Switch the Color struct back to storing unassociated alpha

editor/src/messages/portfolio/document/document_message_handler.rs

@@ -802,8 +802,6 @@ impl MessageHandler<DocumentMessage, DocumentMessageContext<'_>> for DocumentMes
 				parent_and_insert_index,
 				place_at_origin,
 			} => {
-				// All the image's pixels have been converted to 0..=1, linear, and premultiplied by `Color::from_rgba8_srgb`
-
 				let layer_parent = self.new_layer_parent(true);
 				let image_size = DVec2::new(image.width as f64, image.height as f64);
 

editor/src/messages/portfolio/document_migration.rs

@@ -2123,7 +2123,9 @@ fn migrate_node(node_id: &NodeId, node: &DocumentNode, network_path: &[NodeId],
 			_ => None,
 		});
 
-		if let Some(image) = image {
+		if let Some(mut image) = image {
+			// Legacy embedded pixel data is premultiplied, so restore straight alpha before encoding it
+			image.data.iter_mut().for_each(|pixel| *pixel = pixel.to_unassociated_alpha());
 			let hash = document.resources.embedded.store(Resource::new(image.to_png()));
 
 			let resource_id = ResourceId::new();

node-graph/libraries/no-std-types/src/blending.rs

@@ -201,7 +201,7 @@ pub fn blend_colors(foreground: Color, background: Color, blend_mode: BlendMode,
 		blend_mode => apply_blend_mode(foreground, background, blend_mode),
 	};
 
-	background.alpha_blend(target_color.apply_opacity(opacity))
+	background.alpha_blend(target_color.with_alpha(target_color.a() * opacity))
 }
 
 /// Mixes the two colors by the blend mode's own formula, leaving the alpha compositing to the caller.
@@ -283,17 +283,29 @@ mod tests {
 	}
 
 	#[test]
-	fn darker_color_compares_unassociated_channels() {
-		// The premultiplied backdrop reads as 0.1 gray but is really 0.5 gray, so the 0.4 gray foreground is the darker color
+	fn darker_color_ignores_backdrop_alpha() {
+		// The backdrop's low alpha doesn't darken its color, so the 0.4 gray foreground is the darker color
 		let foreground = Color::from_rgbaf32_unchecked(0.4, 0.4, 0.4, 1.);
-		let background = Color::from_rgbaf32_unchecked(0.1, 0.1, 0.1, 0.2);
+		let background = Color::from_rgbaf32_unchecked(0.5, 0.5, 0.5, 0.2);
 
 		let blended = apply_blend_mode(foreground, background, BlendMode::DarkerColor);
 
 		assert!((blended.r() - 0.4).abs() < 1e-5, "red was {}", blended.r());
 		assert!((blended.a() - 1.).abs() < 1e-5, "alpha was {}", blended.a());
 	}
 
+	#[test]
+	fn source_over_weights_straight_colors_by_alpha() {
+		let over = Color::from_rgbaf32_unchecked(1., 0., 0., 0.5);
+		let under = Color::from_rgbaf32_unchecked(0., 0., 1., 1.);
+
+		let blended = under.alpha_blend(over);
+
+		assert!((blended.r() - 0.5).abs() < 1e-5, "red was {}", blended.r());
+		assert!((blended.b() - 0.5).abs() < 1e-5, "blue was {}", blended.b());
+		assert!((blended.a() - 1.).abs() < 1e-5, "alpha was {}", blended.a());
+	}
+
 	#[test]
 	fn alpha_only_modes_fade_with_opacity() {
 		let foreground = Color::from_rgbaf32_unchecked(0.9, 0.9, 0.9, 1.);

node-graph/libraries/no-std-types/src/color/color_traits.rs

@@ -123,14 +123,6 @@ pub trait RGBMut: RGB {
 	fn set_blue(&mut self, blue: Self::ColorChannel);
 }
 
