[v3,2/3] libcamera: software_isp: Normalize statistics sums to 8-bit
diff mbox series

Message ID 20260305-agc-proportional-v3-2-25abc1bfacca@jetm.me
State New
Headers show
Series
  • Simple pipeline: proportional AGC, AWB stats fix, OV2740 black level
Related show

Commit Message

Javier Tia March 5, 2026, 8:10 p.m. UTC 
The SWSTATS_ACCUMULATE_LINE_STATS() macro divides the luminance value
for the histogram to normalize it to 8-bit range, but does not apply
the same normalization to the RGB sums. For 10-bit and 12-bit unpacked
Bayer formats this means the sums are accumulated at native bit depth
(0-1023 or 0-4095 per pixel) while the AWB algorithm subtracts an
8-bit black level from them, under-correcting by 4x or 16x
respectively.

This mismatch between the AWB's gain calculation (using incorrectly
BLC-subtracted sums) and the debayer's correct normalized BLC
subtraction produces a visible color cast. For example, with the
OV2740 sensor (10-bit, BLC=16), the under-subtraction skews R/G gain
by ~9%.

Fix this by right-shifting the RGB sums in finishFrame() to normalize
them to 8-bit scale, matching the histogram and the 8-bit black level
used by AWB. A per-format sumShift_ value is set in configure():
0 for 8-bit and CSI-2 packed formats (already 8-bit), 2 for 10-bit,
and 4 for 12-bit unpacked formats.

Signed-off-by: Javier Tia <floss@jetm.me>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Tested-by: Milan Zamazal <mzamazal@redhat.com>
---
 include/libcamera/internal/software_isp/swstats_cpu.h | 1 +
 src/libcamera/software_isp/swstats_cpu.cpp            | 9 +++++++++
 2 files changed, 10 insertions(+)

Patch
diff mbox series 

diff --git a/include/libcamera/internal/software_isp/swstats_cpu.h b/include/libcamera/internal/software_isp/swstats_cpu.h
index 64b3e23f..37bfde53 100644
--- a/include/libcamera/internal/software_isp/swstats_cpu.h
+++ b/include/libcamera/internal/software_isp/swstats_cpu.h
@@ -115,6 +115,7 @@  private:
 
 	unsigned int xShift_;
 	unsigned int stride_;
+	unsigned int sumShift_;
 
 	SharedMemObject<SwIspStats> sharedStats_;
 	SwIspStats stats_;
diff --git a/src/libcamera/software_isp/swstats_cpu.cpp b/src/libcamera/software_isp/swstats_cpu.cpp
index 1cedcfbc..6f834207 100644
--- a/src/libcamera/software_isp/swstats_cpu.cpp
+++ b/src/libcamera/software_isp/swstats_cpu.cpp
@@ -346,6 +346,11 @@  void SwStatsCpu::startFrame(uint32_t frame)
 void SwStatsCpu::finishFrame(uint32_t frame, uint32_t bufferId)
 {
 	stats_.valid = frame % kStatPerNumFrames == 0;
+	if (stats_.valid && sumShift_) {
+		stats_.sum_.r() >>= sumShift_;
+		stats_.sum_.g() >>= sumShift_;
+		stats_.sum_.b() >>= sumShift_;
+	}
 	*sharedStats_ = stats_;
 	statsReady.emit(frame, bufferId);
 }
@@ -405,12 +410,15 @@  int SwStatsCpu::configure(const StreamConfiguration &inputCfg)
 		switch (bayerFormat.bitDepth) {
 		case 8:
 			stats0_ = &SwStatsCpu::statsBGGR8Line0;
+			sumShift_ = 0;
 			return 0;
 		case 10:
 			stats0_ = &SwStatsCpu::statsBGGR10Line0;
+			sumShift_ = 2;
 			return 0;
 		case 12:
 			stats0_ = &SwStatsCpu::statsBGGR12Line0;
+			sumShift_ = 4;
 			return 0;
 		}
 	}
@@ -422,6 +430,7 @@  int SwStatsCpu::configure(const StreamConfiguration &inputCfg)
 		/* Skip every 3th and 4th line, sample every other 2x2 block */
 		ySkipMask_ = 0x02;
 		xShift_ = 0;
+		sumShift_ = 0;
 		processFrame_ = &SwStatsCpu::processBayerFrame2;
 
 		switch (bayerFormat.order) {