improve description

sample/alphaToCoverage/meta.ts

@@ -1,23 +1,20 @@
 export default {
   name: 'Alpha-to-Coverage',
   description: `
-Visualizes how alpha-to-coverage translates alpha values into sample
-coverage on your device. It draws two full-screen quads into a 4-sample
-texture, with the configured color and alpha values. Then, it visualizes the
-contents of that 4-sample texture: the circles show the 4 samples of each
-pixel; the background checkerboard shows where the pixels are.
-
-The algorithm that converts alpha to a coverage sample mask varies per
-device. This results in different proportions between the black background,
-the first draw, and the second draw.
+Alpha-to-coverage is an alternative to alpha testing and alpha blending. See:
+<https://bgolus.medium.com/anti-aliased-alpha-test-the-esoteric-alpha-to-coverage-8b177335ae4f>
 
-Examples: Some devices use 1x1 patterns, others 2x2, others 4x4. Not all devices
-guarantee that once a sample "pops in", it will stay there at higher alpha
-values. Some devices "move" samples around at certain alpha thresholds even
-without increasing the total sample count.
+This sample visualizes how alpha-to-coverage translates alpha values into sample
+coverage on your device. It draws two full-screen quads into a 4-sample
+texture, each with the configured color and alpha value. Then, it visualizes the
+contents of the resulting 4-sample texture: the circles show the 4 samples of
+each texel; the background shows the "resolved" results (average of 4 samples).
 
-More info on alpha-to-coverage:
-<https://bgolus.medium.com/anti-aliased-alpha-test-the-esoteric-alpha-to-coverage-8b177335ae4f>
+The algorithm that converts alpha to a coverage sample mask varies per device.
+This results in different average "blending" proportions between the black
+background, the first draw, and the second draw.
+Device differences include different tile sizes (e.g. 1x1, 2x2, or 4x4),
+"moving" samples (or not) around with in the tile as alpha increases, etc.
 `,
   filename: __DIRNAME__,
   sources: [

sample/alphaToCoverage/showMultisampleTexture.wgsl

@@ -50,7 +50,7 @@ fn fmain(vary: Varying) -> @location(0) vec4f {
   let xyFrac = xy % vec2f(1, 1);
 
   // Show the visualization only if the resolution is large enough to see it
-  if (dpdx(xy.x) < kGridEdgeHalfWidth) & (dpdy(xy.y) < kGridEdgeHalfWidth) {
+  if (dpdx(xy.x) < kGridEdgeHalfWidth * 2) & (dpdy(xy.y) < kGridEdgeHalfWidth * 2) {
     // Check if we're close to a sample; if so, visualize the sample value
     for (var sampleIndex = 0; sampleIndex < kSampleCount; sampleIndex += 1) {
       let distanceFromSample = distance(xyFrac, kSamplePositions[sampleIndex]);