Merge pull request #4534 from dotCooCoo/fix/issue-3161

Fix #2249: macOS converts the canvas from sRGB to the display's profile

Cargo.lock

@@ -3282,6 +3282,7 @@ dependencies = [
  "objc2-core-foundation",
  "objc2-core-graphics",
  "objc2-foundation 0.3.2",
+ "objc2-quartz-core 0.3.2",
 ]
 
 [[package]]
@@ -6052,8 +6053,10 @@ dependencies = [
  "objc2 0.6.5",
  "objc2-app-kit 0.3.2",
  "objc2-core-foundation",
+ "objc2-core-graphics",
  "objc2-core-text",
  "objc2-foundation 0.3.2",
+ "objc2-quartz-core 0.3.2",
  "pollster 1.0.1",
  "raw-window-handle",
  "rfd",

apps/vectorcraft/Cargo.toml

@@ -58,7 +58,7 @@ windows = { version = "0.62", features = ["Win32_Graphics_DirectWrite", "Win32_G
 # with default features off). Pinned exactly: review the diff before bumping.
 muda = { version = "=0.21.2", default-features = false }
 objc2 = "0.6"
-objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "block2", "objc2-core-foundation", "NSApplication", "NSButton", "NSColor", "NSColorSampler", "NSColorSpace", "NSControl", "NSMenu", "NSEvent", "NSPasteboard", "NSPopUpButton", "NSResponder", "NSSavePanel", "NSTextField", "NSTextInputContext", "NSView", "NSWindow"] }
+objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "block2", "objc2-core-foundation", "objc2-quartz-core", "NSApplication", "NSButton", "NSColor", "NSColorSampler", "NSColorSpace", "NSControl", "NSMenu", "NSEvent", "NSPasteboard", "NSPopUpButton", "NSResponder", "NSSavePanel", "NSTextField", "NSTextInputContext", "NSView", "NSWindow"] }
 # Files opened from Finder and the Dock (src/open_documents.rs), pasted bitmaps (src/clipboard.rs) and the control channel's App Nap activity (src/control_server.rs); both already come with winit, muda and rfd.
 objc2-foundation = { version = "0.3", default-features = false, features = ["std", "block2", "NSArray", "NSData", "NSGeometry", "NSNotification", "NSOperation", "NSProcessInfo", "NSString", "NSURL"] }
 block2 = "0.6"
@@ -70,6 +70,10 @@ raw-window-handle = "0.6"
 # objc2-foundation.
 objc2-core-foundation = { version = "0.3", default-features = false, features = ["std", "CFArray", "CFString", "CFURL"] }
 objc2-core-text = { version = "0.3", default-features = false, features = ["std", "CTFontManager"] }
+# The windows' Metal layers and the sRGB color space they are tagged with (src/mac_color_space.rs).
+# Zlib OR Apache-2.0 OR MIT, already in the build through wgpu with these features.
+objc2-quartz-core = { version = "0.3", default-features = false, features = ["std", "objc2-core-foundation", "objc2-core-graphics", "CALayer", "CAMetalLayer"] }
+objc2-core-graphics = { version = "0.3", default-features = false, features = ["std", "CGColorSpace"] }
 # Waits for the answer of a file dialog shown as a sheet (src/main.rs, #867). MIT OR Apache-2.0,
 # already in the build through eframe.
 pollster = "1"

apps/vectorcraft/src/app.rs

@@ -63,6 +63,9 @@ impl eframe::App for App {
             let inset = mac_window::center_buttons(frame, ui.ctx().zoom_factor(), bar);
             self.app.titlebar_inset = inset.unwrap_or(mac_window::DEFAULT_INSET);
         }
+        // macOS converts the frame from sRGB to the display's profile (#2249).
+        #[cfg(all(target_os = "macos", feature = "wgpu"))]
+        super::mac_color_space::tag(ui);
         file_access::unconfined(|| self.app.ui(ui));
         // Preferences are written when confirmed, not only at exit: a session ended any other way
         // (the system shutting down, the process ended or crashing) used to lose them (#1747).

