examples/shader: bug fix: wrong usages of position

Closes #2638
Updates #1431

examples/shader/water.go

@@ -21,22 +21,23 @@ var Cursor vec2
 var ScreenSize vec2
 
 func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
-	srcsize := imageSrcTextureSize()
-	ypx := texCoord.y * srcsize.y
+	srcOrigin, srcSize := imageSrcRegionOnTexture()
+	pos := (texCoord - srcOrigin) / srcSize
+	pos *= ScreenSize
 
-	border := ScreenSize.y*0.6 + 4*cos(Time*3+ypx/10)
-	if position.y < border {
+	border := ScreenSize.y*0.6 + 4*cos(Time*3+pos.y/10)
+	if pos.y < border {
 		return imageSrc2UnsafeAt(texCoord)
 	}
 
-	rorigin, _ := imageSrcRegionOnTexture()
-
-	xoffset := (4 / srcsize.x) * cos(Time*3+ypx/10)
-	yoffset := (20 / srcsize.y) * (1.0 + cos(Time*3+ypx/40))
-	bordertex := border / srcsize.y
+	// TODO: As the texture size can vary, using srcTexSize here seems wrong (#1431).
+	srcTexSize := imageSrcTextureSize()
+	xoffset := (4 / srcTexSize.x) * cos(Time*3+pos.y/10)
+	yoffset := (20 / srcTexSize.y) * (1.0 + cos(Time*3+pos.y/40))
+	bordertex := border / srcTexSize.y
 	clr := imageSrc2At(vec2(
 		texCoord.x+xoffset,
-		-(texCoord.y+yoffset-rorigin.y)+bordertex*2+rorigin.y,
+		-(texCoord.y+yoffset-srcOrigin.y)+bordertex*2+srcOrigin.y,
 	)).rgb
 
 	overlay := vec3(0.5, 1, 1)