-pub trait AssociatedAlpha: RGB + Alpha {
-	fn to_unassociated<Out: UnassociatedAlpha>(&self) -> Out;
-}
-
-pub trait UnassociatedAlpha: RGB + Alpha {
-	fn to_associated<Out: AssociatedAlpha>(&self) -> Out;
-}
-
 pub trait Alpha {
 	type AlphaChannel: LinearChannel;
 	const TRANSPARENT: Self;

node-graph/libraries/no-std-types/src/color/color_types.rs

@@ -1,4 +1,4 @@
-use super::color_traits::{Alpha, AlphaMut, AssociatedAlpha, Luminance, Pixel, RGB, RGBMut, Rec709Primaries, SRGB};
+use super::color_traits::{Alpha, AlphaMut, Luminance, Pixel, RGB, RGBMut, Rec709Primaries, SRGB};
 use super::discrete_srgb::{float_to_srgb_u8, srgb_u8_to_float};
 use bytemuck::{Pod, Zeroable};
 use core::fmt::Debug;
@@ -72,7 +72,7 @@ impl Alpha for RGBA16F {
 	type AlphaChannel = f32;
 	#[inline(always)]
 	fn alpha(&self) -> f32 {
-		self.alpha.to_f32() / 255.
+		self.alpha.to_f32()
 	}
 
 	const TRANSPARENT: Self = RGBA16F {
@@ -83,9 +83,8 @@ impl Alpha for RGBA16F {
 	};
 
 	fn multiplied_alpha(&self, alpha: Self::AlphaChannel) -> Self {
-		let alpha = alpha * 255.;
 		let mut result = *self;
-		result.alpha = f16::from_f32(alpha * self.alpha());
+		result.alpha = f16::from_f32(self.alpha() * alpha);
 		result
 	}
 }
@@ -254,7 +253,7 @@ impl RGB for Luma {
 
 impl Pixel for Luma {}
 
-/// Linear-light sRGB color with `f32` channels (alpha unassociated for swatch/UI colors, associated/premultiplied for pixel data inside [`Image<Color>`]).
+/// Linear-light sRGB color with `f32` channels and unassociated (straight) alpha.
 ///
 /// Channels range from `0.` to `f32::MAX`, encoding brightness proportional to light intensity (cd/m² nits in HDR, or `0..=1` mapped to white for SDR).
 ///
@@ -359,9 +358,7 @@ impl Pixel for Color {
 	}
 
 	fn from_bytes(bytes: &[u8]) -> Self {
-		// `Image<Color>` pixel convention is linear-light with associated (premultiplied) alpha.
-		let srgba = SRGBA8::new(bytes[0], bytes[1], bytes[2], bytes[3]);
-		Color::from(srgba).apply_opacity(bytes[3] as f32 / 255.)
+		SRGBA8::new(bytes[0], bytes[1], bytes[2], bytes[3]).into()
 	}
 	fn byte_size() -> usize {
 		4
@@ -378,18 +375,7 @@ impl Alpha for Color {
 	}
 	#[inline(always)]
 	fn multiplied_alpha(&self, alpha: Self::AlphaChannel) -> Self {
-		Self {
-			red: self.red * alpha,
-			green: self.green * alpha,
-			blue: self.blue * alpha,
-			alpha: self.alpha * alpha,
-		}
-	}
-}
-
-impl AssociatedAlpha for Color {
-	fn to_unassociated<Out: super::UnassociatedAlpha>(&self) -> Out {
-		todo!()
+		Self { alpha: self.alpha * alpha, ..*self }
 	}
 }
 
@@ -443,12 +429,6 @@ impl Color {
 		Color { red, green, blue, alpha }
 	}
 
-	/// Construct a `Color` from unassociated (straight) RGBA channels, premultiplying the RGB channels by alpha.
-	#[inline(always)]
-	pub fn new_from_unassociated_rgba(red: f32, green: f32, blue: f32, alpha: f32) -> Color {
-		Color::from_rgbaf32_unchecked(red * alpha, green * alpha, blue * alpha, alpha)
-	}
-
 	/// Create a linear-light `Color` from HSL coordinates (all between 0 and 1).
 	/// HSL is defined on sRGB display values, so the RGB produced by the HSL math is gamma-encoded and decoded to linear before being wrapped in `Color`.
 	///
@@ -707,8 +687,7 @@ impl Color {
 	/// Whole-color "Darker Color" blend: keeps whichever color has the lower mean RGB, with `other`'s alpha.
 	#[inline(always)]
 	pub fn blend_darker_color(&self, other: Color) -> Color {
-		let background = self.to_unassociated_alpha();
-		let darker = if background.average_rgb_channels() <= other.average_rgb_channels() { background } else { other };
+		let darker = if self.average_rgb_channels() <= other.average_rgb_channels() { *self } else { other };
 