apps/vectorcraft/src/mac_color_space.rs

@@ -0,0 +1,139 @@
+//! macOS: the windows' Metal layers are tagged sRGB, so macOS converts what VectorCraft draws to
+//! the display's color profile (#2249, #3161).
+//!
+//! VectorCraft draws sRGB values. Apple's documentation of `CAMetalLayer.colorspace` says: "When a
+//! color space is present, Core Animation performs any necessary color space transformations when
+//! compositing this content. The default value is `nil`, indicating that the rendered content
+//! isn’t color-matched"
+//! (<https://developer.apple.com/documentation/quartzcore/cametallayer/colorspace>). Without a
+//! color space, a wide-gamut display shows the sRGB values as its own, so `#FF0000` is drawn as
+//! the display's most saturated red, brighter than the same color in an exported PNG.
+//!
+//! wgpu 30 sets the layer's color space to nil each time it configures the surface: at start, on
+//! a resize and when the window moves to a display with another scale (gfx-rs/wgpu#10013). egui
+//! configures the surface while it paints a frame, after [`eframe::App::ui`] has run, so [`tag`]
+//! adds a paint callback that draws nothing. egui calls it after the configuration and before the
+//! frame is presented, and it sets sRGB on every window's Metal layer that has no color space. A
+//! layer that already has a color space keeps it.
+//!
+//! Reading a layer's sublayers has no safe API, hence the scoped `unsafe_code` allowance.
+#![allow(unsafe_code)]
+
+use eframe::egui_wgpu::{self, CallbackResources, CallbackTrait, wgpu};
+use objc2::MainThreadMarker;
+use objc2::rc::Retained;
+use objc2::{DowncastTarget, Message};
+use objc2_app_kit::NSApplication;
+use objc2_core_graphics::{CGColorSpace, kCGColorSpaceSRGB};
+use objc2_quartz_core::{CALayer, CAMetalLayer};
+
+/// Adds the callback that tags the windows' Metal layers to the frame `ui` draws: a point in its
+/// top-left corner, in the background layer, with nothing drawn.
+pub fn tag(ui: &egui::Ui) {
+    let ctx = ui.ctx();
+    let rect = egui::Rect::from_min_size(ctx.content_rect().min, egui::vec2(1.0, 1.0));
+    ctx.layer_painter(egui::LayerId::background()).add(egui_wgpu::Callback::new_paint_callback(rect, TagLayers));
+}
+
+struct TagLayers;
+
+impl CallbackTrait for TagLayers {
+    fn paint(&self, _: egui::PaintCallbackInfo, _: &mut wgpu::RenderPass<'static>, _: &CallbackResources) {
+        // AppKit's windows and views are read on the main thread only, where eframe paints.
+        let Some(mtm) = MainThreadMarker::new() else {
+            return;
+        };
+        for window in NSApplication::sharedApplication(mtm).windows() {
+            if let Some(root) = window.contentView().and_then(|view| view.layer()) {
+                for layer in metal_layers(&root) {
+                    tag_srgb(&layer);
+                }
+            }
+        }
+    }
+}
+
+/// The Metal layers that draw a view whose layer is `root`: `root` itself, or the sublayers wgpu
+/// adds to it when it is a plain layer.
+fn metal_layers(root: &CALayer) -> Vec<Retained<CAMetalLayer>> {
+    if let Some(layer) = downcast(root) {
+        return vec![layer];
+    }
+    // SAFETY: the returned array is a copy; the layers in it are retained while it lives.
+    let Some(sublayers) = (unsafe { root.sublayers() }) else {
+        return Vec::new();
+    };
+    sublayers.iter().filter_map(|layer| downcast(&layer)).collect()
+}
+
+fn downcast<T: DowncastTarget>(layer: &CALayer) -> Option<Retained<T>> {
+    layer.retain().downcast::<T>().ok()
+}
+
+/// Sets sRGB as `layer`'s color space when it has none. Returns whether it was set.
+fn tag_srgb(layer: &CAMetalLayer) -> bool {
+    if layer.colorspace().is_some() {
+        return false;
+    }
+    // SAFETY: a constant that CoreGraphics exports.
+    let Some(srgb) = CGColorSpace::with_name(Some(unsafe { kCGColorSpaceSRGB })) else {
+        return false;
+    };
+    layer.setColorspace(Some(&srgb));
+    true
+}
+
+#[cfg(test)]
+mod tests {
+    use objc2_core_foundation::CFString;
+    use objc2_core_graphics::kCGColorSpaceDisplayP3;
+
+    use super::*;
+
+    fn space_name(layer: &CAMetalLayer) -> Option<String> {
+        CGColorSpace::name(layer.colorspace().as_deref()).map(|name| name.to_string())
+    }
+
+    fn name(constant: &CFString) -> String {
+        constant.to_string()
+    }
+
+    /// A layer without a color space is tagged sRGB once; the second call leaves it as it is.
+    #[test]
+    fn an_untagged_layer_becomes_srgb() {
+        let layer = CAMetalLayer::new();
+        assert_eq!(space_name(&layer), None, "a new layer has no color space");
+        assert!(tag_srgb(&layer));
+        assert_eq!(space_name(&layer), Some(name(unsafe { kCGColorSpaceSRGB })));
+        assert!(!tag_srgb(&layer));
+        assert_eq!(space_name(&layer), Some(name(unsafe { kCGColorSpaceSRGB })));
+    }
+
+    /// A layer with a color space of its own keeps it.
+    #[test]
+    fn a_layer_with_a_color_space_keeps_it() {
+        let layer = CAMetalLayer::new();
+        let p3 = CGColorSpace::with_name(Some(unsafe { kCGColorSpaceDisplayP3 })).unwrap();
+        layer.setColorspace(Some(&p3));
+        assert!(!tag_srgb(&layer));
+        assert_eq!(space_name(&layer), Some(name(unsafe { kCGColorSpaceDisplayP3 })));
+    }
+
+    /// The Metal layer is found as the view's layer or as one of its sublayers, and only Metal
+    /// layers are returned.
+    #[test]
+    fn metal_layers_are_found_under_the_view_layer() {
+        let metal = CAMetalLayer::new();
+        assert_eq!(metal_layers(&metal), vec![metal.clone()]);
+
+        let root = CALayer::new();
+        assert!(metal_layers(&root).is_empty());
+        root.addSublayer(&CALayer::new());
+        root.addSublayer(&metal);
+        assert_eq!(metal_layers(&root), vec![metal.clone()]);
+        for layer in metal_layers(&root) {
+            tag_srgb(&layer);
+        }
+        assert_eq!(space_name(&metal), Some(name(unsafe { kCGColorSpaceSRGB })));
+    }
+}

apps/vectorcraft/src/main.rs

@@ -26,6 +26,8 @@ mod gpu;
 #[cfg(target_os = "linux")]
 mod linux_stylus;
 mod logging;
+#[cfg(all(target_os = "macos", feature = "wgpu"))]
+mod mac_color_space;
 #[cfg(target_os = "macos")]
 mod mac_fonts;
 #[cfg(target_os = "macos")]