[PATCH 5/8] ipa: rkisp1: Switch histogram to RGB combined mode

[Paul Elder]

Hi Stefan,

Thanks for the patch.

Quoting Stefan Klug (2025-04-11 15:04:12)
> The Y mode of the histogram get's captured at the ISP output, before the

s/get's/gets/

> output formatter.  This has the side effect, that the first and the last

s/,//

> bins of are empty in case of limited YUV range.  Another side effect is

s/bins of/bins/

> that gamma and GWDR processing is included in the histogram which makes
> algorithm development very difficult. In RGB mode the histogram is taken
> after xtalk (CCM) and is therefore independent of gamma and WDR. The
> limited range issue also does not apply. In the ISP reference it is
> however stated that "it is not possible to calculate a luminance or
> grayscale histogram from an RGB histogram since the position information
> is lost during its generation".

Hm, I see, so we need to use RGB mode so that we can tap before gamma and wdr.
Those are both probably nasty to reverse. It's probably better to use RGB mode
then. I suppose in the future we should use hist64, though the rk3399 will be
stuck with this.

> During testing the RGB histogram provided good data and better
> algorithmic stability at a possible (but not measured) inaccuracy.

Good, we get better data :)

> Another option would be to pass the color space information into the IPA
> and strip the histogram accordingly. For ease of implementation switch to
> the RGB mode.
> 
> Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>

Reviewed-by: Paul Elder <paul.elder at ideasonboard.com>

> ---
>  src/ipa/rkisp1/algorithms/agc.cpp | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/ipa/rkisp1/algorithms/agc.cpp b/src/ipa/rkisp1/algorithms/agc.cpp
> index 0e4fd3663167..a2c8d5403ba2 100644
> --- a/src/ipa/rkisp1/algorithms/agc.cpp
> +++ b/src/ipa/rkisp1/algorithms/agc.cpp
> @@ -379,7 +379,7 @@ void Agc::prepare(IPAContext &context, const uint32_t frame,
>         hstConfig.setEnabled(true);
>  
>         hstConfig->meas_window = context.configuration.agc.measureWindow;
> -       hstConfig->mode = RKISP1_CIF_ISP_HISTOGRAM_MODE_Y_HISTOGRAM;
> +       hstConfig->mode = RKISP1_CIF_ISP_HISTOGRAM_MODE_RGB_COMBINED;
>  
>         Span<uint8_t> weights{
>                 hstConfig->hist_weight,
> -- 
> 2.43.0
>