[PATCH 3/5] fixup: ipa: rkisp1: Remove bespoke Agc functions

[Paul Elder]

Remove some constants that should have been removed, and restore a
fairly informative comment that got removed.

Although the removal of one of the constants also removes a todo
comment, this will be addressed imminently so it should be fine.

Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
---
 src/ipa/rkisp1/algorithms/agc.cpp | 32 +++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/src/ipa/rkisp1/algorithms/agc.cpp b/src/ipa/rkisp1/algorithms/agc.cpp
index 7220f00a..1dfc4aaa 100644
--- a/src/ipa/rkisp1/algorithms/agc.cpp
+++ b/src/ipa/rkisp1/algorithms/agc.cpp
@@ -39,12 +39,6 @@ namespace ipa::rkisp1::algorithms {
 
 LOG_DEFINE_CATEGORY(RkISP1Agc)
 
-/* Minimum limit for analogue gain value */
-static constexpr double kMinAnalogueGain = 1.0;
-
-/* \todo Honour the FrameDurationLimits control instead of hardcoding a limit */
-static constexpr utils::Duration kMaxShutterSpeed = 60ms;
-
 int Agc::parseMeteringModes(IPAContext &context, const YamlObject &tuningData,
 			    const char *prop)
 {
@@ -315,6 +309,32 @@ void Agc::parseStatistics(const rkisp1_stat_buffer *stats,
 					       context.hw->numHistogramBins), 4);
 }
 
+/**
+ * \brief Estimate the relative luminance of the frame with a given gain
+ * \param[in] expMeans The mean luminance values, from the RkISP1 statistics
+ * \param[in] gain The gain to apply to the frame
+ *
+ * This function estimates the average relative luminance of the frame that
+ * would be output by the sensor if an additional \a gain was applied.
+ *
+ * The estimation is based on the AE statistics for the current frame. Y
+ * averages for all cells are first multiplied by the gain, and then saturated
+ * to approximate the sensor behaviour at high brightness values. The
+ * approximation is quite rough, as it doesn't take into account non-linearities
+ * when approaching saturation. In this case, saturating after the conversion to
+ * YUV doesn't take into account the fact that the R, G and B components
+ * contribute differently to the relative luminance.
+ *
+ * \todo Have a dedicated YUV algorithm ?
+ *
+ * The values are normalized to the [0.0, 1.0] range, where 1.0 corresponds to a
+ * theoretical perfect reflector of 100% reference white.
+ *
+ * More detailed information can be found in:
+ * https://en.wikipedia.org/wiki/Relative_luminance
+ *
+ * \return The relative luminance
+ */
 double Agc::estimateLuminance(double gain)
 {
 	double ySum = 0.0;
-- 
2.39.2