[v4,13/19] ipa: rkisp1: Switch histogram to RGB combined mode
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Message ID 20250918144333.108695-14-stefan.klug@ideasonboard.com
State New
Headers show
Series
  • Implement WDR algorithm
Related show

Commit Message

Stefan Klug Sept. 18, 2025, 2:43 p.m. UTC 
The Y mode of the histogram gets captured at the ISP output, before the
output formatter.  This has the side effect that the first and the last
bins are empty in case of limited YUV range.  Another side effect is
that gamma and GWDR processing is included in the histogram which makes
algorithm development very difficult. In RGB mode the histogram is taken
after xtalk (CCM) and is therefore independent of gamma and WDR. The
limited range issue also does not apply. In the ISP reference it is
however stated that "it is not possible to calculate a luminance or
grayscale histogram from an RGB histogram since the position information
is lost during its generation".

During testing the RGB histogram provided good data and better
algorithmic stability at a possible (but not measured) inaccuracy.

Another option would be to pass the color space information into the IPA
and strip the histogram accordingly. For ease of implementation switch to
the RGB mode.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

---

Changes in v4:
- Tiny fixes in the block comment (no doxygen & whitespace)
- Collected tags

Changes in v3:
- Added block comment inside the code
---
 src/ipa/rkisp1/algorithms/agc.cpp | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

Patch
diff mbox series 

diff --git a/src/ipa/rkisp1/algorithms/agc.cpp b/src/ipa/rkisp1/algorithms/agc.cpp
index fe2f66cf6ee4..174d810edc65 100644
--- a/src/ipa/rkisp1/algorithms/agc.cpp
+++ b/src/ipa/rkisp1/algorithms/agc.cpp
@@ -397,7 +397,24 @@  void Agc::prepare(IPAContext &context, const uint32_t frame,
 	hstConfig.setEnabled(true);
 
 	hstConfig->meas_window = context.configuration.agc.measureWindow;
-	hstConfig->mode = RKISP1_CIF_ISP_HISTOGRAM_MODE_Y_HISTOGRAM;
+	/*
+	 * The Y mode of the histogram gets captured at the ISP output, before
+	 * the output formatter.  This has the side effect that the first and
+	 * the last bins are empty in case of limited YUV range.  Another side
+	 * effect is that gamma and GWDR processing is included in the histogram
+	 * which makes algorithm development very difficult. In RGB mode the
+	 * histogram is taken after xtalk (CCM) and is therefore independent of
+	 * gamma and WDR. The limited range issue also does not apply. In the
+	 * ISP reference it is however stated that "it is not possible to
+	 * calculate a luminance or grayscale histogram from an RGB histogram
+	 * since the position information is lost during its generation".
+	 *
+	 * During testing the RGB histogram provided good data and better
+	 * algorithmic stability at a possible (but not measured) inaccuracy.
+	 *
+	 * \todo For a proper fix support for HIST64 is needed.
+	 */
+	hstConfig->mode = RKISP1_CIF_ISP_HISTOGRAM_MODE_RGB_COMBINED;
 
 	Span<uint8_t> weights{
 		hstConfig->hist_weight,