wgpu: Implement Screen blend mode as a trivial blend

render/wgpu/shaders/blend/screen.wgsl

@@ -1,48 +0,0 @@
-#import common
-
-struct VertexOutput {
-    @builtin(position) position: vec4<f32>,
-    @location(0) uv: vec2<f32>,
-};
-
-#if use_push_constants == true
-    var<push_constant> transforms: common::Transforms;
-    @group(1) @binding(0) var parent_texture: texture_2d<f32>;
-    @group(1) @binding(1) var current_texture: texture_2d<f32>;
-    @group(1) @binding(2) var texture_sampler: sampler;
-#else
-    @group(1) @binding(0) var<uniform> transforms: common::Transforms;
-    @group(2) @binding(0) var parent_texture: texture_2d<f32>;
-    @group(2) @binding(1) var current_texture: texture_2d<f32>;
-    @group(2) @binding(2) var texture_sampler: sampler;
-#endif
-
-@vertex
-fn main_vertex(in: common::VertexInput) -> VertexOutput {
-    let pos = common::globals.view_matrix * transforms.world_matrix * vec4<f32>(in.position.x, in.position.y, 1.0, 1.0);
-    let uv = vec2<f32>((pos.x + 1.0) / 2.0, -((pos.y - 1.0) / 2.0));
-    return VertexOutput(pos, uv);
-}
-
-fn blend_func(src: vec3<f32>, dst: vec3<f32>) -> vec3<f32> {
-    return (dst + src) - (dst * src);
-}
-
-@fragment
-fn main_fragment(in: VertexOutput) -> @location(0) vec4<f32> {
-    // dst is the parent pixel we're blending onto
-    var dst: vec4<f32> = textureSample(parent_texture, texture_sampler, in.uv);
-    // src is the pixel that we want to apply
-    var src: vec4<f32> = textureSample(current_texture, texture_sampler, in.uv);
-
-    if (src.a > 0.0) {
-        return vec4<f32>(src.rgb * (1.0 - dst.a) + dst.rgb * (1.0 - src.a) + src.a * dst.a * blend_func(src.rgb / src.a, dst.rgb / dst.a), src.a + dst.a * (1.0 - src.a));
-    } else {
-        if (true) {
-            // This needs to be in a branch because... reasons. Bug in naga.
-            // https://github.com/gfx-rs/naga/issues/2168
-            discard;
-        }
-        return dst;
-    }
-}

render/wgpu/src/blend.rs

@@ -5,7 +5,6 @@ use swf::BlendMode;
 #[derive(Enum, Debug, Copy, Clone)]
 pub enum ComplexBlend {
     Multiply,   // Can't be trivial, 0 alpha is special case
-    Screen,     // Can't be trivial. (dst + src) - (dst * src)
     Lighten,    // Might be trivial but I can't reproduce the right colors
     Darken,     // Might be trivial but I can't reproduce the right colors
     Difference, // Can't be trivial, relies on abs operation
@@ -31,7 +30,7 @@ impl BlendType {
             BlendMode::Normal => BlendType::Trivial(TrivialBlend::Normal),
             BlendMode::Layer => BlendType::Trivial(TrivialBlend::Normal),
             BlendMode::Multiply => BlendType::Complex(ComplexBlend::Multiply),
-            BlendMode::Screen => BlendType::Complex(ComplexBlend::Screen),
+            BlendMode::Screen => BlendType::Trivial(TrivialBlend::Screen),
             BlendMode::Lighten => BlendType::Complex(ComplexBlend::Lighten),
             BlendMode::Darken => BlendType::Complex(ComplexBlend::Darken),
             BlendMode::Difference => BlendType::Complex(ComplexBlend::Difference),
@@ -55,6 +54,7 @@ pub enum TrivialBlend {
     Normal,
     Add,
     Subtract,
+    Screen,
 }
 
 impl TrivialBlend {
@@ -70,6 +70,14 @@ impl TrivialBlend {
                 },
                 alpha: wgpu::BlendComponent::OVER,
             },
+            TrivialBlend::Screen => wgpu::BlendState {
+                color: wgpu::BlendComponent {
+                    src_factor: wgpu::BlendFactor::One,
+                    dst_factor: wgpu::BlendFactor::OneMinusSrc,
+                    operation: wgpu::BlendOperation::Add,
+                },
+                alpha: wgpu::BlendComponent::OVER,
+            },
             TrivialBlend::Subtract => wgpu::BlendState {
                 color: wgpu::BlendComponent {
                     src_factor: wgpu::BlendFactor::One,

render/wgpu/src/shaders.rs

@@ -61,7 +61,6 @@ impl Shaders {
 
         let blend_shaders = enum_map! {
             ComplexBlend::Multiply => make_shader(device, &mut composer, &shader_defs, "blend/multiply.wgsl", include_str!("../shaders/blend/multiply.wgsl")),
-            ComplexBlend::Screen => make_shader(device, &mut composer, &shader_defs, "blend/screen.wgsl", include_str!("../shaders/blend/screen.wgsl")),
             ComplexBlend::Lighten => make_shader(device, &mut composer, &shader_defs, "blend/lighten.wgsl", include_str!("../shaders/blend/lighten.wgsl")),
             ComplexBlend::Darken => make_shader(device, &mut composer, &shader_defs, "blend/darken.wgsl", include_str!("../shaders/blend/darken.wgsl")),
             ComplexBlend::Difference => make_shader(device, &mut composer, &shader_defs, "blend/difference.wgsl", include_str!("../shaders/blend/difference.wgsl")),