h264 video decoding: i-frames strike back

Last week I mentioned I had the basics of h264 decode using the proposed vulkan video on radv. This week I attempted to do the same thing with Intel's Mesa vulkan driver "anv".

Now I'd previously unsuccessfully tried to get vaapi on crocus working but got sidetracked back into other projects. The Intel h264 decoder hasn't changed a lot between ivb/hsw/gen8/gen9 era. I ported what I had from crocus to anv and started trying to get something to decode on my WhiskeyLake.

I wrote the code pretty early on, figured out all the things I had to send the hardware.

The first anv side bridge to cross was Vulkan is doing H264 Picture level decode API, so it means you get handed the encoded slice data. However to program the Intel hw you need to decode the slice header. I wrote a slice header decoder in some common code. The other thing you need to give the intel hw is a number of bits of slice header, which in some encoding schemes is rounded to bytes and in some isn't. Slice headers also have a 3-byte header on them, which Intel hardware wants you to discard or skip before handing it to it.

Once I'd fixed up that sort of thing in anv + crocus, I started getting grey I-frames decoded with later B/P frames using the grey frames as references so you'd see this kinda wierd motion.

That was I think 3 days ago. I've have stared at this intently for those 3 days blaming everything from bitstream encoding to rechecking all my packets (not enough times though). I had someone else verify they could see grey frames.

Today after a long discussion about possibilities, I was randomly comparing a frame from the intel-vaapi-driver and from crocus, and I spotted a packet header, the docs say is 34 dwords long, but intel-vaapi was only encoding 16 dwords, I switched crocus to explicitly state a 16-dword length and I started seeing my I-frames.

Now the B/P frames still have issues. I don't think I'm getting the ref frames logic right yet, but it felt like a decent win after 3 days of staring at it.

The crocus code is [1]. The anv code isn't cleaned up enough to post a pointer to yet, enterprising people might find it. Next week I'll clean it all up, and then start to ponder upstream paths and shared code for radv + anv. Then h265 maybe.



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