[libcamera-devel] [PATCH] ipa: ipu3: awb: Correct the coefficient factor

[Jean-Michel Hautbois]

The factor used right now in the IPU3 is 8192, as a multiplier of the
estimated gain. This is wrong, as the isp is adding 1.0 to the gain
applied, ie Pout = { Pin * (1 + Gx) }.

Fix it, and to ease the reading, introduce a small helper function.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
---
 src/ipa/ipu3/algorithms/awb.cpp | 21 +++++++++++++++++----
 src/ipa/ipu3/algorithms/awb.h   |  1 +
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/ipa/ipu3/algorithms/awb.cpp b/src/ipa/ipu3/algorithms/awb.cpp
index 5c232d92..6abaf75f 100644
--- a/src/ipa/ipu3/algorithms/awb.cpp
+++ b/src/ipa/ipu3/algorithms/awb.cpp
@@ -409,6 +409,19 @@ constexpr uint16_t Awb::threshold(float value)
 	return value * 8191;
 }
 
+constexpr uint16_t Awb::fixedGain(double gain)
+{
+	/*
+	 * For BNR parameters WB gain factor for the three channels
+	 * [Ggr, Ggb, Gb, Gr]. Their precision is U3.13 and the range is (0, 8)
+	 * and the actual gain is Gx + 1, it is typically Gx = 1.
+	 *
+	 * Pout = {Pin * (1 + Gx)}.
+	 */
+	gain = std::clamp((gain - 1.0), 0.0, 8.0);
+	return gain * 8192;
+}
+
 /**
  * \copydoc libcamera::ipa::Algorithm::prepare
  */
@@ -451,10 +464,10 @@ void Awb::prepare(IPAContext &context, ipu3_uapi_params *params)
 	params->acc_param.bnr.opt_center_sqr.y_sqr_reset = params->acc_param.bnr.opt_center.y_reset
 							* params->acc_param.bnr.opt_center.y_reset;
 	/* Convert to u3.13 fixed point values */
-	params->acc_param.bnr.wb_gains.gr = 8192 * context.activeState.awb.gains.green;
-	params->acc_param.bnr.wb_gains.r  = 8192 * context.activeState.awb.gains.red;
-	params->acc_param.bnr.wb_gains.b  = 8192 * context.activeState.awb.gains.blue;
-	params->acc_param.bnr.wb_gains.gb = 8192 * context.activeState.awb.gains.green;
+	params->acc_param.bnr.wb_gains.gr = fixedGain(context.activeState.awb.gains.green);
+	params->acc_param.bnr.wb_gains.r  = fixedGain(context.activeState.awb.gains.red);
+	params->acc_param.bnr.wb_gains.b  = fixedGain(context.activeState.awb.gains.blue);
+	params->acc_param.bnr.wb_gains.gb = fixedGain(context.activeState.awb.gains.green);
 
 	LOG(IPU3Awb, Debug) << "Color temperature estimated: " << asyncResults_.temperatureK;
 
diff --git a/src/ipa/ipu3/algorithms/awb.h b/src/ipa/ipu3/algorithms/awb.h
index 9a50a985..3154541d 100644
--- a/src/ipa/ipu3/algorithms/awb.h
+++ b/src/ipa/ipu3/algorithms/awb.h
@@ -73,6 +73,7 @@ private:
 	void awbGreyWorld();
 	uint32_t estimateCCT(double red, double green, double blue);
 	static constexpr uint16_t threshold(float value);
+	static constexpr uint16_t fixedGain(double gain);
 
 	std::vector<RGB> zones_;
 	Accumulator awbStats_[kAwbStatsSizeX * kAwbStatsSizeY];
-- 
2.34.1