[FFmpeg-devel] [PATCH 2/3] avfilter/vf_geq: use per-thread state for expression evaluation

[Marton Balint]

On Sun, 29 Dec 2019, Michael Niedermayer wrote:

> On Sat, Dec 28, 2019 at 03:46:24PM +0100, Marton Balint wrote:
>> Fixes ticket #7528.
>>
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  libavfilter/vf_geq.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/libavfilter/vf_geq.c b/libavfilter/vf_geq.c
>> index 2905efae24..417b92222d 100644
>> --- a/libavfilter/vf_geq.c
>> +++ b/libavfilter/vf_geq.c
>> @@ -377,6 +377,7 @@ static int slice_geq_filter(AVFilterContext *ctx, void *arg, int jobnr, int nb_j
>>      int x, y;
>>      uint8_t *ptr;
>>      uint16_t *ptr16;
>> +    AVExprState *state = av_expr_state_alloc();
>>
>>      double values[VAR_VARS_NB];
>>      values[VAR_W] = geq->values[VAR_W];
>> @@ -386,6 +387,9 @@ static int slice_geq_filter(AVFilterContext *ctx, void *arg, int jobnr, int nb_j
>>      values[VAR_SH] = geq->values[VAR_SH];
>>      values[VAR_T] = geq->values[VAR_T];
>>
>> +    if (!state)
>> +        return AVERROR(ENOMEM);
>> +
>>      if (geq->bps == 8) {
>>          for (y = slice_start; y < slice_end; y++) {
>>              ptr = geq->dst + linesize * y;
>> @@ -393,7 +397,7 @@ static int slice_geq_filter(AVFilterContext *ctx, void *arg, int jobnr, int nb_j
>>
>>              for (x = 0; x < width; x++) {
>>                  values[VAR_X] = x;
>> -                ptr[x] = av_expr_eval(geq->e[plane], values, geq);
>> +                ptr[x] = av_expr_eval2(geq->e[plane], state, values, geq);
>>              }
>>              ptr += linesize;
>>          }
>> @@ -404,10 +408,11 @@ static int slice_geq_filter(AVFilterContext *ctx, void *arg, int jobnr, int nb_j
>>              values[VAR_Y] = y;
>>              for (x = 0; x < width; x++) {
>>                  values[VAR_X] = x;
>> -                ptr16[x] = av_expr_eval(geq->e[plane], values, geq);
>> +                ptr16[x] = av_expr_eval2(geq->e[plane], state, values, geq);
>>              }
>>          }
>>      }
>> +    av_expr_state_free(&state);
>
> a filter which adds random noise would get its seed reset on each frame and
> slice with this, unless iam missing something.
>
> The random values produced by random should be uncorrelatec between frame n and m when n!=m

I guess I can make one AVExprState for each thread and keep them during 
the lifetime of the filter. That would resolve the frame correlation, but 
not the slice correlation.

I don't see too many simple ways to deal with this:

1) accept slice correlation and keep the AVExprStates across executions

2) document that the number of filter threads has to be 1 if 
uncorrelated random is required

3) initialize the state with the line number at the beginning of each 
line, which is kind of ugly but would make the filter invariant of the 
thread count used.

Do you have something else in mind? Do you prefer any of these?

Thanks,
Marton