kernel4: assume colors in alpha-premultiplied sRGB format

See http://ssp.impulsetrain.com/gamma-premult.html for a description of
the format.

Pre-multiplied alpha only matters for translucent objects; draw a few
such shapes in the test render.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

Author: eliasnaur

piet-gpu/shader/kernel4.comp

@@ -207,17 +207,19 @@ void main() {
             break;
         case Cmd_Color:
             CmdColor color = Cmd_Color_read(cmd_alloc, cmd_ref);
-            vec4 fg_rgba = unpacksRGB(color.rgba_color);
+            vec4 fg = unpacksRGB(color.rgba_color);
             for (uint k = 0; k < CHUNK; k++) {
-                rgb[k] = mix(rgb[k], fg_rgba.rgb, area[k] * fg_rgba.a);
+                vec4 fg_k = fg * area[k];
+                rgb[k] = rgb[k] * (1.0 - fg_k.a) + fg_k.rgb;
             }
             cmd_ref.offset += 4 + CmdColor_size;
             break;
         case Cmd_Image:
             CmdImage fill_img = Cmd_Image_read(cmd_alloc, cmd_ref);
-            vec4 rgba[CHUNK] = fillImage(xy_uint, fill_img);
+            vec4 img[CHUNK] = fillImage(xy_uint, fill_img);
             for (uint k = 0; k < CHUNK; k++) {
-                rgb[k] = mix(rgb[k], rgba[k].rgb, area[k] * rgba[k].a);
+                vec4 fg_k = img[k] * area[k];
+                rgb[k] = rgb[k] * (1.0 - fg_k.a) + fg_k.rgb;
             }
             cmd_ref.offset += 4 + CmdImage_size;
             break;

piet-gpu/src/lib.rs

@@ -7,7 +7,7 @@ pub use render_ctx::PietGpuRenderContext;
 
 use rand::{Rng, RngCore};
 
-use piet::kurbo::{BezPath, Circle, Point, Vec2};
+use piet::kurbo::{BezPath, Circle, Point, Shape, Vec2};
 use piet::{Color, ImageFormat, RenderContext};
 
 use piet_gpu_types::encoder::Encode;
@@ -74,6 +74,7 @@ pub fn render_scene(rc: &mut impl RenderContext) {
     );
     //render_cardioid(rc);
     render_clip_test(rc);
+    render_alpha_test(rc);
     //render_tiger(rc);
 }
 
@@ -124,6 +125,33 @@ fn render_clip_test(rc: &mut impl RenderContext) {
     }
 }
 
+#[allow(unused)]
+fn render_alpha_test(rc: &mut impl RenderContext) {
+    // Alpha compositing tests. kernel4 expects colors encoded in alpha-premultiplied sRGB:
+    //
+    // [α,sRGB(α⋅R),sRGB(α⋅G),sRGB(α⋅B)]
+    //
+    // See also http://ssp.impulsetrain.com/gamma-premult.html.
+    rc.fill(diamond(Point::new(1024.0, 100.0)), &Color::Rgba32(0xff0000ff));
+    rc.fill(diamond(Point::new(1024.0, 125.0)), &Color::Rgba32(0x00ba0080));
+    rc.save();
+    rc.clip(diamond(Point::new(1024.0, 150.0)));
+    rc.fill(diamond(Point::new(1024.0, 175.0)), &Color::Rgba32(0x0000ba80));
+    rc.restore();
+}
+
+fn diamond(origin: Point) -> impl Shape {
+    let mut path = BezPath::new();
+    const SIZE: f64 = 50.0;
+    path.move_to((origin.x, origin.y - SIZE));
+    path.line_to((origin.x + SIZE, origin.y));
+    path.line_to((origin.x, origin.y + SIZE));
+    path.line_to((origin.x - SIZE, origin.y));
+    path.close_path();
+    return path;
+}
+
+#[allow(unused)]
 fn render_tiger(rc: &mut impl RenderContext) {
     let xml_str = std::str::from_utf8(include_bytes!("../Ghostscript_Tiger.svg")).unwrap();
     let start = std::time::Instant::now();