[libcamera-devel] [PATCH v3 04/14] ipa: ipu3: awb: Change minimal green threshold value

[Jean-Michel Hautbois]

When zones are used for the grey world algorithm, they are only
considered if their average green value is at least 32/255 to exclude
zones that are too dark and don't provide relevant colour information
(on the opposite side of the spectrum, saturated regions are excluded by
the ImgU statistics engine).

The algorithm requires a minimal number of zones that meet this criteria
in order to run. Now that we correct the black level, the 32/255 minimal
value is a bit high and prevents the algorithm for running in low-light
conditions. Lower the value to 16/255 to fix it.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 src/ipa/ipu3/algorithms/awb.cpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/ipa/ipu3/algorithms/awb.cpp b/src/ipa/ipu3/algorithms/awb.cpp
index ce01791b..ae78cb87 100644
--- a/src/ipa/ipu3/algorithms/awb.cpp
+++ b/src/ipa/ipu3/algorithms/awb.cpp
@@ -17,7 +17,14 @@ namespace ipa::ipu3::algorithms {
 
 LOG_DEFINE_CATEGORY(IPU3Awb)
 
-static constexpr uint32_t kMinGreenLevelInZone = 32;
+/*
+ * When zones are used for the grey world algorithm, they are only considered if
+ * their average green value is at least 16/255 (after black level subtraction)
+ * to exclude zones that are too dark and don't provide relevant colour
+ * information (on the opposite side of the spectrum, saturated regions are
+ * excluded by the ImgU statistics engine).
+ */
+static constexpr uint32_t kMinGreenLevelInZone = 16;
 
 /*
  * Minimum proportion of non-saturated cells in a zone for the zone to be used
-- 
2.32.0