LR-HDR-EXPORT: HLG HDR AVIF export (hdrTransfer) and a tested SDR/HDR display decision

AVIF HDR export takes hdrTransfer = pq | hlg (export dialog: HDR curve;
MCP export schema lists it). HLG writes CICP transfer 18 and no clli box.
The window logs whether it shows HDR; today always SDR (eframe configures
an 8-bit surface), with the platform / surface format / headroom / opt-out
rules unit-tested. Also satisfies clippy 1.99's chunks_exact_to_as_chunks
in spots.rs.

Part of #868.

Sources: ITU-R BT.2100 (HLG OETF, inverse OOTF with system gamma 1.2),
ITU-R BT.2408 (203 cd/m2 reference white at 75 % HLG); own design for
the display decision.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/lightcraft/src/hdr_display.rs

@@ -0,0 +1,174 @@
+//! HDR display output (LR-VIEW-HDR-DISPLAY): whether the window can show the range above SDR
+//! white, and with which surface format. Pure decisions, unit-tested here; the window always
+//! falls back to SDR when any condition is missing, so an HDR edit is never shown wrong.
+//!
+//! HDR needs three things: a platform path (macOS EDR via an extended-range float Metal layer,
+//! Windows HDR via scRGB float or HDR10 swap chains, Wayland via the colour-management protocol),
+//! a surface format that carries values above 1.0 (`Rgba16Float`) or a 10-bit PQ signal
+//! (`Rgb10a2Unorm`), and a renderer that actually configured that format. eframe 0.36 picks the
+//! surface format itself (8-bit only, `egui_wgpu::preferred_framebuffer_format`), so today the
+//! choice reports SDR with the reason; the moment the renderer's target is an HDR format the same
+//! decision turns HDR on.
+
+use eframe::wgpu::TextureFormat;
+
+/// Windowing platform, as far as HDR output is concerned.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Platform {
+    MacOs,
+    Windows,
+    Wayland,
+    X11,
+    Web,
+    Other,
+}
+
+impl Platform {
+    /// The platform this build runs on (`wayland` from the session environment on Linux / BSD).
+    pub fn current() -> Self {
+        if cfg!(target_os = "macos") {
+            Self::MacOs
+        } else if cfg!(target_os = "windows") {
+            Self::Windows
+        } else if cfg!(target_arch = "wasm32") {
+            Self::Web
+        } else if std::env::var_os("WAYLAND_DISPLAY").is_some() {
+            Self::Wayland
+        } else if std::env::var_os("DISPLAY").is_some() {
+            Self::X11
+        } else {
+            Self::Other
+        }
+    }
+
+    /// Whether the platform has an HDR compositing path wgpu can reach at all.
+    fn has_hdr_path(self) -> bool {
+        matches!(self, Self::MacOs | Self::Windows | Self::Wayland)
+    }
+}
+
+/// How HDR values reach the display.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum HdrEncoding {
+    /// Linear float, SDR white at 1.0 and the headroom above it (macOS EDR, Windows scRGB,
+    /// Wayland extended linear).
+    ExtendedLinear,
+    /// 10-bit BT.2100 PQ (Windows HDR10, Wayland PQ).
+    Pq10,
+}
+
+/// The display output decision.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum DisplayOutput {
+    Sdr { reason: &'static str },
+    Hdr { format: TextureFormat, encoding: HdrEncoding },
+}
+
+impl DisplayOutput {
+    #[cfg_attr(not(test), allow(dead_code))] // read by the HDR view once the surface can be HDR
+    pub fn is_hdr(self) -> bool {
+        matches!(self, Self::Hdr { .. })
+    }
+}
+
+/// The encoding a surface format carries on `platform`, if it is an HDR one.
+fn encoding_of(format: TextureFormat, platform: Platform) -> Option<HdrEncoding> {
+    match format {
+        TextureFormat::Rgba16Float => Some(HdrEncoding::ExtendedLinear),
+        // macOS takes HDR through EDR floats only
+        TextureFormat::Rgb10a2Unorm if platform != Platform::MacOs => Some(HdrEncoding::Pq10),
+        _ => None,
+    }
+}
+
+/// The best HDR surface format among those a surface offers (`available`), for `platform`:
+/// float first (keeps SDR white at 1.0 and needs no PQ encode), then 10-bit PQ. `None` = SDR only.
+/// (For configuring the surface once eframe lets the app pick its format.)
+#[cfg_attr(not(test), allow(dead_code))]
+pub fn preferred_hdr_format(available: &[TextureFormat], platform: Platform) -> Option<TextureFormat> {
+    if !platform.has_hdr_path() {
+        return None;
+    }
+    [TextureFormat::Rgba16Float, TextureFormat::Rgb10a2Unorm].into_iter().find(|f| available.contains(f) && encoding_of(*f, platform).is_some())
+}
+
+/// The decision for the window: `target` is the format the renderer configured (`None`: the glow
+/// renderer or no wgpu state), `display_headroom` the display's current peak over SDR white as the
+/// OS reports it (`None`: unknown, `Some(1.0)`: an SDR display or HDR switched off in the OS),
