wgpu: Slight opimitsation to blur filter by not needing to calculate full_width per fragment

render/wgpu/shaders/filter/blur.wgsl

@@ -5,10 +5,11 @@ struct Filter {
     dir_x: f32,
     dir_y: f32,
 
-    // Size of the blur kernel
-    size: f32,
+    // Full size of the blur kernel (from left to right, ie)
+    full_size: f32,
 
-    _padding: f32,
+    // Half size of the blur kernel (from center to right, ie) - without center piece
+    half_size: f32,
 }
 
 @group(0) @binding(0) var texture: texture_2d<f32>;
@@ -25,11 +26,10 @@ fn main_fragment(in: filter::VertexOutput) -> @location(0) vec4<f32> {
     let direction = vec2<f32>(filter_args.dir_x, filter_args.dir_y);
     var color = vec4<f32>();
 
-    let num_samples = 2.0 * filter_args.size + 1.0;
-    let weight = 1.0 / num_samples;
+    let weight = 1.0 / filter_args.full_size;
 
-    for (var i = 0.0; i < num_samples; i += 1.0) {
-        color += textureSample(texture, texture_sampler, in.uv + direction * (i - filter_args.size)) * weight;
+    for (var i = 0.0; i < filter_args.full_size; i += 1.0) {
+        color += textureSample(texture, texture_sampler, in.uv + direction * (i - filter_args.half_size)) * weight;
     }
 
     return color;

render/wgpu/src/filters/blur.rs

@@ -21,8 +21,8 @@ const PASS_SCALES: [f32; 15] = [
 #[derive(Copy, Clone, Debug, Pod, Zeroable, PartialEq)]
 struct BlurUniform {
     direction: [f32; 2],
-    size: f32,
-    _padding: f32,
+    full_size: f32,
+    half_size: f32,
 }
 
 pub struct BlurFilter {
@@ -190,12 +190,14 @@ impl BlurFilter {
                 } else {
                     filter.blur_y.to_f32()
                 };
-                // blur_x/y is the total kernel width; change it to just half excluding center
-                let strength = ((strength.min(255.0) * pass_scale - 1.0) / 2.0).floor();
-                if strength <= 0.0 {
-                    // A strength of 0 is a noop
+                // Full width of the kernel (left edge to right edge)
+                let full_size = (strength.min(255.0) * pass_scale).round();
+                if full_size <= 1.0 {
+                    // A width of 1 or less is a noop (it'd just sample itself and nothing else)
                     continue;
                 }
+                // Radius of the kernel (edge of the center, to right edge), excludes center pixel
+                let half_size = (full_size - 1.0) / 2.0;
 
                 let (previous_view, previous_vertices, previous_width, previous_height) = if first {
                     first = false;
@@ -224,8 +226,8 @@ impl BlurFilter {
                                 } else {
                                     [0.0, 1.0 / previous_height]
                                 },
-                                size: strength,
-                                _padding: 0.0,
+                                full_size,
+                                half_size,
                             }]),
                             usage: wgpu::BufferUsages::UNIFORM,
                         });