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

[Pekka Paalanen <pekka.paalanen@collabora.com>]

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On Tue, 23 Dec 2025 16:44:48 -0300
Nícolas F. R. A. Prado <nfraprado@collabora.com> wrote:

> Introduce definition of a hybrid log-gamma electro-optical transfer
> function for 1D Curve colorops.
> 
> Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> ---
>  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?


[1]: https://gitlab.freedesktop.org/pq/color-and-hdr/-/blob/main/doc/specs.md#itu-r-bt2100


Thanks,
pq

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