I’ve wasted enough time on AVC decoder for On2 family so while it’s not working properly for those special cases I’m moving to VP7 regardless.
For those who don’t know (or forgot; or never had a reason to care) On2 AAC is AAC-LC rip-off with some creative reconstruction modes added to the usual long/short windows. I’ve failed to understand how it works before and I fail to understand how it works still. But at least some details are a bit clearer now that I’ve analysed the whole codec from scratch with less guesswork.
The codec has three IDs that it recognizes: 0x500, 0x501 and 0x1234. First two are different only in the aspect that one handles singular packets and another one handles several packets glued together prefixed with size. The last ID is simply recognized but it does not have any special handling.
The tricky part is some special modes that do some heavy processing of data. For most modes you invoke IMDCT and that’s all, here you do some QMF-like filtering (probably for transients extraction), then you perform RDFT (previously I thought it was plain FFT but after long investigation it turned out to be RDFT after all) on quarters, merge those quarters using filters that look like convolution filters for four sub-bands, perform RDFT again on the whole block and add some transients. And after that you still may need to reverse the data before using permuted window for overlap-add operation. In other words it’s not fun and I lack education for recognizing all those algorithms used, why they’re used and where it goes wrong.
So hopefully I’ll return to it some day to fix it for good but now VP7 awaits (so I can at least formally declare Duck codecs family done and move to implementing missing bits in the framework itself).
Yes, that codec is insanse.