Avid_HD (1920×1080@8) progressive
• 1239: Avid_HD (1920×1080@8) interlaced (marked as debug format)
• 1240: Avid_HD (1920×1080@8) interlaced (marked as debug format)
• 1241: Avid_HD (1920×1080@10) interlaced
• 1242: Avid_HD (1920×1080@8) interlaced
• 1243: Avid_HD (1920×1080@8) interlaced
• 1244: Avid_HD (1440×1080@8) interlaced
• 1250: Avid_HD (1280×720@10) progressive
• 1251: Avid_HD (1280×720@8) progressive
• 1252: Avid_HD (1280×720@8) progressive
• 1253: Avid_HD (1920×1080@8) progressive
• 1254: Avid_HD (1920×1080@8) interlaced
• 1256: DNx444 (1920×1080@10) progressive
• 1257: DNx444 (1920×1080@10) interlaced
• 1258: DNx100 (960×720@8) progressive
• 1259: DNx100 (1440×1080@8) progressive
• 1260: DNx100 (1440×1080@8) interlaced
• 32768: AHD-DBG-1 Avid_HD (64×32@8) interlaced
• 32769: AHD-DBG-2 Avid_HD (128×128@8) interlaced
• 32770: AHD-DBG-3 Avid_HD (480×320@8) interlaced
• 32771: AHD-DBG-4 Avid_HD (64×32@10) interlaced
• 32772: AHD-DBG-5 Avid_HD (128×128@10) interlaced
• 32773: AHD-DBG-6 Avid_HD (480×320@10) interlaced
• 36864: AHD-DBG-3 Avid_HD (720×512@8) interlaced

If you look at this table you can see more formats than supported by libavcodec currently. Unsupported formats being debug ones, interlaced ones and not belonging to Avid_HD family.

While I fully approve not having interlaced formats support, the rest can be supported (especially if samples are provided).

Sigh, too much intermediate codecs I had looked at.

(luckily there’s not much left of this month of intermediate codecs)

So I’ve looked at another intermediate codec, post title hints on both its name and design. Coding scheme is rather simple: you code lines either in raw form or with prediction (from the left neighbour for the top line or (3 * L + 3 * T - 2 * TL) >> 2 for other lines, prediction error is coded with fixed Huffman codes.

Simple, right?

Here’s the catch: there is an insane number of formats it supports, both for storage and output and there’s an insane number of decoding functions for decoding format X into format Y.

So quite probably no decoder — not interesting and too tedious.

I’ve finally brought myself into looking at alpha plane decoding support for ProRes. It was a bit peculiar but rather easy to reverse engineer. Now I only need to update my ConsumerRes decoder to support it.

And that’s probably enough for the month of intermediate codecs.

The adjective is referring to the hype that the company that made this codec is run by designers (unlike some other companies where even design is made by developers or — even worse — marketers). And let’s call it AWIC or iNtermediate codec for short. Let’s not mention its name at all.

It is a rather old codec and it codes 8-bit YUV420 in 16×16 macroblocks with DCT, quantisation and static codes. Frame is divided into slices in such way so that there are not more than 32 slices on one line (and slice height is one macroblock). The main peculiarity is having scalable mode — every macroblock is partitioned into 8×8 sub-macroblock (i.e. 8×8 luma block and two 4×4 chroma blocks) with the following data for the rest of the block, and this is exploited for decoding frames in half-width, half-height or half-width half-height modes.

Maybe I’ll write a decoder for it after all.

Astrologers proclaim month of intermediate codecs. Number of blog posts about intermediate codecs doubles! (from zero)

Let’s look at Canopus codecs and their development.

Canopus Lossless

This is very simple lossless video codec: you have code tables description and coded difference from the left neighbour (or top one for the first pixel) for each component. For RGBA and YUV there are slight improvements but the overall coding remains the same.

Canopus HQ

This is an ordinary intermediate codec employing IDCT with 16×16 macroblocks in 4:2:2 format and interlacing support.
It has predefined profiles with frame sizes (from 160×120 to 1920×1080), number of slices and macroblock shuffling order (yes, like DV it decodes macroblocks in shuffled order).

Block coding is nothing special but quantising is. For every macroblock there can be one of sixteen quantiser sets selected. And for each block one of four quantising matrices can be selected from that set. Of course there are different quantiser matrices for luma and chroma (i.e. 128 quantising matrices in total, about 80 of them are unique).

Interlacing is signalled per block (in case it’s enabled for the frame).

Canopus HQA

This is Canopus HQ with alpha support. The main differences are flexible frame size (no hardcoded profiles), alpha component in macroblocks and coded block pattern. Coding and tables seem to be the same as in HQ.

Coded block pattern specifies which of 4 luma blocks are coded (along with corresponding alpha and chroma blocks). Uncoded blocks are filled with zeroes (i.e. totally transparent).

Canopus HQX

This codec combines both previous codecs and extends them for more formats support. While HQ was 4:2:2 8-bit, this one can be 4:2:2 or 4:4:4 and 9-, 10- or 11-bit support (with or without alpha).

There are changes in overall and block coding.

Frame is now partitioned into slices of 480 macroblocks and every 16 macroblocks are shuffled.

Blocks now have more adaptive coding. DCs are coded as the differences to the previous ones (inside macroblock component) and instead of being coded as 9-bit number they are now Huffman-coded and table is selected depending on component bit depth. Quantisation is split: now there’s a selectable quantiser and two quantiser matrices (for luma/alpha and chroma). AC codes are selected depending on quantiser selected for the block. So there are less quantiser matrices (two instead of seventy eight) but more VLC tables (CBP + 3 DC + 6 AC tables instead of CBP and single AC table).

Conclusion

Reverse engineering all those formats was obvious because they are not complex, obfuscated or C++ (which is usually both).

Shall I write decoders for them? Unlikely. The codecs are not too interesting (I’ve seen only one Canopus HQ sample and no HQA or HQX samples at all) and rather tedious to implement because of all those tables. And we have Canopus Lossless decoder already.