ipa: ipu3: agc: Improve gain calculation

When an image is partially saturated, its brightness is not increasing
linearly when the shutter time or gain increases. It is a big issue with
a backlight as the algorithm is fading to darkness right now.

Introduce a function to estimate the brightness of the frame, based on
the current exposure/gain and loop on it several times to estimate it
again and approach the non linear function.

Inspired-by: 7de5506c30b3 ("libcamera: src: ipa: raspberrypi: agc: Improve gain update calculation for partly saturated images")
Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
index 99c60d46..8f99dab9 100644
--- a/src/ipa/ipu3/algorithms/agc.h
+++ b/src/ipa/ipu3/algorithms/agc.h
@@ -34,7 +34,11 @@ private:
 	void measureBrightness(const ipu3_uapi_stats_3a *stats,
 			       const ipu3_uapi_grid_config &grid);
 	void filterExposure();
-	void computeExposure(IPAFrameContext &frameContext);
+	void computeExposure(IPAFrameContext &frameContext, double currentYGain);
+	double computeInitialY(IPAFrameContext &frameContext,
+			       const ipu3_uapi_grid_config &grid,
+			       const ipu3_uapi_stats_3a *stats,
+			       double currentYGain);
 
 	uint64_t frameCount_;