[PATCH 3/3] ipa: ipu3: Use a Y histogram instead of green

[Dan Scally]

Hi Jean-Michel - sorry for the delay replying to you

On 18/04/2024 13:54, Jean-Michel Hautbois wrote:
> Hi Daniel,
>
> On 18/04/2024 14:46, Daniel Scally wrote:
>> The IPU3 IPA module uses a green histogram for the AGC algorithm's
>> constraint mode calculations. Switch instead to a luminance histogram
>> calculated using the Rec.601 coefficients.
>>
>> Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
>> ---
>>   src/ipa/ipu3/algorithms/agc.cpp | 15 ++++++++-------
>>   1 file changed, 8 insertions(+), 7 deletions(-)
>>
>> diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
>> index a59b73fb..76d2bb60 100644
>> --- a/src/ipa/ipu3/algorithms/agc.cpp
>> +++ b/src/ipa/ipu3/algorithms/agc.cpp
>> @@ -152,18 +152,19 @@ Histogram Agc::parseStatistics(const ipu3_uapi_stats_3a *stats,
>>                   reinterpret_cast<const ipu3_uapi_awb_set_item *>(
>> &stats->awb_raw_buffer.meta_data[cellPosition]);
>>   -            rgbTriples_.push_back({
>> -                cell->R_avg,
>> -                (cell->Gr_avg + cell->Gb_avg) / 2,
>> -                cell->B_avg
>> -            });
>> +            uint8_t G_avg = (cell->Gr_avg + cell->Gb_avg) / 2;
>> +
>> +            rgbTriples_.push_back({cell->R_avg, G_avg, cell->B_avg});
>>                 /*
>> -             * Store the average green value to estimate the
>> +             * Store the average luminance value to estimate the
>>                * brightness. Even the overexposed pixels are
>>                * taken into account.
>>                */
>> -            hist[(cell->Gr_avg + cell->Gb_avg) / 2]++;
>> +            uint8_t y = (cell->R_avg * .299) +
>> +                    (G_avg * .587) +
>> +                    (cell->B_avg * .114);
>> +            hist[y]++;
>
> Is there a benefit to have the "real" Y value and not only the green parts (which reflect this Y 
> value quite nicely as it is ~60% of the value) ?


Only that it more accurately matches the Y-value that's used in the rest of the algorithm; 
estimateLuminance() uses the same Rec.601 coefficients and the gain that's derived from that 
calculation is compared against one derived from this Histogram.


> Could you measure it ? No increase in CPU time for big stats grids ?


I didn't check specifically but I can do - I didn't lose any framerate on my Surface Go2...but I 
imagine that the Imgu can run much faster than that is doing currently.


Cheers

Dan



>
> JM