Attempt to implement faster comparison algorithm

compare.glsl

@@ -1,13 +1,48 @@
-// Compute the pixelwise diff between two textures.
-// Computes the diff for each color chanel separately (r, g, b).
+// Compute the pixelwise diff between two textures, one row at a time.
 // Assumes both images are not transparent.
+// Warning: Only works on rows <= ((2**8)**4)/255 = 16843009
 
 // try isampler2D, ivec4 etc.? Probably not
 uniform vec2 size;
+uniform bool horizontal;
 // uniform vec2 offs;
 uniform sampler2D base;
 uniform sampler2D curr;
 
+void main(void)
+{
+	// calculate total diff of row in either direction
+	float total = 0.0;
+	
+	if (horizontal) {
+		float posy = gl_FragCoord.y/size.y;
+		for (float x=0.0; x<size.x; x+=1.0) {
+			vec4 color     = texture2D(curr, vec2(x,   posy));
+			vec4 basecolor = texture2D(base, vec2(x, 1.0-posy));
+			color.rgb = abs(color.rgb - basecolor.rgb);
+			total = total + color.r + color.g + color.b;
+		}
+	} else {
+		float posx = gl_FragCoord.x/size.x;
+		for (float y=0.0; y<size.y; y+=1.0) {
+			vec4 color     = texture2D(curr, vec2(posx,   y));
+			vec4 basecolor = texture2D(base, vec2(posx, 1.0-y));
+			color.rgb = abs(color.rgb - basecolor.rgb);
+			total = total + color.r + color.g + color.b;
+		}
+	}
+	
+	// encode to rgba channels in current pixel
+	vec4 color;
+	color.r = mod(total, 256.0/255.0); total = (total - color.r) / (256.0/255.0);
+	color.g = mod(total, 256.0/255.0); total = (total - color.g) / (256.0/255.0);
+	color.b = mod(total, 256.0/255.0); total = (total - color.b) / (256.0/255.0);
+	color.a = mod(total, 256.0/255.0);
+	
+	gl_FragColor = color;
+}
+
+/*
 void main(void)
 {
 	vec2 pos = gl_FragCoord.xy/size;
@@ -17,3 +52,4 @@ void main(void)
 	color.rgb = abs(color.rgb - basecolor.rgb);
 	gl_FragColor = color;
 }
+*/