Merging two images as if they are overlayed..... Taken from: http://forum.openframeworks.cc/index.php?topic=4431.0 Some small bugs in the original code have been fixed. This assumes both images are the same size and each pixel has RGBA values

gistfile1.txt

// a and b are both RGBA images
int totalPixels = a.width * a.height;
unsigned char * aPixels = a.getPixels();
unsigned char * bPixels = b.getPixels();

for(int i=0; i<totalPixels; i++) {
	int base = i*4;
	
	// deal with r,g,b
	float bAlpha = float(bPixels[base+3])/256.0f;
	for (int j=0; j<3; j++ )
	aPixels[base+j] = (aPixels[base+j]*(1.0f-bAlpha) + bPixels[base+j]*bAlpha);
	
	// full alpha (no transparency)
	aPixels[base+3] = 255;
}