 		darker.with_alpha(other.alpha)
 	}
@@ -740,8 +719,7 @@ impl Color {
 	/// Whole-color "Lighter Color" blend: keeps whichever color has the higher mean RGB, with `other`'s alpha.
 	#[inline(always)]
 	pub fn blend_lighter_color(&self, other: Color) -> Color {
-		let background = self.to_unassociated_alpha();
-		let lighter = if background.average_rgb_channels() >= other.average_rgb_channels() { background } else { other };
+		let lighter = if self.average_rgb_channels() >= other.average_rgb_channels() { *self } else { other };
 
 		lighter.with_alpha(other.alpha)
 	}
@@ -824,25 +802,23 @@ impl Color {
 
 	/// Whole-color "Hue" blend: source hue with this color's saturation and Rec.601 luma, with `c_s`'s alpha.
 	pub fn blend_hue(&self, c_s: Color) -> Color {
-		let background = self.to_unassociated_alpha();
-		let sat_b = background.chroma_range();
-		let lum_b = background.luminance_rec_601();
+		let sat_b = self.chroma_range();
+		let lum_b = self.luminance_rec_601();
 
 		c_s.with_saturation(sat_b).with_luminance(lum_b).with_alpha(c_s.alpha)
 	}
 
 	/// Whole-color "Saturation" blend: this color's hue/luma with source saturation, with `c_s`'s alpha.
 	pub fn blend_saturation(&self, c_s: Color) -> Color {
-		let background = self.to_unassociated_alpha();
 		let sat_s = c_s.chroma_range();
-		let lum_b = background.luminance_rec_601();
+		let lum_b = self.luminance_rec_601();
 
-		background.with_saturation(sat_s).with_luminance(lum_b).with_alpha(c_s.alpha)
+		self.with_saturation(sat_s).with_luminance(lum_b).with_alpha(c_s.alpha)
 	}
 
 	/// Whole-color "Color" blend: source hue/saturation with this color's luma, with `c_s`'s alpha.
 	pub fn blend_color(&self, c_s: Color) -> Color {
-		let lum_b = self.to_unassociated_alpha().luminance_rec_601();
+		let lum_b = self.luminance_rec_601();
 
 		c_s.with_luminance(lum_b).with_alpha(c_s.alpha)
 	}
@@ -851,7 +827,7 @@ impl Color {
 	pub fn blend_luminosity(&self, c_s: Color) -> Color {
 		let lum_s = c_s.luminance_rec_601();
 
-		self.to_unassociated_alpha().with_luminance(lum_s).with_alpha(c_s.alpha)
+		self.with_luminance(lum_s).with_alpha(c_s.alpha)
 	}
 
 	/// All four channels as `(red, green, blue, alpha)`.
@@ -990,13 +966,13 @@ impl Color {
 		Self::from_rgbaf32_unchecked(f(self.r()), f(self.g()), f(self.b()), self.a())
 	}
 
-	/// Multiply all four channels (including alpha) by `opacity`, applying an additional premultiplication factor to this Color.
+	/// Multiply RGB by alpha, giving the associated (premultiplied) form for compositing and filtering.
 	#[inline(always)]
-	pub fn apply_opacity(&self, opacity: f32) -> Self {
-		Self::from_rgbaf32_unchecked(self.r() * opacity, self.g() * opacity, self.b() * opacity, self.a() * opacity)
+	pub fn to_associated_alpha(&self) -> Self {
+		self.map_rgb(|channel| channel * self.alpha)
 	}
 
-	/// Divide RGB by alpha to recover unassociated (straight-alpha) channels; no-op if alpha is zero.
+	/// Divide RGB by alpha, undoing [`Self::to_associated_alpha`]; no-op if alpha is zero.
 	#[inline(always)]
 	pub fn to_unassociated_alpha(&self) -> Self {
 		if self.alpha == 0. {
@@ -1011,27 +987,30 @@ impl Color {
 		}
 	}
 
