## Introduction
We are looking to enable HDR support for a couple of single-plane and
multi-plane scenarios. To do this effectively we recommend new interfaces to
drm_plane. Below I'll give a bit of background on HDR and why we propose these
interfaces.
## Defining a pixel's luminance
Currently the luminance space of pixels in a framebuffer/plane presented to the
display is not well defined. It's usually assumed to be in a 2.2 or 2.4 gamma
space and has no mapping to an absolute luminance value but is interpreted in
relative terms.
Luminance can be measured and described in absolute terms as candela per meter
squared, or cd/m2, or nits. Even though a pixel value can be mapped to
luminance in a linear fashion to do so without losing a lot of detail requires
16-bpc color depth. The reason for this is that human perception can
distinguish roughly between a 0.5-1% luminance delta. A linear representation
is suboptimal, wasting precision in the highlights and losing precision in the
shadows.
A gamma curve is a decent approximation to a human's perception of luminance,
but the PQ (perceptual quantizer) function [1] improves on it. It also defines
the luminance values in absolute terms, with the highest value being 10,000
nits and the lowest 0.0005 nits.
Using a content that's defined in PQ space we can approximate the real world in
a much better way.
Here are some examples of real-life objects and their approximate luminance
values:
| Object | Luminance in nits |
| ----------------- | ----------------- |
| Sun | 1.6 million |
| Fluorescent light | 10,000 |
| Highlights | 1,000 - sunlight |
| White Objects | 250 - 1,000 |
| Typical objects | 1 - 250 |
| Shadows | 0.01 - 1 |
| Ultra Blacks | 0 - 0.0005 |
## Describing the luminance space
**We propose a new drm_plane property to describe the Eletro-Optical Transfer
Function (EOTF) with which its framebuffer was composed.** Examples of EOTF are:
| EOTF | Description
|
| ---------
|:------------------------------------------------------------------------- |
| Gamma 2.2 | a simple 2.2 gamma
|
| sRGB | 2.4 gamma with small initial linear section
|
| PQ 2084 | SMPTE ST 2084; used for HDR video and allows for up to 10,000 nit
support |
| Linear | Linear relationship between pixel value and luminance value
|
## Mastering Luminances
Now we are able to use the PQ 2084 EOTF to define the luminance of pixels in
absolute terms. Unfortunately we're again presented with physical limitations
of the display technologies on the market today. Here are a few examples of
luminance ranges of displays.
| Display | Luminance range in nits |
| ------------------------ | ----------------------- |
| Typical PC display | 0.3 - 200 |
| Excellent LCD HDTV | 0.3 - 400 |
| HDR LCD w/ local dimming | 0.05 - 1,500 |
Since no display can currently show the full 0.0005 to 10,000 nits luminance
range the display will need to tonemap the HDR content, i.e to fit the content
within a display's capabilities. To assist with tonemapping HDR content is
usually accompanied with a metadata that describes (among other things) the
minimum and maximum mastering luminance, i.e. the maximum and minimum luminance
of the display that was used to master the HDR content.
The HDR metadata is currently defined on the drm_connector via the
hdr_output_metadata blob property.
It might be useful to define per-plane hdr metadata, as different planes might
have been mastered differently.
## SDR Luminance
Since SDR covers a smaller luminance range than HDR, an SDR plane might look
dark when blended with HDR content. Since the max HDR luminance can be quite
variable (200-1,500 nits on actual displays) it is best to make the SDR maximum
luminance value configurable.
**We propose a drm_plane property to specfy the desired maximum luminance of
the SDR plane in nits.** This allows us to map the SDR content predictably into
HDR's absolute luminance space.
## Let There Be Color
So far we've only talked about luminance, ignoring colors altogether. Just like
in the luminance space, traditionally the color space of display outputs has
not been well defined. Similar to how an EOTF defines a mapping of pixel data
to an absolute luminance value, the color space maps color information for each
pixel onto the CIE 1931 chromaticity space. This can be thought of as a mapping
to an absolute, real-life, color value.
A color space is defined by its primaries and white point. The primaries and
white point are expressed as coordinates in the CIE 1931 color space. Think of
the red primary as the reddest red that can be displayed within the color
space. Same for green and blue.
Examples of color spaces are:
| Color Space | Description |
| ----------- | ------------------------------------------ |
| BT 601 | similar to BT 709 |
| BT 709 | used by sRGB content; ~53% of BT 2020 |
| DCI-P3 | used by most HDR displays; ~72% of BT 2020 |
| BT 2020 | standard for most HDR content |
The color space is defined in DRM for YCbCr planes via the color_encoding
property of the drm_plane.
