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Clean up prev experiment.

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- Only need 4 (not 5) samples
- Document some of the math
- Use round of CTM to better match 'brute-force' version

Change-Id: Ibcc8e8eabc10158da8eecaba303385ed79218126
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/379847
Reviewed-by: Mike Klein <mtklein@google.com>
This commit is contained in:
Mike Reed 2021-03-05 10:55:24 -05:00
parent a42ed48e84
commit 319565ac6e
1 changed files with 21 additions and 14 deletions
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35
demos.skia.org/demos/up_scaling/index.html
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@ -47,27 +47,34 @@ function init() {
uniform float sharp; // slope of the lerp section of the kernel (steeper == sharper)
float2 sharpen(float2 w) {
return saturate(sharp * (w - 0.5) + 0.5);
// we think of sharp as a slope on a shifted line
// y = sharp * (w - 0.5) + 0.5
// Rewrite with mix needed for some GPUs to be correct
return saturate(mix(float2(0.5), w, sharp));
}
bool nearly_center(float2 p) {
float tolerance = 0.01; // not sure what to use here
float tolerance = 1/255.0;
p = abs(fract(p) - 0.5);
return p.x < tolerance && p.y < tolerance;
}
half4 main(float2 p) {
float h = 0.5;
if (nearly_center(p)) {
return sample(image, p);
} else {
half4 pa = sample(image, float2(p.x-0.5, p.y-0.5));
half4 pb = sample(image, float2(p.x+0.5, p.y-0.5));
half4 pc = sample(image, float2(p.x-0.5, p.y+0.5));
half4 pd = sample(image, float2(p.x+0.5, p.y+0.5));
float2 w = sharpen(fract(p + 0.5));
return mix(mix(pa, pb, w.x), mix(pc, pd, w.x), w.y);
}
// p+1/2, p-1/2 can be numerically unstable when near the center, so we
// detect that case, and just sample at our center.
float h = nearly_center(p) ? 0.0 : 0.5;
// Manual bilerp logic
half4 pa = sample(image, float2(p.x-h, p.y-h));
half4 pb = sample(image, float2(p.x+h, p.y-h));
half4 pc = sample(image, float2(p.x-h, p.y+h));
half4 pd = sample(image, float2(p.x+h, p.y+h));
// Now 'sharpen' the weighting. This is the magic sauce where we different
// from a normal bilerp
float2 w = sharpen(fract(p + 0.5));
return mix(mix(pa, pb, w.x),
mix(pc, pd, w.x), w.y);
}
`;
const effect = CanvasKit.RuntimeEffect.Make(prog);
@ -111,7 +118,7 @@ function init() {
// Draw the prog version.
canvas.save();
canvas.scale(scale, 1.0);
shader_paint.setShader(effect.makeShaderWithChildren([scale], true, [imageShader], null));
shader_paint.setShader(effect.makeShaderWithChildren([Math.round(scale)], true, [imageShader], null));
canvas.drawRect([0, 0, size, 100], shader_paint);
canvas.restore();