-	/// Apply a per-channel blend function to this color (unmultiplied) and `other`, returning a color with `other`'s alpha; channels are clamped to 0..1.
+	/// Apply a per-channel blend function to this color and `other`, returning a color with `other`'s alpha; channels are clamped to 0..1.
 	#[inline(always)]
 	pub fn blend_rgb<F: Fn(f32, f32) -> f32>(&self, other: Color, f: F) -> Self {
-		let background = self.to_unassociated_alpha();
 		Color {
-			red: f(background.red, other.red).clamp(0., 1.),
-			green: f(background.green, other.green).clamp(0., 1.),
-			blue: f(background.blue, other.blue).clamp(0., 1.),
+			red: f(self.red, other.red).clamp(0., 1.),
+			green: f(self.green, other.green).clamp(0., 1.),
+			blue: f(self.blue, other.blue).clamp(0., 1.),
 			alpha: other.alpha,
 		}
 	}
 
-	/// Porter-Duff "source over" composite of `other` over `self`. Both colors must use associated (premultiplied) alpha.
+	/// Porter-Duff "source over" composite of `other` over `self`.
 	#[inline(always)]
 	pub fn alpha_blend(&self, other: Color) -> Self {
-		let inv_alpha = 1. - other.alpha;
+		let under_weight = self.alpha * (1. - other.alpha);
+		let alpha = other.alpha + under_weight;
+		if alpha == 0. {
+			return Self::TRANSPARENT;
+		}
 		Self {
-			red: self.red * inv_alpha + other.red,
-			green: self.green * inv_alpha + other.green,
-			blue: self.blue * inv_alpha + other.blue,
-			alpha: self.alpha * inv_alpha + other.alpha,
+			red: (other.red * other.alpha + self.red * under_weight) / alpha,
+			green: (other.green * other.alpha + self.green * under_weight) / alpha,
+			blue: (other.blue * other.alpha + self.blue * under_weight) / alpha,
+			alpha,
 		}
 	}
 

node-graph/libraries/raster-types/src/image.rs

@@ -144,14 +144,7 @@ impl<P: Pixel> Image<P> {
 impl Image<Color> {
 	/// Generate Image from some frontend image data (the canvas pixels as u8s in a flat array)
 	pub fn from_image_data(image_data: &[u8], width: u32, height: u32) -> Self {
-		let data = image_data
-			.chunks_exact(4)
-			.map(|v| {
-				// `Image<Color>` pixels are stored linear-light with premultiplied alpha
-				let srgba = SRGBA8::new(v[0], v[1], v[2], v[3]);
-				Color::from(srgba).apply_opacity(v[3] as f32 / 255.)
-			})
-			.collect();
+		let data = image_data.chunks_exact(4).map(|v| SRGBA8::new(v[0], v[1], v[2], v[3]).into()).collect();
 		Image {
 			width,
 			height,
@@ -171,7 +164,7 @@ impl Image<Color> {
 }
 
 use super::*;
-impl<P: Alpha + RGB + AssociatedAlpha> Image<P>
+impl<P: Alpha + RGB> Image<P>
 where
 	P::ColorChannel: Linear,
 	<P as Alpha>::AlphaChannel: Linear,
@@ -195,10 +188,9 @@ where
 			// Smaller alpha values than this would map to fully transparent
 			// anyway, avoid expensive encoding.
 			if a >= 0.5 / 255. {
-				let undo_premultiply = 1. / a;
-				let r = color.r().to_f32() * undo_premultiply;
-				let g = color.g().to_f32() * undo_premultiply;
-				let b = color.b().to_f32() * undo_premultiply;
+				let r = color.r().to_f32();
+				let g = color.g().to_f32();
+				let b = color.b().to_f32();
 
