[PATCH v5 02/10] drm/vkms: Fix limited-range YCbCr to RGB conversion scaling
Harry Wentland <[email protected]> Fri, 31 Jul 2026 14:15:13 -0400
| Newsgroups | org.freedesktop.lists.amd-gfx,org.freedesktop.lists.dri-devel |
|---|---|
| Message-ID | <[email protected]> |
The limited-/studio-range YCbCr to RGB conversion matrices were generated
with colour.matrix_YCbCr(is_legal=True, bits=8), which normalises the
narrow range by 2^bits (256 for 8-bit) rather than by the full-range
maximum 2^bits - 1 (255). As a result the luma scale was 256/219 and the
chroma scale 256/224 instead of the correct 255/219 and 255/224.
This over-scales every limited-range conversion by a factor of 256/255,
producing an error of up to ~3/255 (8-bit) and causing IGT tests that
compare software-computed surfaces against VKMS-processed ones to fail.
Recompute the three limited-range matrices from first principles rather
than relying on colour's is_legal scaling:
1. Start from the standard ITU-R YCbCr -> RGB relations for the luma
weights Kr, Kb (Kg = 1 - Kr - Kb) of each encoding (BT.601, BT.709
and BT.2020 non-constant luminance). These are the same relations
that produce the existing full-range matrices.
2. Expand the studio input range to full range relative to a full-range
maximum of 2^n - 1: the luma coefficient by 255/(235 - 16) and the
chroma coefficients by 255/(240 - 16). This matches the DRM UAPI
definition and IGT's igt_ycbcr_to_rgb_matrix().
3. Convert each coefficient to S31.32 fixed point (round(coeff * 2^32)).
Update the limited-range reference values in the vkms-format KUnit test
accordingly. Provide a python script to show how the kunit test value
are calculated.
Fixes: fe22d21e9342 ("drm/vkms: Add YUV support")
Assisted-by: Copilot:claude-opus-4.8
Signed-off-by: Harry Wentland <[email protected]>
Reviewed-by: Alex Hung <[email protected]>
Tested-by: Robert Mader <[email protected]>
---
.../gpu/drm/vkms/tests/gen_yuv_conversion.py | 87 +++++++++++++++++++
drivers/gpu/drm/vkms/tests/vkms_format_test.c | 38 ++++----
drivers/gpu/drm/vkms/vkms_formats.c | 55 ++++++++----
3 files changed, 144 insertions(+), 36 deletions(-)
create mode 100755 drivers/gpu/drm/vkms/tests/gen_yuv_conversion.py
diff --git a/drivers/gpu/drm/vkms/tests/gen_yuv_conversion.py b/drivers/gpu/drm/vkms/tests/gen_yuv_conversion.py
new file mode 100755
index 000000000000..f4d226137cc2
--- /dev/null
+++ b/drivers/gpu/drm/vkms/tests/gen_yuv_conversion.py
@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0+
+"""
+Derive the VKMS YCbCr -> RGB conversion constants and the vkms-format KUnit
+reference values.
+
+It produces:
+ 1. The S31.32 fixed-point conversion matrices used in
+ drivers/gpu/drm/vkms/vkms_formats.c and asserted by
+ vkms_format_test_conversion_matrix.
+ 2. The 16-bit reference YCbCr inputs for the limited-range
+ yuv_u16_to_argb_u16 round-trip cases (the values changed by the fix).
+
+Requires numpy.
+"""
+
+import numpy as np
+
+FP = 1 << 32 # S31.32 scale: fixed = round(coeff * 2^32)
+
+# ITU-R luma weights (Kr, Kb); Kg = 1 - Kr - Kb.
+ENCODINGS = {
+ "BT601": (0.299, 0.114),
+ "BT709": (0.2126, 0.0722),
+ "BT2020": (0.2627, 0.0593), # non-constant luminance
+}
+
+# 8-bit studio-range levels: luma 16..235 (range 219), chroma 16..240
+# (range 224, neutral 128); full-range maximum is 2^n - 1 = 255.
+FULL_MAX = 255
+Y_RANGE = 235 - 16 # 219
+C_RANGE = 240 - 16 # 224
+
+# 16-bit RGB test colors.
