HDR & Color Management

Jay supports HDR (High Dynamic Range) output and the Wayland color management protocol. This chapter explains how HDR works in Jay and walks through enabling it.

Prerequisites

HDR in Jay requires:

The Vulkan renderer. The OpenGL renderer does not support color management or HDR. Vulkan is the default when available; you can verify with jay randr show (look for the “Api” field under your GPU) or switch to it in the control center GPUs pane.
A monitor that supports HDR. The display must advertise PQ (Perceptual Quantizer) support and BT.2020 colorimetry through its EDID. Run jay randr show to see which color spaces and EOTFs your display supports.

Quick start

Enable HDR10 on a specific output using the CLI:

~$ jay randr output DP-1 colors set bt2020 pq

Or in your configuration file:

[[outputs]]
match.serial-number = "33K03894SL0"
color-space = "bt2020"
transfer-function = "pq"

Enable the color management protocol so that applications can communicate their color spaces to the compositor:

~$ jay color-management enable

Or in your configuration file:

[color-management]
enabled = true

To return to SDR:

~$ jay randr output DP-1 colors set default default

Note

Run jay randr to find connector names and serial numbers for your displays.

Concepts

Color space

The color space determines the range of colors (gamut) the output uses.

default: sRGB gamut. This is the standard gamut for desktop displays.
bt2020: BT.2020 gamut. A much wider gamut used by HDR10 and modern cinema standards.

Transfer function (EOTF)

The transfer function (technically the Electro-Optical Transfer Function) controls how encoded pixel values are mapped to display luminance.

default: Gamma 2.2. The traditional SDR transfer function.
pq: Perceptual Quantizer (SMPTE ST 2084). The HDR10 transfer function, designed to cover luminance levels from 0 to 10,000 cd/m^2.

Setting color-space = "bt2020" and transfer-function = "pq" together activates HDR10 mode. Jay sends the appropriate HDR metadata infoframe to the display, including the display’s mastering luminance and primaries from its EDID.

Brightness

The brightness setting controls SDR content luminance in cd/m^2. This determines how bright non-HDR content appears on an HDR display.

With the default (gamma 2.2) transfer function, the default brightness is the maximum the display supports, anchored at 80 cd/m^2. Setting brightness below 80 makes SDR content dimmer while creating HDR headroom above it.

With the pq transfer function, the default brightness is 203 cd/m^2 (the ITU reference level for SDR content in an HDR signal).

[[outputs]]
match.serial-number = "33K03894SL0"
color-space = "bt2020"
transfer-function = "pq"
brightness = 250

~$ jay randr output DP-1 brightness 250
~$ jay randr output DP-1 brightness default

Note

Brightness has no effect unless the Vulkan renderer is in use.

Blend space

When Jay composites overlapping translucent surfaces, the blend space determines in which color space the alpha blending is performed.

srgb: Classic desktop blending in sRGB space. This is the default and matches the behavior of most other compositors.
linear: Blending in linear light. This is physically correct but can make semi-transparent elements appear brighter than expected.

[[outputs]]
match.serial-number = "33K03894SL0"
blend-space = "linear"

~$ jay randr output DP-1 blend-space linear

Native gamut

By default, Jay assumes all displays use sRGB primaries. This matches the behavior of most other compositors, but most modern displays actually have a wider gamut (e.g. 95% DCI-P3 coverage). Because the display interprets sRGB-intended color values in its wider native gamut, colors appear more saturated than they should.

Setting use-native-gamut = true tells Jay the display’s actual primaries from its EDID, allowing the compositor to map colors correctly. This produces less saturated but more accurate colors, and allows color-managed applications to address the full display gamut.

[[outputs]]
match.serial-number = "33K03894SL0"
use-native-gamut = true

~$ jay randr output DP-1 use-native-gamut true

This setting has no effect when the display is already operating in a wide color space like BT.2020.

Framebuffer format

The default framebuffer format (xrgb8888) uses 8 bits per channel, which can cause banding in HDR content. For HDR, consider a higher-precision format:

xrgb2101010 or argb2101010 – 10 bits per channel. Good balance between precision and performance.
xbgr16161616f or abgr16161616f – 16-bit floating point per channel. Maximum precision.

[[outputs]]
match.serial-number = "33K03894SL0"
color-space = "bt2020"
transfer-function = "pq"
format = "xrgb2101010"

~$ jay randr output DP-1 format set xrgb2101010

Tip

Run jay randr show --formats to see which formats your display supports.

Color management protocol

The Wayland color management protocol (wp_color_manager_v1) lets applications communicate their color space to the compositor. This enables color-aware applications to render correctly regardless of the output’s color settings – for example, an image viewer can tag its surfaces as sRGB and Jay will map the colors to the output’s actual gamut and transfer function.

The protocol is disabled by default and must be enabled explicitly:

[color-management]
enabled = true

Or at runtime:

~$ jay color-management enable

Note

Changing this setting has no effect on applications that are already running. They must be restarted to discover the protocol.

Color management availability requires both the protocol to be enabled and the Vulkan renderer to be active. Check with:

~$ jay color-management status

This prints one of:

Enabled: The protocol is enabled and available to clients.
Enabled (Unavailable): The protocol is enabled but unavailable – usually because the OpenGL renderer is in use.
Disabled: The protocol is disabled.

The control center Color Management pane shows the same information as a toggle and a read-only availability indicator.

Supported capabilities

Jay’s color management implementation supports:

Parametric image descriptions with custom primaries, luminances, and transfer functions (including power curves).
Named primaries: sRGB, BT.2020, DCI-P3, Display P3, Adobe RGB, and others.
Named transfer functions: sRGB, PQ (ST 2084), gamma 2.2, gamma 2.4, BT.1886, linear, and others.
Mastering display metadata for HDR content.
Windows scRGB compatibility.

ICC profile creation is not supported.

Checking display capabilities

Use jay randr show to inspect what your display supports:

~$ jay randr show

The output for each connector includes:

Color spaces – which color spaces the display supports (e.g. default, bt2020), with the current setting marked.
EOTFs – which transfer functions the display supports (e.g. default, pq), with the current setting marked.
Brightness range – minimum and maximum luminance from the EDID, in cd/m^2.
Current brightness – displayed if a custom value is set.
Blend space – current blend space setting.
Native gamut – the display’s CIE xy primaries for red, green, blue, and white point.

Complete example

A typical HDR configuration for a monitor that supports HDR10:

[[outputs]]
match.serial-number = "33K03894SL0"
color-space = "bt2020"
transfer-function = "pq"
brightness = 203
format = "xrgb2101010"
blend-space = "linear"

[color-management]
enabled = true

This sets the output to HDR10 with 10-bit color, physically correct blending, and enables the color management protocol so applications can communicate their color spaces.

Control center

The Outputs pane in the control center (alt-c by default) provides GUI controls for all HDR-related settings per output: Colorimetry, EOTF, Custom Brightness, Format, Blend Space, Use Native Gamut, and a read-only Native Gamut display. See Outputs (Monitors) for details on each field.

The Color Management pane has a toggle to enable/disable the protocol and a read-only indicator showing whether color management is currently available.

The Jay Book