Re: [FFmpeg-devel] [PATCH 01/17] avcodec/ac3enc: Don't presume ch_layout to be AV_CHANNEL_ORDER_NATIVE

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

James Almer:
> On 4/7/2024 6:53 PM, Andreas Rheinhardt wrote:
>> James Almer:
>>> On 4/7/2024 5:39 PM, Andreas Rheinhardt wrote:
>>>> It is perfectly legal for users to use a custom layout
>>>> that is equivalent to a supported native one.
>>>
>>> Is that really the case? FFCodec.p.ch_layouts[] has a list of native
>>> ones, and the generic encode.c code will reject anything not in it.
>>>
>>
>> This is not true. It allows everything that is equivalent (in the
>> av_channel_layout_compare() sense) to one of the channel layouts in
>> AVCodec.ch_layouts. This means that if ch_layout.u.map[i].id is strictly
>> ascending for 0 <= i <nb_channels and < 64, then it is equivalent to the
>> native channel layout with the same nb_channels whose mask is the
>> bitwise or of all (1ULL << ch_layout.u.map[i].id).
> 
> Right, misread av_channel_layout_compare() as rejecting layouts that
> were not the same order.
> 
>>
>>> I guess the encode.c check could be improved with
>>> av_channel_layout_retype().
>>>
>>
>> ch_layout is not supposed to be set by lavc for encoders.
> 
> I didn't mean to set the layout for encoders, i meant it as a way for
> encode.c to compare input layouts with encoder-supported native layouts,
> because of my misunderstanding of how av_channel_layout_compare() behaved.
> 
>>
>>>> In this case the union in AVChannelLayout is not an uint64_t mask,
>>>> but a pointer to a custom map.
>>>>
>>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>>>> ---
>>>>    libavcodec/ac3enc.c | 6 +-----
>>>>    1 file changed, 1 insertion(+), 5 deletions(-)
>>>>
>>>> diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c
>>>> index 7a6bcf7900..dc197c1517 100644
>>>> --- a/libavcodec/ac3enc.c
>>>> +++ b/libavcodec/ac3enc.c
>>>> @@ -2192,18 +2192,14 @@ av_cold int ff_ac3_encode_close(AVCodecContext
>>>> *avctx)
>>>>    static av_cold int set_channel_info(AVCodecContext *avctx)
>>>>    {
>>>>        AC3EncodeContext *s = avctx->priv_data;
>>>> +    uint64_t mask = av_channel_layout_subset(&avctx->ch_layout,
>>>> ~(uint64_t)0);
>>>
>>> Might as well use UINT64_MAX.
>>>
>>
>> This is a bitfield, so I intentionally used a bit-operation.
> 
> Then ~UINT64_C(0).
> 

And the advantage of this is?

- Andreas

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