examples/shader: bug fix: wrong usages of position

Closes #2638
Updates #1431

examples/shader/default.go

@@ -21,6 +21,7 @@ var Cursor vec2
 var ScreenSize vec2
 
 func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	// Note that the value of position.xy is indeterministic as the destination image is somewhere on a texture atlas.
 	pos := position.xy/imageDstTextureSize() + Cursor/ScreenSize/4
 	clr := 0.0
 	clr += sin(pos.x*cos(Time/15)*80) + cos(pos.y*cos(Time/15)*10)

examples/shader/lighting.go

@@ -21,8 +21,12 @@ var Cursor vec2
 var ScreenSize vec2
 
 func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	srcOrigin, srcSize := imageSrcRegionOnTexture()
+	pos := (texCoord - srcOrigin) / srcSize
+	pos *= ScreenSize
+
 	lightpos := vec3(Cursor, 50)
-	lightdir := normalize(lightpos - position.xyz)
+	lightdir := normalize(lightpos - vec3(pos, 0))
 	normal := normalize(imageSrc1UnsafeAt(texCoord) - 0.5)
 	const ambient = 0.25
 	diffuse := 0.75 * max(0.0, dot(normal.xyz, lightdir))

examples/shader/radialblur.go

@@ -21,7 +21,11 @@ var Cursor vec2
 var ScreenSize vec2
 
 func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
-	dir := normalize(position.xy - Cursor)
+	srcOrigin, srcSize := imageSrcRegionOnTexture()
+	pos := (texCoord - srcOrigin) / srcSize
+	pos *= ScreenSize
+
+	dir := normalize(pos - Cursor)
 	clr := imageSrc2UnsafeAt(texCoord)
 
 	samples := [...]float{
@@ -34,7 +38,7 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	}
 	sum /= 10 + 1
 
-	dist := distance(position.xy, Cursor)
+	dist := distance(pos, Cursor)
 	t := clamp(dist/256, 0, 1)
 	return mix(clr, sum, t)
 }

examples/shader/water.go

@@ -21,22 +21,22 @@ 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
+	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)