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

[Daniel Scally]

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]++;
 		}
 	}
 
-- 
2.34.1