internal/builtinshader: refactoring

internal/builtinshader/shader.go

@@ -173,17 +173,15 @@ package main
 func Fragment(dstPos vec4, srcPos vec2) vec4 {
 	// Blend source colors in a square region, which size is 1/scale.
 	scale := imageDstSize()/imageSrc0Size()
-	pos := srcPos
-	p0 := pos - 1/2.0/scale
-	p1 := pos + 1/2.0/scale
+	p0 := srcPos - 1/2.0/scale
+	p1 := srcPos + 1/2.0/scale
 
 	// Texels must be in the source rect, so it is not necessary to check.
 	c0 := imageSrc0UnsafeAt(p0)
 	c1 := imageSrc0UnsafeAt(vec2(p1.x, p0.y))
 	c2 := imageSrc0UnsafeAt(vec2(p0.x, p1.y))
 	c3 := imageSrc0UnsafeAt(p1)
 
-	// p is the p1 value in one pixel assuming that the pixel's upper-left is (0, 0) and the lower-right is (1, 1).
 	rate := clamp(fract(p1)*scale, 0, 1)
 	return mix(mix(c0, c1, rate.x), mix(c2, c3, rate.x), rate.y)
 }