[PATCH v7 15/18] libcamera: software_isp: Use floating point for color parameters

[Milan Zamazal]

It's more natural to represent color gains as floating point numbers
rather than using a particular pixel-related representation.

double is used rather than float because it's a more common floating
point type in libcamera algorithms.  Otherwise there is no obvious
reason to select one over the other here.

The constructed color tables still use integer representation for
efficiency.

Black level still uses pixel (integer) values, for consistency with
other libcamera parts.

Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
---
 src/ipa/simple/algorithms/awb.cpp |  9 ++++-----
 src/ipa/simple/algorithms/lut.cpp | 13 ++++++++-----
 src/ipa/simple/ipa_context.h      |  6 +++---
 3 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/src/ipa/simple/algorithms/awb.cpp b/src/ipa/simple/algorithms/awb.cpp
index 3d76cc2f..195de41d 100644
--- a/src/ipa/simple/algorithms/awb.cpp
+++ b/src/ipa/simple/algorithms/awb.cpp
@@ -24,7 +24,7 @@ int Awb::configure(IPAContext &context,
 		   [[maybe_unused]] const IPAConfigInfo &configInfo)
 {
 	auto &gains = context.activeState.gains;
-	gains.red = gains.green = gains.blue = 256;
+	gains.red = gains.green = gains.blue = 1.0;
 
 	return 0;
 }
@@ -53,12 +53,11 @@ void Awb::process(IPAContext &context,
 	/*
 	 * Calculate red and blue gains for AWB.
 	 * Clamp max gain at 4.0, this also avoids 0 division.
-	 * Gain: 128 = 0.5, 256 = 1.0, 512 = 2.0, etc.
 	 */
 	auto &gains = context.activeState.gains;
-	gains.red = sumR <= sumG / 4 ? 1024 : 256 * sumG / sumR;
-	gains.blue = sumB <= sumG / 4 ? 1024 : 256 * sumG / sumB;
-	/* Green gain is fixed to 256 */
+	gains.red = sumR <= sumG / 4 ? 4.0 : static_cast<double>(sumG) / sumR;
+	gains.blue = sumB <= sumG / 4 ? 4.0 : static_cast<double>(sumG) / sumB;
+	/* Green gain is fixed to 1.0 */
 
 	LOG(IPASoftAwb, Debug) << "gain R/B " << gains.red << "/" << gains.blue;
 }
diff --git a/src/ipa/simple/algorithms/lut.cpp b/src/ipa/simple/algorithms/lut.cpp
index 44e13864..794d3644 100644
--- a/src/ipa/simple/algorithms/lut.cpp
+++ b/src/ipa/simple/algorithms/lut.cpp
@@ -59,15 +59,18 @@ void Lut::prepare(IPAContext &context,
 	auto &gammaTable = context.activeState.gamma.gammaTable;
 	const unsigned int gammaTableSize = gammaTable.size();
 	for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
-		const unsigned int div = static_cast<double>(DebayerParams::kRGBLookupSize) *
-					 256 / gammaTableSize;
+		const double div = static_cast<double>(DebayerParams::kRGBLookupSize) /
+				   gammaTableSize;
 		/* Apply gamma after gain! */
 		unsigned int idx;
-		idx = std::min({ i * gains.red / div, gammaTableSize - 1 });
+		idx = std::min({ static_cast<unsigned int>(i * gains.red / div),
+				 gammaTableSize - 1 });
 		params->red[i] = gammaTable[idx];
-		idx = std::min({ i * gains.green / div, gammaTableSize - 1 });
+		idx = std::min({ static_cast<unsigned int>(i * gains.green / div),
+				 gammaTableSize - 1 });
 		params->green[i] = gammaTable[idx];
-		idx = std::min({ i * gains.blue / div, gammaTableSize - 1 });
+		idx = std::min({ static_cast<unsigned int>(i * gains.blue / div),
+				 gammaTableSize - 1 });
 		params->blue[i] = gammaTable[idx];
 	}
 }
diff --git a/src/ipa/simple/ipa_context.h b/src/ipa/simple/ipa_context.h
index 737bbbc3..cf1ef3fd 100644
--- a/src/ipa/simple/ipa_context.h
+++ b/src/ipa/simple/ipa_context.h
@@ -26,9 +26,9 @@ struct IPAActiveState {
 	} blc;
 
 	struct {
-		unsigned int red;
-		unsigned int green;
-		unsigned int blue;
+		double red;
+		double green;
+		double blue;
 	} gains;
 
 	static constexpr unsigned int kGammaLookupSize = 1024;
-- 
2.44.1