cmm: ignore backend luminance with the default EOTF

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);

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,

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));

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

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.