[FFmpeg-devel] [PATCH] avcodec/vc1: fix B predictor validity for 4-MV MBs

[Jerome Borsboom]

> 2019-01-14 9:05 GMT+01:00, Jerome Borsboom <jerome.borsboom at carpalis.nl>:
>> The B predictor for 4-MV MBs in interlace field pictures is not used
>> for blocks 0 and 2 when the picture is 1 MB wide.
>>
>> Signed-off-by: Jerome Borsboom <jerome.borsboom at carpalis.nl>
>> ---
>> My 'shuffle calculation of MV predictor candidates' patch overlooked the
>> corner case of 1 MB wide field interlace pictures. According to VC-1 spec
>> and the reference decoder, the B predictor is not used for for block 0 and
>> block 2 when the picture is 1 MB wide. This patch corrects this.
>>
>>  libavcodec/vc1_pred.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/libavcodec/vc1_pred.c b/libavcodec/vc1_pred.c
>> index e1758a3817..77dda86cd0 100644
>> --- a/libavcodec/vc1_pred.c
>> +++ b/libavcodec/vc1_pred.c
>> @@ -289,6 +289,8 @@ void ff_vc1_pred_mv(VC1Context *v, int n, int dmv_x, int
>> dmv_y,
>>          case 3:
>>              off = -1;
>>          }
>> +        if (v->field_mode && s->mb_width == 1)
>> +            b_valid = b_valid && c_valid;
> 
> I will push this if you don't request commit rights
> but shouldn't this be "b_valid &= c_valid;"?
> 
> Carl Eugen

Please push. As b_valid and c_valid both expressions would give the same
result. I really meant logical comparison, so 'b_valid = b_valid &&
c_valid' is correct and will short-circuit where &= would not.

Regards,
Jerome