 				// Compute new sRGB value if necessary.
 				if r != last_r {
@@ -287,4 +279,14 @@ mod test {
 
 		assert_eq!(image, deserialized);
 	}
+
+	#[test]
+	fn image_data_round_trips_translucent_pixels() {
+		use super::*;
+		let bytes = [255, 0, 0, 128, 0, 255, 0, 1, 255, 255, 255, 41, 10, 20, 30, 255];
+
+		let image = Image::from_image_data(&bytes, 4, 1);
+
+		assert_eq!(image.to_flat_u8().0, bytes);
+	}
 }

node-graph/libraries/rendering/src/renderer.rs

@@ -22,7 +22,7 @@ use dyn_any::DynAny;
 use glam::{DAffine2, DMat2, DVec2};
 use graphene_hash::CacheHashWrapper;
 use graphene_resource::Resource;
-use graphic_types::raster_types::{BitmapMut, CPU, GPU, Image, Raster, Texture};
+use graphic_types::raster_types::{CPU, GPU, Image, Raster, Texture};
 use graphic_types::vector_types::gradient::{Gradient, GradientForm};
 use graphic_types::vector_types::vector::click_target::{ClickTarget, FreePoint};
 use graphic_types::vector_types::vector::misc::dvec2_to_point;
@@ -130,9 +130,7 @@ fn composite_paint_over(over: Color, under: Color, blend_mode: BlendMode) -> Col
 		return Color::TRANSPARENT;
 	}
 
-	// The blend formulas read their backdrop premultiplied
-	let premultiplied_under = Color::from_rgbaf32_unchecked(under.r() * under_alpha, under.g() * under_alpha, under.b() * under_alpha, under_alpha);
-	let mixed = apply_blend_mode(over, premultiplied_under, blend_mode);
+	let mixed = apply_blend_mode(over, under, blend_mode);
 
 	// The mode only mixes where the backdrop has coverage, so its alpha interpolates each source channel from the raw color to the mixed color
 	let source_channel = |over_channel: f32, mixed_channel: f32| over_channel * (1. - under_alpha) + mixed_channel * under_alpha;
@@ -428,17 +426,8 @@ fn singular_values(transform: DAffine2) -> (f64, f64) {
 pub fn black_or_white_for_best_contrast(background: Option<Color>) -> Color {
 	let Some(bg) = background else { return core_types::consts::LAYER_OUTLINE_STROKE_COLOR };
 
-	let alpha = bg.a();
-
-	// Un-premultiply, then encode to gamma sRGB to do the composite in display space.
-	let (gamma_r, gamma_g, gamma_b) = if alpha > f32::EPSILON {
-		let [r, g, b, _] = Color::from_rgbaf32_unchecked(bg.r() / alpha, bg.g() / alpha, bg.b() / alpha, alpha).to_gamma_srgb_channels();
-		(r, g, b)
-	} else {
-		(0., 0., 0.)
-	};
-
-	// Composite over black in sRGB space (premultiplied by alpha), then decode to linear for the luminance test.
+	// Composite over black in gamma sRGB space, then decode to linear for the luminance test.
+	let [gamma_r, gamma_g, gamma_b, alpha] = bg.to_gamma_srgb_channels();
 	let composited = Color::from_gamma_srgb_channels(gamma_r * alpha, gamma_g * alpha, gamma_b * alpha, 1.);
 
 	let threshold = (1.05 * 0.05f32).sqrt() - 0.05;
@@ -2295,9 +2284,7 @@ fn render_raster_cpu_item_svg(item: ItemRef<'_, Raster<CPU>>, render: &mut SvgRe
 	}
 
 	if render_params.to_canvas() {
-		let mut image_copy = image.clone();
-		image_copy.data_mut().map_pixels(|p| p.to_unassociated_alpha());
-		let id = *render.image_data.entry(CacheHashWrapper(image_copy.into_data())).or_insert_with(generate_uuid);
+		let id = *render.image_data.entry(CacheHashWrapper(image.clone().into_data())).or_insert_with(generate_uuid);
 
 		render.parent_tag(
 			"foreignObject",

node-graph/libraries/wgpu-executor/src/texture_conversion.rs

@@ -115,9 +115,7 @@ impl RasterGpuToRasterCpuConverter {
 			let start = row * row_stride;
 			let row_slice = &view[start..start + row_bytes];
 			for px in row_slice.chunks_exact(4) {
-				// `Image<Color>` pixels are stored linear-light with associated (premultiplied) alpha
-				let srgba = SRGBA8::new(px[0], px[1], px[2], px[3]);
-				cpu_data.push(Color::from(srgba).apply_opacity(px[3] as f32 / 255.));
+				cpu_data.push(SRGBA8::new(px[0], px[1], px[2], px[3]).into());
 			}
 		}