libcamera: software_isp: Use floating point for color parameters

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@redhat.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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.red = sumR <= sumG / 4 ? 4.0 : static_cast<double>(sumG) / sumR;
-	gains.blue = sumB <= sumG / 4 ? 1024 : 256 * sumG / sumB;
+	gains.blue = sumB <= sumG / 4 ? 4.0 : static_cast<double>(sumG) / sumB;
-	/* Green gain is fixed to 256 */
+	/* Green gain is fixed to 1.0 */
 
 	LOG(IPASoftAwb, Debug) << "gain R/B " << gains.red << "/" << gains.blue;
 }

src/ipa/simple/algorithms/lut.cpp

@@ -63,15 +63,18 @@ void Lut::prepare(IPAContext &context,
 	const unsigned int gammaTableSize = gammaTable.size();
 
 	for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
-		const unsigned int div = static_cast<double>(DebayerParams::kRGBLookupSize) *
+		const double div = static_cast<double>(DebayerParams::kRGBLookupSize) /
-					 256 / gammaTableSize;
+				   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];
 	}
 }

src/ipa/simple/ipa_context.h

@@ -26,9 +26,9 @@ struct IPAActiveState {
 	} blc;
 
 	struct {
-		unsigned int red;
+		double red;
-		unsigned int green;
+		double green;
-		unsigned int blue;
+		double blue;
 	} gains;
 
 	static constexpr unsigned int kGammaLookupSize = 1024;