[libcamera-devel] [PATCH v2 03/13] ipa: ipu3: awb: Use saturation under 90%

[Jean-Michel Hautbois]

The AWB grey world algorithm tries to find a grey value and it can't do
it on over-exposed images. To exclude those, the saturation ratio is
used for each cell, and the cell is included only if this ratio is 0.

Now that we have changed the threshold, more cells may be considered as
partially saturated and excluded, preventing the algorithm from running
efficiently.

Change that behaviour, and consider 90% as a good enough ratio.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
---
 src/ipa/ipu3/algorithms/awb.cpp | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/src/ipa/ipu3/algorithms/awb.cpp b/src/ipa/ipu3/algorithms/awb.cpp
index e44bbd6c..62c72cce 100644
--- a/src/ipa/ipu3/algorithms/awb.cpp
+++ b/src/ipa/ipu3/algorithms/awb.cpp
@@ -19,6 +19,12 @@ LOG_DEFINE_CATEGORY(IPU3Awb)
 
 static constexpr uint32_t kMinGreenLevelInZone = 32;
 
+/* Minimum proportion of cells counted within a zone for it to be relevant */
+static constexpr double kMinRelevantCellsRatio = 0.8;
+
+/* Number of cells below the saturation ratio */
+static constexpr uint32_t kSaturationThreshold = 255 * 90 / 100;
+
 /**
  * \struct Accumulator
  * \brief RGB statistics for a given zone
@@ -160,7 +166,8 @@ int Awb::configure(IPAContext &context,
 	 * for it to be relevant for the grey world algorithm.
 	 * \todo This proportion could be configured.
 	 */
-	cellsPerZoneThreshold_ = cellsPerZoneX_ * cellsPerZoneY_ * 80 / 100;
+	cellsPerZoneThreshold_ = cellsPerZoneX_ * cellsPerZoneY_ * kMinRelevantCellsRatio;
+	LOG(IPU3Awb, Debug) << "Threshold for AWB is set to " << cellsPerZoneThreshold_;
 
 	return 0;
 }
@@ -234,7 +241,18 @@ void Awb::generateAwbStats(const ipu3_uapi_stats_3a *stats)
 				reinterpret_cast<const ipu3_uapi_awb_set_item *>(
 					&stats->awb_raw_buffer.meta_data[cellPosition]
 				);
-			if (currentCell->sat_ratio == 0) {
+
+			/*
+			 * Use cells which have less than 90%
+			 * saturation as an initial means to include
+			 * otherwise bright cells which are not fully
+			 * saturated.
+			 *
+			 * \todo The 90% saturation rate may require
+			 * further empirical measurements and
+			 * optimisation during camera tuning phases.
+			 */
+			if (currentCell->sat_ratio <= kSaturationThreshold) {
 				/* The cell is not saturated, use the current cell */
 				awbStats_[awbZonePosition].counted++;
 				uint32_t greenValue = currentCell->Gr_avg + currentCell->Gb_avg;
-- 
2.32.0