Re: [PATCH 07/11] drm/colorop: Introduce HLG EOTF

On Fri, 2026-02-06 at 10:51 +0200, Pekka Paalanen wrote:

On Tue, 23 Dec 2025 16:44:48 -0300
Nícolas F. R. A. Prado [off-list ref] wrote:

quoted

Introduce definition of a hybrid log-gamma electro-optical transfer
function for 1D Curve colorops.

Signed-off-by: Nícolas F. R. A. Prado <redacted>
---
 drivers/gpu/drm/drm_colorop.c |  1 +
 include/drm/drm_colorop.h     | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/drivers/gpu/drm/drm_colorop.c

b/drivers/gpu/drm/drm_colorop.c
index fafe45b93ff8..a19e03fb9c7c 100644

--- a/drivers/gpu/drm/drm_colorop.c
+++ b/drivers/gpu/drm/drm_colorop.c

@@ -79,6 +79,7 @@ static const char * const

colorop_curve_1d_type_names[] = {
 	[DRM_COLOROP_1D_CURVE_BT2020_OETF] = "BT.2020 OETF",
 	[DRM_COLOROP_1D_CURVE_GAMMA22] = "Gamma 2.2",
 	[DRM_COLOROP_1D_CURVE_GAMMA22_INV] = "Gamma 2.2 Inverse",
+	[DRM_COLOROP_1D_CURVE_HLG_EOTF] = "HLG",
 };
 
 static const struct drm_prop_enum_list
drm_colorop_lut1d_interpolation_list[] = {

diff --git a/include/drm/drm_colorop.h b/include/drm/drm_colorop.h
index ee6454b08b2d..8ec98521607d 100644
--- a/include/drm/drm_colorop.h
+++ b/include/drm/drm_colorop.h

@@ -126,6 +126,16 @@ enum drm_colorop_curve_1d_type {

 	 * The inverse of &DRM_COLOROP_1D_CURVE_GAMMA22
 	 */
 	DRM_COLOROP_1D_CURVE_GAMMA22_INV,
+
+	/**
+	 * @DRM_COLOROP_1D_CURVE_HLG_EOTF:
+	 *
+	 * enum string "HLG"
+	 *
+	 * Hybrid log-gamma transfer function.
+	 */
+	DRM_COLOROP_1D_CURVE_HLG_EOTF,
+

Hi,

if you read ITU-R BT.2100[1], you will see that the HLG EOTF has
three
parameters: the system gamma, the display nominal peak luminance, and
the display black luminance. You cannot define a HLG EOTF or its
inverse without these parameters. One might assume to derive the
system
gamma from the peak luminance, but I would like to leave that as a
free
parameter if possible, because I'm not convinced the formulas for it
are
universally applicable.

The HLG OETF on the other hand is a fixed function. Did you mean that
instead?

Thank you for the insights! But as it's obvious by now I don't have
enough information to answer that. Hopefully someone from MediaTek
could clarify what is the precise function implemented in hardware.

I didn't know about this difference between the HLG EOTF and OETF, but
what made me guess it would be the EOTF is that this curve is available
at the GAMMA block, which is right before blending the planes (the
pipeline topology I reverse-engineered from testing is on the cover-
letter), which I believe is usually done in the optical domain. But I
suppose one option would be that this curve really is the Inverse HLG
OETF, which wouldn't require additional parameters and also brings the
content to the optical domain.

-- 
Thanks,

Nícolas