+/// `disabled` the user's opt-out (`LIGHTCRAFT_HDR_DISPLAY=0`).
+pub fn choose(target: Option<TextureFormat>, platform: Platform, display_headroom: Option<f32>, disabled: bool) -> DisplayOutput {
+    if disabled {
+        return DisplayOutput::Sdr { reason: "HDR display is switched off" };
+    }
+    if !platform.has_hdr_path() {
+        return DisplayOutput::Sdr { reason: "this windowing system has no HDR output" };
+    }
+    let Some(format) = target else {
+        return DisplayOutput::Sdr { reason: "the window renderer is not wgpu" };
+    };
+    let Some(encoding) = encoding_of(format, platform) else {
+        return DisplayOutput::Sdr { reason: "the window surface is 8-bit" };
+    };
+    match display_headroom {
+        Some(h) if h.is_finite() && h > 1.0 => DisplayOutput::Hdr { format, encoding },
+        Some(_) => DisplayOutput::Sdr { reason: "the display is in SDR mode" },
+        None => DisplayOutput::Sdr { reason: "the display's HDR headroom is unknown" },
+    }
+}
+
+/// The decision for this start, logged once (the window renderer's `target` format).
+pub fn report(target: Option<TextureFormat>) -> DisplayOutput {
+    let disabled = std::env::var("LIGHTCRAFT_HDR_DISPLAY").ok().is_some_and(|v| matches!(v.trim(), "0" | "off" | "false" | "no"));
+    // no OS headroom query is wired yet: unknown, so SDR unless everything else is HDR-ready
+    let out = choose(target, Platform::current(), None, disabled);
+    match out {
+        DisplayOutput::Sdr { reason } => log::info!("display: SDR ({reason})"),
+        DisplayOutput::Hdr { format, encoding } => log::info!("display: HDR {encoding:?} ({format:?})"),
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use TextureFormat as F;
+
+    #[test]
+    fn eight_bit_surfaces_are_sdr() {
+        for p in [Platform::MacOs, Platform::Windows, Platform::Wayland] {
+            for f in [F::Bgra8Unorm, F::Bgra8UnormSrgb, F::Rgba8Unorm] {
+                assert!(!choose(Some(f), p, Some(4.0), false).is_hdr(), "{p:?} {f:?}");
+            }
+        }
+    }
+
+    #[test]
+    fn hdr_needs_platform_format_and_headroom() {
+        let hdr = DisplayOutput::Hdr { format: F::Rgba16Float, encoding: HdrEncoding::ExtendedLinear };
+        assert_eq!(choose(Some(F::Rgba16Float), Platform::MacOs, Some(2.0), false), hdr);
+        assert_eq!(
+            choose(Some(F::Rgb10a2Unorm), Platform::Windows, Some(4.0), false),
+            DisplayOutput::Hdr { format: F::Rgb10a2Unorm, encoding: HdrEncoding::Pq10 }
+        );
+        // any missing condition falls back to SDR
+        assert!(!choose(Some(F::Rgba16Float), Platform::X11, Some(4.0), false).is_hdr());
+        assert!(!choose(Some(F::Rgba16Float), Platform::Web, Some(4.0), false).is_hdr());
+        assert!(!choose(Some(F::Rgba16Float), Platform::MacOs, Some(1.0), false).is_hdr());
+        assert!(!choose(Some(F::Rgba16Float), Platform::MacOs, None, false).is_hdr());
+        assert!(!choose(Some(F::Rgba16Float), Platform::MacOs, Some(f32::NAN), false).is_hdr());
+        assert!(!choose(Some(F::Rgba16Float), Platform::MacOs, Some(2.0), true).is_hdr());
+        assert!(!choose(None, Platform::Windows, Some(4.0), false).is_hdr());
+        // macOS has no PQ swap chain path
+        assert!(!choose(Some(F::Rgb10a2Unorm), Platform::MacOs, Some(4.0), false).is_hdr());
+    }
+
+    #[test]
+    fn preferred_format_picks_float_then_pq() {
+        let all = [F::Bgra8Unorm, F::Rgb10a2Unorm, F::Rgba16Float];
+        assert_eq!(preferred_hdr_format(&all, Platform::Windows), Some(F::Rgba16Float));
+        assert_eq!(preferred_hdr_format(&[F::Bgra8Unorm, F::Rgb10a2Unorm], Platform::Wayland), Some(F::Rgb10a2Unorm));
+        assert_eq!(preferred_hdr_format(&[F::Bgra8Unorm, F::Rgb10a2Unorm], Platform::MacOs), None);
+        assert_eq!(preferred_hdr_format(&all, Platform::X11), None);
+        assert_eq!(preferred_hdr_format(&[], Platform::MacOs), None);
+    }
+}

apps/lightcraft/src/main.rs

@@ -27,6 +27,7 @@ mod appearance;
 mod control_server;
 mod dialog_filter;
 mod editors;
+mod hdr_display;
 mod logging;
 #[cfg(target_os = "macos")]
 mod native_menu;
@@ -833,6 +834,8 @@ fn main() -> eframe::Result {
                 // (never required: without it, AI masks offer to download it; see docs/ai-masks.md)
                 // and the user's own download locations, one base URL per line (LIGHTCRAFT_SAM3_MIRRORS too)
                 session = session.with_default_sam3_model();
+                // HDR display: SDR unless the surface, platform and display all allow HDR
+                hdr_display::report(cc.wgpu_render_state.as_ref().map(|r| r.target_format));
                 let mut app = LightcraftApp::new(session, services(cc, app_log_file.as_deref()));
                 if let Some(ui) = prefs {
                     app.ui = ui;