+COLORS = {
+ "white": (0xffff, 0xffff, 0xffff),
+ "gray": (0x8080, 0x8080, 0x8080),
+ "black": (0x0000, 0x0000, 0x0000),
+ "red": (0xffff, 0x0000, 0x0000),
+ "green": (0x0000, 0xffff, 0x0000),
+ "blue": (0x0000, 0x0000, 0xffff),
+}
+
+
+def rgb_matrix(kr, kb, limited):
+ """Normalized YCbCr -> RGB (Y in [0,1], Cb/Cr in [-0.5, 0.5])."""
+ kg = 1.0 - kr - kb
+ # Standard full-range relations.
+ m = [[1.0, 0.0, 2 * (1 - kr)],
+ [1.0, -2 * (1 - kb) * kb / kg, -2 * (1 - kr) * kr / kg],
+ [1.0, 2 * (1 - kb), 0.0]]
+ if limited:
+ # Expand studio input range to full range: luma by 255/219,
+ # chroma by 255/224 (relative to full-range maximum 2^n - 1).
+ ys, cs = FULL_MAX / Y_RANGE, FULL_MAX / C_RANGE
+ m = [[r[0] * ys, r[1] * cs, r[2] * cs] for r in m]
+ return m
+
+
+def ref_yuv(m, y_offset):
+ """16-bit YCbCr that VKMS decodes back to each RGB color.
+
+ VKMS decode: [R,G,B] = M . [Y - y_offset*257, Cb - 128*257, Cr - 128*257]
+ (8-bit offsets lifted to 16-bit by *257 = 65535/255), so invert and
+ re-add the offsets.
+ """
+ inv = np.linalg.inv(np.array(m, float))
+ off = np.array([y_offset * 257, 128 * 257, 128 * 257])
+ return {name: tuple(int(min(max(round(v), 0), 0xffff))
+ for v in inv @ np.array(rgb, float) + off)
+ for name, rgb in COLORS.items()}
+
+print(f"== RGB REFERENCE VALUES ==")
+for name, rgb in COLORS.items():
+ print(f" {name:6} {{ {rgb[0]:#06x}, {rgb[1]:#06x}, {rgb[2]:#06x} }}")
+print()
+
+for enc, (kr, kb) in ENCODINGS.items():
+ for limited in (False, True):
+ m = rgb_matrix(kr, kb, limited)
+ y_offset = 16 if limited else 0
+ print(f"== {enc} {'LIMITED' if limited else 'FULL'} (y_offset={y_offset}) ==")
+ for row in m:
+ print(" { " + ", ".join(str(round(v * FP)) for v in row) + " },")
+ for name, yuv in ref_yuv(m, y_offset).items():
+ print(f" {name:6} {{ {yuv[0]:#06x}, {yuv[1]:#06x}, {yuv[2]:#06x} }}")
+ print()
diff --git a/drivers/gpu/drm/vkms/tests/vkms_format_test.c b/drivers/gpu/drm/vkms/tests/vkms_format_test.c
index a7788fbc45dc..d2ae6321383b 100644
--- a/drivers/gpu/drm/vkms/tests/vkms_format_test.c
+++ b/drivers/gpu/drm/vkms/tests/vkms_format_test.c
@@ -54,6 +54,8 @@ struct yuv_u16_to_argb_u16_case {
* The YUV color representation were acquired via the colour python framework.
* Below are the function calls used for generating each case.
*
+ * The limited-range cases are generated by gen_yuv_conversion.py.
