[VDPAU] [PATCH 2/2] [PATCH] Enable lossless video playback via VDPAU

[José Hiram Soltren]

Hello,

On 11/03/2014 04:39 AM, Rémi Denis-Courmont wrote:
>    Hello,
> 
> Le 2014-10-31 21:11, Jose Soltren a écrit :
>>>> Added H264_HIGH_444PP profile which was not present before.
>>>
>>> Since all defined VDPAU video surface chroma types so far have 8-bits
>>> depth, I assume this only supports 8-bits... Probably that should be
>>> documented.
>>
>> Perhaps. The VDPAU API could conceivably provide enumerations for 10-bit,
>> 12-bit, etc. decoding; it is up to an implementation to support them. My
>> understanding is that lossless encoding is a only defined in the H.264
>> High 444PP profile, which also defines higher bit depth encodings.
> 
> I know. But at the very least, new chroma types would need to be defined,
> otherwise applications cannot create video surfaces to render into.

Yes. I support adding addition bit depths here. To be somewhat future-proof we
could add 10, 12, 14, and 16 bits.

>>>> +typedef struct {
>>>> +    /**
>>>> +     * This field must be filled with VDP_H264_EXTENDED_VERSION
>>>> +     */
>>>> +    uint32_t struct_version;
>>>> +    /** Performs lossless decode
>>>> +     * 0: no lossless decode
>>>> +     * 1: x264 lossless decode
>>>> +     * 2: JM lossless decode
>>>> +     * >2: unsupported lossless decode stream
>>>> +     */
>>>> +    uint8_t transform_bypass;
>>>> +} VdpPictureInfoH264Extended;
>>>> +
>>>
>>> It's not clear where/when that structure should be used by application.
>>> I think the documentation should probably explain that.
>>
>> I believe this is a workaround for the fact that VdpPictureInfoH264 is not
>> versioned. Ideally we would add this new transform_bypass structure to the
>> end of VdpPictureInfoH264, were it versioned.
>>
>> There are a number of workable solutions, and I am curious to know what
>> others on this list believe would be a good solution.
> (...)
>> I'm curious to know, which of the proposals (regarding both versioning,
>> and the intra8x8 workaround) are most favorable to a VDPAU implementation
>> implementor? To a decoder application maintainer?
> 
> Considering how it's done in libavcodec, I would suggest:
> 
> - Append the new members at the end of the existing structure.
> - Leave VDPAU_INTERFACE_VERSION as it is.
> - Forbid drivers from accessing the appended members if the decoder profile
> value is less than 22.

That is a simple enough solution. Since we only support lossless decode with
H264_HIGH_444PP that seems workable for an implementation as well. We would
need to document this of course.

> That should preserve both source and binary compatibility, with minimal
> difficulty for applications. This does however require well-behaving drivers.

Yes. Again, it only works well here due to the quirk of the H.264 specification
that ties lossless decoding to H264_HIGH_444PP.

> Regarding source compatibility, an old application would allocate space for the
> new members, but fail to initialize them. Regarding binary compatibility, an
> old application will not allocate space for the new members. Either way, the
> new driver should detect that the decode profile is less than 22 and not
> attempt to read the uninitialized structure members.

Yes.

> If an application wants to support both the old and new vdpau.h header versions
> at source level, it can use an #ifdef on the codec profile.

That does work, yes.

Thanks,
--José
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