**We propose to add definitions for the RGB variants of the BT color spaces.**
## Color Primaries and White Point
Just like displays can currently not represent the entire 0.0005 - 10,000 nits
HDR range of the PQ 2084 EOTF, they are currently not capable of representing
the entire BT.2020 color Gamut. For this reason video content will often
specify the color primaries and white point used to master the video, in order
to allow displays to be able to map the image as best as possible onto the
display's gamut.
## Displays and Tonemapping
External displays are able to do their own tone and color mapping, based on the
mastering luminance, color primaries, and white space defined in the HDR
metadata.
Internal panels (which are currently few and far between) usually don't include
the complex HW to do tone and color mapping on their own and will require the
display driver to perform appropriate mapping.
## Pixel Formats
The pixel formats, such as ARGB8888, ARGB2101010, P010, or FP16 are unrelated
to color space and EOTF definitions. HDR pixels can be formatted in different
ways but in order to not lose precision HDR content requires at least 10 bpc
precision. For this reason ARGB2101010, P010, and FP16 are the obvious
candidates for HDR. ARGB2101010 and P010 have the advantage of requiring only
half the bandwidth as FP16, while FP16 has the advantage of enough precision to
operate in a linear space, i.e. without EOTF.
## Proposed use-cases
Although the userspace side of this work is still in the early stages it is
clear that we will want to support the following two use-cases:
**One XRGB2101010 HDR Plane:** A single, composited plane of HDR content. The
use-case is a video player on a desktop with the compositor owning the
composition of SDR and HDR content. The content shall be PQ BT.2020 formatted.
The drm_connector's hdr_output_metadata shall be set.
**One ARGB8888 SDR Plane + One P010 HDR Plane:** A normal 8bpc desktop plane,
with a P010 HDR video plane underlayed. The HDR plane shall be PQ BT.2020
formatted. The desktop plane shall specify an SDR boost value. The
drm_connector's hdr_output_metadata shall be set.
**One XRGB8888 SDR Plane - HDR output:** In order to support a smooth
transition we recommend an OS that supports HDR output to provide the
hdr_output_metadata on the drm_connector to configure the output for HDR, even
when the content is only SDR. This will allow for a smooth transition between
SDR-only and HDR content. In this use-case the SDR max luminance value should
be provided on the drm_plane.
In DCN we will de-PQ or de-Gamma all input in order to blend in linear space.
For SDR content we will also apply any desired boost before blending. After
blending we will then re-apply the PQ EOTF and do RGB to YCbCr conversion if
needed.
## Summary of proposed interface changes
per drm_plane:
- new RGB color space definitions, mirroring the existing YUV color space
definitions
- new transfer function property
- new SDR maximum white level property
## References
[1] https://en.wikipedia.org/wiki/High-dynamic-range_video#Perceptual_Quantizer
## Further Reading
https://gitlab.freedesktop.org/swick/wayland-protocols/-/blob/color/unstable/color-management/color.rst
http://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP309.pdf
https://app.spectracal.com/Documents/White%20Papers/HDR_Demystified.pdf
Bhawanpreet Lakha (3):
drm/color: Add RGB Color encodings
drm/color: Add Color transfer functions for HDR/SDR
drm/color: Add sdr boost property
.../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 4 +-
.../gpu/drm/arm/display/komeda/komeda_plane.c | 4 +-
drivers/gpu/drm/arm/malidp_planes.c | 4 +-
drivers/gpu/drm/armada/armada_overlay.c | 4 +-
drivers/gpu/drm/drm_atomic_uapi.c | 8 ++
drivers/gpu/drm/drm_color_mgmt.c | 84 +++++++++++++++++--
drivers/gpu/drm/i915/display/intel_sprite.c | 4 +-
.../drm/i915/display/skl_universal_plane.c | 4 +-
drivers/gpu/drm/nouveau/dispnv04/overlay.c | 4 +-
drivers/gpu/drm/omapdrm/omap_plane.c | 4 +-
drivers/gpu/drm/sun4i/sun8i_vi_layer.c | 4 +-
drivers/gpu/drm/tidss/tidss_plane.c | 6 +-
include/drm/drm_color_mgmt.h | 25 +++++-
include/drm/drm_plane.h | 30 +++++++
14 files changed, 173 insertions(+), 16 deletions(-)
--
2.31.0
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