+ *
* For more information got to the docs:
* https://colour.readthedocs.io/en/master/generated/colour.RGB_to_YCbCr.html
*/
@@ -101,12 +103,12 @@ static struct yuv_u16_to_argb_u16_case yuv_u16_to_argb_u16_cases[] = {
.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
.n_colors = 6,
.colors = {
- { "white", { 0xeb00, 0x8000, 0x8000 }, { 0xffff, 0xffff, 0xffff, 0xffff }},
- { "gray", { 0x7dee, 0x8000, 0x8000 }, { 0xffff, 0x8080, 0x8080, 0x8080 }},
- { "black", { 0x1000, 0x8000, 0x8000 }, { 0xffff, 0x0000, 0x0000, 0x0000 }},
- { "red", { 0x517b, 0x5a34, 0xf000 }, { 0xffff, 0xffff, 0x0000, 0x0000 }},
- { "green", { 0x908e, 0x35cc, 0x2237 }, { 0xffff, 0x0000, 0xffff, 0x0000 }},
- { "blue", { 0x28f7, 0xf000, 0x6dc9 }, { 0xffff, 0x0000, 0x0000, 0xffff }},
+ { "white", { 0xebeb, 0x8080, 0x8080 }, { 0xffff, 0xffff, 0xffff, 0xffff }},
+ { "gray", { 0x7e6c, 0x8080, 0x8080 }, { 0xffff, 0x8080, 0x8080, 0x8080 }},
+ { "black", { 0x1010, 0x8080, 0x8080 }, { 0xffff, 0x0000, 0x0000, 0x0000 }},
+ { "red", { 0x51cd, 0x5a8e, 0xf0f0 }, { 0xffff, 0xffff, 0x0000, 0x0000 }},
+ { "green", { 0x911e, 0x3602, 0x2259 }, { 0xffff, 0x0000, 0xffff, 0x0000 }},
+ { "blue", { 0x2920, 0xf0f0, 0x6e37 }, { 0xffff, 0x0000, 0x0000, 0xffff }},
}
},
/*
@@ -151,12 +153,12 @@ static struct yuv_u16_to_argb_u16_case yuv_u16_to_argb_u16_cases[] = {
.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
.n_colors = 6,
.colors = {
- { "white", { 0xeb00, 0x8000, 0x8000 }, { 0xffff, 0xffff, 0xffff, 0xffff }},
- { "gray", { 0x7dee, 0x8000, 0x8000 }, { 0xffff, 0x8080, 0x8080, 0x8080 }},
- { "black", { 0x1000, 0x8000, 0x8000 }, { 0xffff, 0x0000, 0x0000, 0x0000 }},
- { "red", { 0x3e8f, 0x6656, 0xf000 }, { 0xffff, 0xffff, 0x0000, 0x0000 }},
- { "green", { 0xaca1, 0x29aa, 0x1a45 }, { 0xffff, 0x0000, 0xffff, 0x0000 }},
- { "blue", { 0x1fd0, 0xf000, 0x75bb }, { 0xffff, 0x0000, 0x0000, 0xffff }},
+ { "white", { 0xebeb, 0x8080, 0x8080 }, { 0xffff, 0xffff, 0xffff, 0xffff }},
+ { "gray", { 0x7e6c, 0x8080, 0x8080 }, { 0xffff, 0x8080, 0x8080, 0x8080 }},
+ { "black", { 0x1010, 0x8080, 0x8080 }, { 0xffff, 0x0000, 0x0000, 0x0000 }},
+ { "red", { 0x3ece, 0x66bc, 0xf0f0 }, { 0xffff, 0xffff, 0x0000, 0x0000 }},
+ { "green", { 0xad4e, 0x29d4, 0x1a5f }, { 0xffff, 0x0000, 0xffff, 0x0000 }},
+ { "blue", { 0x1ff0, 0xf0f0, 0x7631 }, { 0xffff, 0x0000, 0x0000, 0xffff }},
}
},
/*
@@ -201,12 +203,12 @@ static struct yuv_u16_to_argb_u16_case yuv_u16_to_argb_u16_cases[] = {
.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
.n_colors = 6,
.colors = {
- { "white", { 0xeb00, 0x8000, 0x8000 }, { 0xffff, 0xffff, 0xffff, 0xffff }},
- { "gray", { 0x7dee, 0x8000, 0x8000 }, { 0xffff, 0x8080, 0x8080, 0x8080 }},
- { "black", { 0x1000, 0x8000, 0x8000 }, { 0xffff, 0x0000, 0x0000, 0x0000 }},
- { "red", { 0x4988, 0x60b9, 0xf000 }, { 0xffff, 0xffff, 0x0000, 0x0000 }},
- { "green", { 0xa47b, 0x2f47, 0x1902 }, { 0xffff, 0x0000, 0xffff, 0x0000 }},
- { "blue", { 0x1cfd, 0xf000, 0x76fe }, { 0xffff, 0x0000, 0x0000, 0xffff }},
+ { "white", { 0xebeb, 0x8080, 0x8080 }, { 0xffff, 0xffff, 0xffff, 0xffff }},
+ { "gray", { 0x7e6c, 0x8080, 0x8080 }, { 0xffff, 0x8080, 0x8080, 0x8080 }},
+ { "black", { 0x1010, 0x8080, 0x8080 }, { 0xffff, 0x0000, 0x0000, 0x0000 }},
+ { "red", { 0x49d2, 0x611a, 0xf0f0 }, { 0xffff, 0xffff, 0x0000, 0x0000 }},
+ { "green", { 0xa520, 0x2f76, 0x191b }, { 0xffff, 0x0000, 0xffff, 0x0000 }},
+ { "blue", { 0x1d1a, 0xf0f0, 0x7775 }, { 0xffff, 0x0000, 0x0000, 0xffff }},
}
},
};
diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
index dfb8e13cba87..4d5fcaeb82c5 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.c
+++ b/drivers/gpu/drm/vkms/vkms_formats.c
@@ -788,15 +788,36 @@ static const struct conversion_matrix yuv_bt601_full = {
};
/*
- * numpy.around(colour.matrix_YCbCr(K=colour.WEIGHTS_YCBCR["ITU-R BT.601"],
- * is_legal = True,
- * bits = 8) * 2**32).astype(int)
+ * BT.601 limited-/studio-range YCbCr to full-range RGB.
