Rework Gradient into a newtype of List<Color> with optional position and midpoint attributes (#4397)

* Rework Gradient into a newtype of List<Color> with optional position and midpoint attributes

* Fix Vello stopless-gradient fallback coverage, empty legacy gradient tables, the node docs gradient swatch, NaN position elision, and wired setter input overwrites

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

@@ -127,6 +127,11 @@ pub fn render_gradient_paint<S: core_types::lane::LaneSource<Element = Gradient>
 			stop.push_str(" />")
 		}
 
+		// A gradient with no stops paints as solid black, matching `Gradient::evaluate` (a stopless def would otherwise render as no paint per the SVG spec)
+		if stop.is_empty() {
+			stop.push_str(r##"<stop stop-color="#000000" />"##);
+		}
+
 		// Need to cancel out the element's transform as it is already applied to the path itself.
 		let element_transform_inverse = if transform_is_invertible(element_transform) {
 			element_transform.inverse()

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

@@ -400,6 +400,31 @@ pub(crate) fn gradient_placement(transform: DAffine2, gradient_type: GradientTyp
 	}
 }
 
+/// Converts a gradient's renderer samples to peniko color stops, duplicating an off-zero first stop at position 0 since Vello ignores the first stop's position and always treats it as 0.
+fn peniko_color_stops(gradient: &Gradient) -> peniko::ColorStops {
+	let mut peniko_stops = peniko::ColorStops::new();
+
+	for (position, color, _) in gradient.interpolated_samples() {
+		let color = peniko::color::DynamicColor::from_alpha_color(SRGBA8::from(color).to_peniko_color());
+
+		if peniko_stops.is_empty() && position > 0. {
+			peniko_stops.push(peniko::ColorStop { offset: 0., color });
+		}
+
+		peniko_stops.push(peniko::ColorStop { offset: position as f32, color });
+	}
+
+	// A gradient with no stops paints as solid black, matching `Gradient::evaluate`
+	if peniko_stops.is_empty() {
+		peniko_stops.push(peniko::ColorStop {
+			offset: 0.,
+			color: peniko::color::DynamicColor::from_alpha_color(SRGBA8::from(Color::BLACK).to_peniko_color()),
+		});
+	}
+
+	peniko_stops
+}
+
 fn create_peniko_gradient_brush<S: LaneSource<Element = Gradient>>(gradient_list: &S, multiplied_transform: &DAffine2) -> Option<(peniko::Brush, DAffine2)> {
 	let stops = gradient_list.element(0)?;
 
@@ -407,13 +432,7 @@ fn create_peniko_gradient_brush<S: LaneSource<Element = Gradient>>(gradient_list
 	let gradient_transform: DAffine2 = gradient_list.attr::<Transform>(0);
 	let spread_method: GradientSpreadMethod = gradient_list.attr::<SpreadMethod>(0);
 
-	let mut peniko_stops = peniko::ColorStops::new();
-	for (position, color, _) in stops.interpolated_samples() {
-		peniko_stops.push(peniko::ColorStop {
-			offset: position as f32,
-			color: peniko::color::DynamicColor::from_alpha_color(SRGBA8::from(color).to_peniko_color()),
-		});
-	}
+	let peniko_stops = peniko_color_stops(stops);
 
 	// The unit gradient is placed by the desheared frame so a non-uniform transform produces the intended ellipse
 	let (start, end, gradient_to_device) = (DVec2::ZERO, DVec2::X, gradient_placement(multiplied_transform * gradient_transform, gradient_type));
@@ -2403,13 +2422,7 @@ fn render_gradient_vello<S: LaneSource<Element = Gradient>>(source: &S, scene: &
 		let blend_mode = blend_mode_attr.to_peniko();
 		let opacity = (opacity_attr * if render_params.for_mask { 1. } else { opacity_fill_attr }) as f32;
 
-		let mut stops: peniko::ColorStops = peniko::ColorStops::new();
-		for (position, color, _) in gradient.interpolated_samples() {
-			stops.push(peniko::ColorStop {
-				offset: position as f32,
-				color: peniko::color::DynamicColor::from_alpha_color(SRGBA8::from(color).to_peniko_color()),
-			})
-		}
+		let stops = peniko_color_stops(gradient);
 
 		let extend = match spread_method {
 			GradientSpreadMethod::Pad => peniko::Extend::Pad,