[Libva] H264 decoder erroneous output, some bytes off-by-one

[Gwenole Beauchesne]

Hi,

2014-06-11 11:51 GMT+02:00 Sam Jansen <sam.jansen at starleaf.com>:
> Hi all,
>
> Thanks for the quick replies!
>
>
> On 11 June 2014 05:54, Gwenole Beauchesne <gb.devel at gmail.com> wrote:
>>
>> Hi,
>>
>> 2014-06-10 20:33 GMT+02:00 Sam Jansen <sam.jansen at starleaf.com>:
>>
>> > I've been working on H264 encode, decode, and JPEG decode VA-API
>> > programs
>> > recently, using the Intel va-driver on Sandy Bridge and Bay Trail.
>> >
>> > This has gone well, but I've hit a problem with the H264 decoder in some
>> > situations where I believe it is producing incorrect output. In some
>> > cases
>> > chroma, and less often luma, data does not agree with the reference
>> > decoder
>> > [1], often in Intra frames (but sometimes P frames too), always baseline
>> > profile. I have an intra-encoded test stream where the 389th frame of a
>> > stream encoded from the standard "paris (cif)" test stream is incorrect
>> > on
>> > both Sandy Bridge and Bay Trail. Many other test streams pass fine,
>> > leaving
>> > me to believe our decoder implementation is largely correct.
>> >
>> > I've also implemented a mode where I am able to get the current encoded
>> > picture out of the VA-API encoder. I can then compare this,
>> > byte-for-byte,
>> > against a decode of the frame just encoded. I use this with out software
>> > codecs to ensure our encode and decode match exactly. Using this
>> > technique I
>> > can easily compare our software codec, and the VA-API H264 codecs in any
>> > combination. I've found the VA-API encode always agrees with our
>> > decoder,
>> > but the VA-API decoder does not always agree with the encode - either
>> > our
>> > software encoder or the VA-API encoder (this testing just on Bay Trail).
>> >
>> > I'd like to get to the bottom of this. The first step is removing our
>> > decode
>> > implementation from the test, and using one you believe to be good. What
>> > tools do you use for regression testing? Is this perhaps ffmpeg invoked
>> > in
>> > some way? Is your test setup open source, such that I can modify it to
>> > include my new test stream?
>>
>> The supported HW decoders that use VA-API and can be serve as
>> reference are either GStreamer/vaapi, or FFmpeg/vaapi (for H.264).
>> <http://gitorious.org/vaapi/gstreamer-vaapi/>
>>
>> Please provide me with a sample. We also have other tools, which I
>> will publish later on.
>>
>
> I've made it temporarily available at:
> http://cam.starleaf.com/paris_cif_intra.264
>
> Feel free to have a look at it. You're welcome to add it to whatever test
> suite you have as well. However, I'm happy right now that I should first use
> your gstreamer-vaapi to decode it, and check I get the same result there. I
> would have thought the most likely explanation at this point in time is that
> I have a bug in my VA-API H264 decoder -- I'd rather you didn't waste your
> time on my bugs!

I have just tried your clip against our internal tools. I can confirm
that gstreamer-vaapi decodes your file correctly on Ivybridge and
produces bitexact output, as compared against JM. Likewise for
FFmpeg/vaapi.

I am afraid, that might be an issue in your code then. :)

>> > PS: here's an extract of some of the chroma bytes where it has gone
>> > right.
>> > On the top, some bytes of reference decoder output, on the bottom, bytes
>> > of
>> > VA-API H264 decoder output. Note the 0x8f83 becomes 0x908f, then 0x8e8e
>> > becomes 0x8d8d, the 0x8e8e becomes 0x8d8e.
>>
>> Note: how do you retrieve the bytes for comparison? If this is through
>> vaGetImage(), the result of the conversion is generally not bitexact.
>>
>
> I use vaDeriveImage(), which gives me a VA_FOURCC_NV12 image. I then
> vaMapBuffer() it to get at the image data.
>
>>
>> Regards,
>> Gwenole.
>
>


Regards,
-- 
Gwenole Beauchesne
Intel Corporation SAS / 2 rue de Paris, 92196 Meudon Cedex, France
Registration Number (RCS): Nanterre B 302 456 199