[PATCH v3 20/23] libcamera: software_isp: Use floating point for color parameters

[Milan Zamazal]

It's more natural to represent color gains and black level 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.

Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
---
 src/ipa/simple/algorithms/blc.cpp    |  8 ++++----
 src/ipa/simple/algorithms/colors.cpp | 26 ++++++++++++++------------
 src/ipa/simple/ipa_context.h         | 11 +++++------
 3 files changed, 23 insertions(+), 22 deletions(-)

diff --git a/src/ipa/simple/algorithms/blc.cpp b/src/ipa/simple/algorithms/blc.cpp
index 3b73d830..ab7e7dd9 100644
--- a/src/ipa/simple/algorithms/blc.cpp
+++ b/src/ipa/simple/algorithms/blc.cpp
@@ -24,7 +24,7 @@ BlackLevel::BlackLevel()
 int BlackLevel::init(IPAContext &context,
 		     [[maybe_unused]] const YamlObject &tuningData)
 {
-	context.activeState.black.level = 255;
+	context.activeState.black.level = 1.0;
 	return 0;
 }
 
@@ -44,16 +44,16 @@ void BlackLevel::process(IPAContext &context,
 	const unsigned int total =
 		std::accumulate(begin(histogram), end(histogram), 0);
 	const unsigned int pixelThreshold = ignoredPercentage_ * total;
-	const unsigned int histogramRatio = 256 / SwIspStats::kYHistogramSize;
 	const unsigned int currentBlackIdx =
-		context.activeState.black.level / histogramRatio;
+		context.activeState.black.level * SwIspStats::kYHistogramSize;
 
 	for (unsigned int i = 0, seen = 0;
 	     i < currentBlackIdx && i < SwIspStats::kYHistogramSize;
 	     i++) {
 		seen += histogram[i];
 		if (seen >= pixelThreshold) {
-			context.activeState.black.level = i * histogramRatio;
+			context.activeState.black.level =
+				static_cast<double>(i) / SwIspStats::kYHistogramSize;
 			LOG(IPASoftBL, Debug)
 				<< "Auto-set black level: "
 				<< i << "/" << SwIspStats::kYHistogramSize
diff --git a/src/ipa/simple/algorithms/colors.cpp b/src/ipa/simple/algorithms/colors.cpp
index 60fdca15..77492d9e 100644
--- a/src/ipa/simple/algorithms/colors.cpp
+++ b/src/ipa/simple/algorithms/colors.cpp
@@ -36,7 +36,7 @@ int Colors::init(IPAContext &context,
 	updateGammaTable(context);
 
 	auto &gains = context.activeState.gains;
-	gains.red = gains.green = gains.blue = 256;
+	gains.red = gains.green = gains.blue = 1.0;
 
 	return 0;
 }
@@ -47,7 +47,7 @@ void Colors::updateGammaTable(IPAContext &context)
 	auto &blackLevel = context.activeState.gamma.blackLevel;
 	blackLevel = context.activeState.black.level;
 	const unsigned int blackIndex =
-		blackLevel * IPAActiveState::kGammaLookupSize / 256;
+		context.activeState.black.level * IPAActiveState::kGammaLookupSize;
 	std::fill(gammaTable.begin(), gammaTable.begin() + blackIndex, 0);
 	const float divisor = kGammaLookupSize - blackIndex - 1.0;
 	for (unsigned int i = blackIndex; i < kGammaLookupSize; i++)
@@ -67,15 +67,18 @@ void Colors::prepare(IPAContext &context,
 	auto &gains = context.activeState.gains;
 	auto &gammaTable = context.activeState.gamma.gammaTable;
 	for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
-		constexpr unsigned int div =
-			static_cast<double>(DebayerParams::kRGBLookupSize) * 256 / kGammaLookupSize;
+		constexpr double div =
+			static_cast<double>(DebayerParams::kRGBLookupSize) / kGammaLookupSize;
 		/* Apply gamma after gain! */
 		unsigned int idx;
-		idx = std::min({ i * gains.red / div, kGammaLookupSize - 1 });
+		idx = std::min({ static_cast<unsigned int>(i * gains.red / div),
+				 kGammaLookupSize - 1 });
 		params->red[i] = gammaTable[idx];
-		idx = std::min({ i * gains.green / div, kGammaLookupSize - 1 });
+		idx = std::min({ static_cast<unsigned int>(i * gains.green / div),
+				 kGammaLookupSize - 1 });
 		params->green[i] = gammaTable[idx];
-		idx = std::min({ i * gains.blue / div, kGammaLookupSize - 1 });
+		idx = std::min({ static_cast<unsigned int>(i * gains.blue / div),
+				 kGammaLookupSize - 1 });
 		params->blue[i] = gammaTable[idx];
 	}
 }
@@ -87,7 +90,7 @@ void Colors::process(IPAContext &context,
 		     [[maybe_unused]] ControlList &metadata)
 {
 	const SwIspStats::Histogram &histogram = stats->yHistogram;
-	const uint8_t blackLevel = context.activeState.black.level;
+	const double blackLevel = context.activeState.black.level;
 
 	/*
 	 * Black level must be subtracted to get the correct AWB ratios, they
@@ -104,12 +107,11 @@ void Colors::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(IPASoftColors, Debug) << "gain R/B " << gains.red << "/" << gains.blue;
 }
diff --git a/src/ipa/simple/ipa_context.h b/src/ipa/simple/ipa_context.h
index 979ddb8f..552d6cde 100644
--- a/src/ipa/simple/ipa_context.h
+++ b/src/ipa/simple/ipa_context.h
@@ -9,7 +9,6 @@
 #pragma once
 
 #include <array>
-#include <stdint.h>
 
 #include <libipa/fc_queue.h>
 
@@ -23,17 +22,17 @@ struct IPASessionConfiguration {
 
 struct IPAActiveState {
 	struct {
-		uint8_t level;
+		double level;
 	} black;
 	struct {
-		unsigned int red;
-		unsigned int green;
-		unsigned int blue;
+		double red;
+		double green;
+		double blue;
 	} gains;
 	static constexpr unsigned int kGammaLookupSize = 1024;
 	struct {
 		std::array<double, kGammaLookupSize> gammaTable;
-		uint8_t blackLevel;
+		double blackLevel;
 	} gamma;
 };
 
-- 
2.44.1