[FFmpeg-devel] [PATCH] mpeg2: fix block_last_index when mismatch control modifies last coeff

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> On Sun, Jun 20, 2010 at 11:41:32PM +0100, Mans Rullgard wrote:
>> ---
>>  libavcodec/mpeg12.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/libavcodec/mpeg12.c b/libavcodec/mpeg12.c
>> index bc9ddcc..bd579ac 100644
>> --- a/libavcodec/mpeg12.c
>> +++ b/libavcodec/mpeg12.c
>> @@ -939,7 +939,7 @@ end:
>>      }
>>      block[63] ^= (mismatch & 1);
>>  
>> -    s->block_last_index[n] = i;
>> +    s->block_last_index[n] = block[63]? 63: i;
>>      return 0;
>>  }
>>  
>> @@ -1090,7 +1090,7 @@ static inline int mpeg2_decode_block_intra(MpegEncContext *s,
>>      }
>>      block[63]^= mismatch&1;
>>  
>> -    s->block_last_index[n] = i;
>> +    s->block_last_index[n] = block[63]? 63: i;
>>      return 0;
>
> is this fixing a bug?

The value of block_last_index is wrong otherwise.  We're trying to add
a dc-only idct (and perhaps other long zero tails).  That requires a
correct block_last_index value.

> because, if it makes no difference, then it would cause a speedloss
> and possibly not small if this is a conditional branch that fails to be
> predicted

Do you have a better suggestion?

-- 
M?ns Rullg?rd
mans at mansr.com