ipa: ipu3: agc: Standardize vocabulary on "relative luminance"

The AGC computes the average relative luminance of the frame and calls
the value "normalized luma", "brightness" or "initialY". The latter is
the most accurate term, as the relative luminance is abbreviated Y, but
the "initial" prefix isn't accurate.

Standardize the vocabulary on "relative luminance" in code and comments,
abbreviating it to Y when needed.

While at it, rename variables to match the libcamera coding style.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

2 files changed, 26 insertions, 26 deletions

diff --git a/src/ipa/ipu3/algorithms/agc.cpp b/src/ipa/ipu3/algorithms/agc.cpp
index 43a39ffd..9cd2ded5 100644
--- a/src/ipa/ipu3/algorithms/agc.cpp
+++ b/src/ipa/ipu3/algorithms/agc.cpp
@@ -62,12 +62,12 @@ static constexpr double kEvGainTarget = 0.5;
 static constexpr uint32_t kNumStartupFrames = 10;
 
 /*
- * Normalized luma value target.
+ * Relative luminance target.
  *
  * It's a number that's chosen so that, when the camera points at a grey
  * target, the resulting image brightness is considered right.
  */
-static constexpr double kNormalizedLumaTarget = 0.16;
+static constexpr double kRelativeLuminanceTarget = 0.16;
 
 Agc::Agc()
 	: frameCount_(0), iqMean_(0.0), lineDuration_(0s), minShutterSpeed_(0s),
@@ -250,12 +250,12 @@ void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
 }
 
 /**
- * \brief Estimate the average brightness of the frame
+ * \brief Estimate the relative luminance of the frame with a given gain
  * \param[in] frameContext The shared IPA frame context
  * \param[in] grid The grid used to store the statistics in the IPU3
  * \param[in] stats The IPU3 statistics and ISP results
  * \param[in] currentYGain The gain calculated on the current brightness level
- * \return The normalized luma
+ * \return The relative luminance
  *
  * Luma is the weighted sum of gamma-compressed R′G′B′ components of a color
  * video. The luma values are normalized as 0.0 to 1.0, with 1.0 being a
@@ -263,12 +263,12 @@ void Agc::computeExposure(IPAFrameContext &frameContext, double currentYGain)
  * luma here.
  *
  * More detailed information can be found in:
- * https://en.wikipedia.org/wiki/Luma_(video)
+ * https://en.wikipedia.org/wiki/Relative_luminance
  */
-double Agc::computeInitialY(IPAFrameContext &frameContext,
-			    const ipu3_uapi_grid_config &grid,
-			    const ipu3_uapi_stats_3a *stats,
-			    double currentYGain)
+double Agc::estimateLuminance(IPAFrameContext &frameContext,
+			      const ipu3_uapi_grid_config &grid,
+			      const ipu3_uapi_stats_3a *stats,
+			      double currentYGain)
 {
 	double redSum = 0, greenSum = 0, blueSum = 0;
 
@@ -288,14 +288,14 @@ double Agc::computeInitialY(IPAFrameContext &frameContext,
 	}
 
 	/*
-	 * Estimate the sum of the brightness values, weighted with the gains
-	 * applied on the channels in AWB as the Rec. 601 luma.
+	 * Apply the AWB gains to approximate colours correctly, use the Rec.
+	 * 601 formula to calculate the relative luminance, and normalize it.
 	 */
-	double Y_sum = redSum * frameContext.awb.gains.red * .299 +
-		       greenSum * frameContext.awb.gains.green * .587 +
-		       blueSum * frameContext.awb.gains.blue * .114;
+	double ySum = redSum * frameContext.awb.gains.red * 0.299
+		    + greenSum * frameContext.awb.gains.green * 0.587
+		    + blueSum * frameContext.awb.gains.blue * 0.114;
 
-	return Y_sum / (grid.height * grid.width) / 255;
+	return ySum / (grid.height * grid.width) / 255;
 }
 
 /**
@@ -311,22 +311,22 @@ void Agc::process(IPAContext &context, const ipu3_uapi_stats_3a *stats)
 	measureBrightness(stats, context.configuration.grid.bdsGrid);
 
 	double currentYGain = 1.0;
-	double targetY = kNormalizedLumaTarget;
+	double yTarget = kRelativeLuminanceTarget;
 
 	/*
 	 * Do this calculation a few times as brightness increase can be
 	 * non-linear when there are saturated regions.
 	 */
-	for (int i = 0; i < 8; i++) {
-		double initialY = computeInitialY(context.frameContext,
+	for (unsigned int i = 0; i < 8; i++) {
+		double yValue = estimateLuminance(context.frameContext,
 						  context.configuration.grid.bdsGrid,
 						  stats, currentYGain);
-		double extra_gain = std::min(10.0, targetY / (initialY + .001));
+		double extra_gain = std::min(10.0, yTarget / (yValue + .001));
 
 		currentYGain *= extra_gain;
-		LOG(IPU3Agc, Debug) << "Initial Y " << initialY
-				    << " target " << targetY
-				    << " gives gain " << currentYGain;
+		LOG(IPU3Agc, Debug) << "Y value: " << yValue
+				    << ", Y target: " << yTarget
+				    << ", gives gain " << currentYGain;
 		if (extra_gain < 1.01)
 			break;
 	}
diff --git a/src/ipa/ipu3/algorithms/agc.h b/src/ipa/ipu3/algorithms/agc.h
index 31c5a6e5..943c354a 100644
--- a/src/ipa/ipu3/algorithms/agc.h
+++ b/src/ipa/ipu3/algorithms/agc.h
@@ -35,10 +35,10 @@ private:
 			       const ipu3_uapi_grid_config &grid);
 	void filterExposure();
 	void computeExposure(IPAFrameContext &frameContext, double currentYGain);
-	double computeInitialY(IPAFrameContext &frameContext,
-			       const ipu3_uapi_grid_config &grid,
-			       const ipu3_uapi_stats_3a *stats,
-			       double currentYGain);
+	double estimateLuminance(IPAFrameContext &frameContext,
+				 const ipu3_uapi_grid_config &grid,
+				 const ipu3_uapi_stats_3a *stats,
+				 double currentYGain);
 
 	uint64_t frameCount_;