+ *
+ * The coefficients are derived as follows:
+ *
+ * 1. Take the standard ITU-R YCbCr -> RGB relations for luma weights
+ * Kr, Kb (Kg = 1 - Kr - Kb), with Y in [0, 1] and Cb, Cr in
+ * [-0.5, 0.5]. For BT.601 Kr = 0.299 and Kb = 0.114:
+ *
+ * R = Y + 2 * (1 - Kr) * Cr
+ * G = Y - 2 * (1 - Kb) * Kb / Kg * Cb - 2 * (1 - Kr) * Kr / Kg * Cr
+ * B = Y + 2 * (1 - Kb) * Cb
+ *
+ * These are exactly the yuv_bt601_full coefficients above.
+ *
+ * 2. Expand the studio input range to full range, relative to a
+ * full-range maximum of 2^n - 1 (255 for 8-bit): the luma
+ * coefficient is scaled by 255/(235 - 16) and the chroma
+ * coefficients by 255/(240 - 16). This matches the DRM UAPI
+ * definition and IGT's igt_ycbcr_to_rgb_matrix(). Note this differs
+ * from colour.matrix_YCbCr(is_legal=True), which normalises by 2^n
+ * and is thus off by a factor of 256/255.
+ *
+ * 3. Convert each coefficient to S31.32 fixed point, i.e.
+ * round(coeff * 2^32).
*/
static const struct conversion_matrix yuv_bt601_limited = {
.matrix = {
- { 5020601039, 0, 6881764740 },
- { 5020601039, -1689204679, -3505362278 },
- { 5020601039, 8697922339, 0 },
+ { 5000989317, 0, 6854882848 },
+ { 5000989317, -1682606224, -3491669458 },
+ { 5000989317, 8663946082, 0 },
},
.y_offset = 16,
};
@@ -816,15 +837,14 @@ static const struct conversion_matrix yuv_bt709_full = {
};
/*
- * numpy.around(colour.matrix_YCbCr(K=colour.WEIGHTS_YCBCR["ITU-R BT.709"],
- * is_legal = True,
- * bits = 8) * 2**32).astype(int)
+ * BT.709 limited-range YCbCr to full-range RGB (Kr = 0.2126, Kb = 0.0722).
+ * Derived as described for yuv_bt601_limited.
*/
static const struct conversion_matrix yuv_bt709_limited = {
.matrix = {
- { 5020601039, 0, 7729959424 },
- { 5020601039, -919487572, -2297803934 },
- { 5020601039, 9108275786, 0 },
+ { 5000989317, 0, 7699764272 },
+ { 5000989317, -915895824, -2288828138 },
+ { 5000989317, 9072696586, 0 },
},
.y_offset = 16,
};
@@ -844,15 +864,14 @@ static const struct conversion_matrix yuv_bt2020_full = {
};
/*
- * numpy.around(colour.matrix_YCbCr(K=colour.WEIGHTS_YCBCR["ITU-R BT.2020"],
- * is_legal = True,
- * bits = 8) * 2**32).astype(int)
+ * BT.2020 non-constant-luminance limited-range YCbCr to full-range RGB
+ * (Kr = 0.2627, Kb = 0.0593). Derived as described for yuv_bt601_limited.
*/
static const struct conversion_matrix yuv_bt2020_limited = {
.matrix = {
- { 5020601039, 0, 7238124312 },
- { 5020601039, -807714626, -2804506279 },
- { 5020601039, 9234915964, 0 },
+ { 5000989317, 0, 7209850391 },
+ { 5000989317, -804559491, -2793551177 },
+ { 5000989317, 9198842076, 0 },
},
.y_offset = 16,
};
--
2.55.0