libcamera: software_isp: Apply CCM in debayering

This patch applies color correction matrix (CCM) in debayering if the
CCM is specified.  Not using CCM must still be supported for performance
reasons.

The CCM is applied as follows:

  [r1 g1 b1]   [r]
  [r2 g2 b2] * [g]
  [r3 g3 b3]   [b]

The CCM matrix (the left side of the multiplication) is constant during
single frame processing, while the input pixel (the right side) changes.
Because each of the color channels is only 8-bit in software ISP, we can
make 9 lookup tables with 256 input values for multiplications of each
of the r_i, g_i, b_i values.  This way we don't have to multiply each
pixel, we can use table lookups and additions instead.  Gamma (which is
non-linear and thus cannot be a part of the 9 lookup tables values) is
applied on the final values rounded to integers using another lookup
table.

Because the changing part is the pixel value with three color elements,
only three dynamic table lookups are needed.  We use three lookup tables
to represent the multiplied matrix values, each of the tables
corresponding to the given matrix column and pixel color.

We use int16_t to store the precomputed multiplications.  This seems to
be noticeably (>10%) faster than `float' for the price of slightly less
accuracy and it covers the range of values that sane CCMs produce.  The
selection and structure of data is performance critical, for example
using bytes would add significant (>10%) speedup but would be too short
to cover the value range.

The color lookup tables can be represented either as unions,
accommodating tables for both the CCM and non-CCM cases, or as separate
tables for each of the cases, leaving the tables for the other case
unused.  The latter is selected as a matter of preference.

The tables are copied (as before), which is not elegant but also not a
big problem.  There are patches posted that use shared buffers for
parameters passing in software ISP (see software ISP TODO #5) and they
can be adjusted for the new parameter format.

Color gains from white balance are supposed not to be a part of the
specified CCM.  They are applied on it using matrix multiplication,
which is simple and in correspondence with future additions in the form
of matrix multiplication, like saturation adjustment.

With this patch, the reported per-frame slowdown when applying CCM is
about 45% on Debix Model A and about 75% on TI AM69 SK.

Using std::clamp in debayering adds some performance penalty (a few
percent).  The clamping is necessary to eliminate out of range values
possibly produced by the CCM.  If it could be avoided by adjusting the
precomputed tables some way then performance could be improved a bit.

Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

1 files changed, 31 insertions, 5 deletions

diff --git a/include/libcamera/internal/software_isp/debayer_params.h b/include/libcamera/internal/software_isp/debayer_params.h
index 7d8fdd48..217cd5d9 100644
--- a/include/libcamera/internal/software_isp/debayer_params.h
+++ b/include/libcamera/internal/software_isp/debayer_params.h
@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 /*
- * Copyright (C) 2023, 2024 Red Hat Inc.
+ * Copyright (C) 2023-2025 Red Hat Inc.
  *
  * Authors:
  * Hans de Goede <hdegoede@redhat.com>
@@ -18,11 +18,37 @@ namespace libcamera {
 struct DebayerParams {
 	static constexpr unsigned int kRGBLookupSize = 256;
 
-	using ColorLookupTable = std::array<uint8_t, kRGBLookupSize>;
+	struct CcmColumn {
+		int16_t r;
+		int16_t g;
+		int16_t b;
+	};
 
-	ColorLookupTable red;
-	ColorLookupTable green;
-	ColorLookupTable blue;
+	using LookupTable = std::array<uint8_t, kRGBLookupSize>;
+	using CcmLookupTable = std::array<CcmColumn, kRGBLookupSize>;
+
+	/*
+	 * Color lookup tables when CCM is not used.
+	 *
+	 * Each color of a debayered pixel is amended by the corresponding
+	 * value in the given table.
+	 */
+	LookupTable red;
+	LookupTable green;
+	LookupTable blue;
+
+	/*
+	 * Color and gamma lookup tables when CCM is used.
+	 *
+	 * Each of the CcmLookupTable's corresponds to a CCM column; together they
+	 * make a complete 3x3 CCM lookup table. The CCM is applied on debayered
+	 * pixels and then the gamma lookup table is used to set the resulting
+	 * values of all the three colors.
+	 */
+	CcmLookupTable redCcm;
+	CcmLookupTable greenCcm;
+	CcmLookupTable blueCcm;
+	LookupTable gammaLut;
 };
 
 } /* namespace libcamera */