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

[Stefan Klug]

The Y mode of the histogram get's captured at the ISP output, before the
output formatter.  This has the side effect, that the first and the last
bins of are empty in case of limited YUV range.  Another side effect is
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".

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

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>
---
 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