Re: [PATCH v6 02/10] drm/vkms: Fix limited-range YCbCr to RGB conversion scaling
Robert Mader <[email protected]>
| Newsgroups | org.freedesktop.lists.amd-gfx,org.freedesktop.lists.dri-devel |
|---|---|
| Message-ID | <[email protected]> |
FYI: Following my previous observation that the Weston
test ("color-representation-drm") that should have detected this error
did not do so, I opened a MR to reduce the fuzziness (accepted error
range) of said test [1]. The test checks all six cases with llvmpipe,
lavapipe, Weston-internal GL fallback shaders and VKMS. All of them
succeed within an error range of 1 per 8bit channel *with* the change
from this patch. Without it the VKMS results have higher errors[2] -
confirming again that this patch clearly is an improvement.
So hope we can move forward with the series soon :)
Best regards,
Robert
1: https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/2184
2: https://gitlab.freedesktop.org/rmader/weston/-/pipelines/1731461
On 14.08.26 23:18, Harry Wentland wrote:
> 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,
> };
--
Robert Mader
Consultant Software Developer
Collabora Ltd.
Platinum Building, St John's Innovation Park, Cambridge CB4 0DS, UK
Registered in England & Wales, no. 5513718