From 4746361cb7bef36a962d9db7539595c562de3bb9 Mon Sep 17 00:00:00 2001
From: Julian Orth <ju.orth@gmail.com>
Date: Wed, 15 Oct 2025 20:59:12 +0200
Subject: [PATCH] cmm: ignore backend luminance with the default EOTF

---
 jay-config/src/video.rs          |  4 ++++
 src/cli/randr.rs                 |  4 ++++
 src/ifs/wl_output.rs             | 29 +++++++++--------------------
 toml-spec/spec/spec.generated.md |  4 ++++
 toml-spec/spec/spec.yaml         |  4 ++++
 5 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/jay-config/src/video.rs b/jay-config/src/video.rs
index 56b8a271..05d6ae0d 100644
--- a/jay-config/src/video.rs
+++ b/jay-config/src/video.rs
@@ -304,6 +304,10 @@ impl Connector {
     /// This should only be used with the PQ transfer function. If the default transfer
     /// function is used, you should instead calibrate the hardware directly.
     ///
+    /// When used with the default transfer function, the default brightness is anchored
+    /// at 80 cd/m^2. That is, setting this to 40 cd/m^2 makes everything appear half as
+    /// bright as normal and creates 50% HDR headroom.
+    ///
     /// This has no effect unless the vulkan renderer is used.
     pub fn set_brightness(self, brightness: Option<f64>) {
         get!().connector_set_brightness(self, brightness);
diff --git a/src/cli/randr.rs b/src/cli/randr.rs
index cec9887a..df63d735 100644
--- a/src/cli/randr.rs
+++ b/src/cli/randr.rs
@@ -386,6 +386,10 @@ pub struct BrightnessArgs {
     /// When using the default EOTF, you likely want to set this to `default`
     /// and adjust the display hardware brightness setting instead.
     ///
+    /// When used with the default transfer function, the default brightness is anchored
+    /// at 80 cd/m^2. That is, setting this to 40 cd/m^2 makes everything appear half as
+    /// bright as normal and creates 50% HDR headroom.
+    ///
     /// This has no effect unless the vulkan renderer is used.
     #[clap(verbatim_doc_comment, value_parser = parse_brightness)]
     brightness: Brightness,
diff --git a/src/ifs/wl_output.rs b/src/ifs/wl_output.rs
index f0dcd5b4..9925ff87 100644
--- a/src/ifs/wl_output.rs
+++ b/src/ifs/wl_output.rs
@@ -361,30 +361,19 @@ impl WlOutputGlobal {
     }
 
     pub fn update_color_description(&self) -> bool {
-        let mut luminance = Luminance::SRGB;
-        let tf = match self.btf.get() {
-            BackendEotfs::Default => {
-                if let Some(brightness) = self.persistent.brightness.get() {
-                    let output_max = match self.luminance {
-                        None => 80.0,
-                        Some(v) => v.max,
-                    };
-                    luminance.white.0 = luminance.max.0 * brightness / output_max;
-                }
-                Eotf::Gamma22
-            }
-            BackendEotfs::Pq => {
-                luminance = Luminance::ST2084_PQ;
-                if let Some(brightness) = self.persistent.brightness.get() {
-                    luminance.white.0 = brightness;
-                }
-                Eotf::St2084Pq
-            }
+        let (mut luminance, tf) = match self.btf.get() {
+            BackendEotfs::Default => (Luminance::SRGB, Eotf::Gamma22),
+            BackendEotfs::Pq => (Luminance::ST2084_PQ, Eotf::St2084Pq),
         };
+        if let Some(brightness) = self.persistent.brightness.get() {
+            luminance.white.0 = brightness;
+        }
         let mut target_luminance = luminance.to_target();
         let mut max_cll = None;
         let mut max_fall = None;
-        if let Some(l) = self.luminance {
+        if let Some(l) = self.luminance
+            && self.btf.get() == BackendEotfs::Pq
+        {
             target_luminance.min = F64(l.min);
             target_luminance.max = F64(l.max);
             max_cll = Some(F64(l.max));
diff --git a/toml-spec/spec/spec.generated.md b/toml-spec/spec/spec.generated.md
index 43a32c02..c6ea9050 100644
--- a/toml-spec/spec/spec.generated.md
+++ b/toml-spec/spec/spec.generated.md
@@ -857,6 +857,10 @@ The string should have one of the following values:
   
   - `default`: The maximum brightness of the output.
   - `PQ`: 203 cd/m^2
+  
+  When used with the default transfer function, the default brightness is anchored
+  at 80 cd/m^2. That is, setting this to 40 cd/m^2 makes everything appear half as
+  bright as normal and creates 50% HDR headroom.
 
 
 #### A number
diff --git a/toml-spec/spec/spec.yaml b/toml-spec/spec/spec.yaml
index 32d122bc..59fa300e 100644
--- a/toml-spec/spec/spec.yaml
+++ b/toml-spec/spec/spec.yaml
@@ -3425,6 +3425,10 @@ Brightness:
             
             - `default`: The maximum brightness of the output.
             - `PQ`: 203 cd/m^2
+            
+            When used with the default transfer function, the default brightness is anchored
+            at 80 cd/m^2. That is, setting this to 40 cd/m^2 makes everything appear half as
+            bright as normal and creates 50% HDR headroom.
     - kind: number
       description: |
         The brightness in cd/m^2.