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

[Jason Garrett-Glaser]

On Sun, Jun 20, 2010 at 7:00 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Mon, Jun 21, 2010 at 02:47:16AM +0100, M?ns Rullg?rd wrote:
>> Michael Niedermayer <michaelni at gmx.at> writes:
>>
>> > On Mon, Jun 21, 2010 at 12:40:14AM +0100, M?ns Rullg?rd wrote:
>> >> 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?
>> >
>> > ignore it?
>>
>> Then the optimisation is impossible.
>
> you are unable to ignore block[63] in the dc idct ?

And have non-bit-exact decoding?

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