I have no idea of your goal, and might be completely off-topic, but why not just using Quartz 2D?
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSetBlendMode(ctx, kCGBlendModeOverlay);
... draw with overlay blending
CGBlendMode
offers via CGContextSetBlendMode
Overlay, Multiply, Screen and much more... :
enum CGBlendMode {
/* Available in Mac OS X 10.4 & later. */
kCGBlendModeNormal,
kCGBlendModeMultiply,
kCGBlendModeScreen,
kCGBlendModeOverlay,
kCGBlendModeDarken,
kCGBlendModeLighten,
kCGBlendModeColorDodge,
kCGBlendModeColorBurn,
kCGBlendModeSoftLight,
kCGBlendModeHardLight,
kCGBlendModeDifference,
kCGBlendModeExclusion,
kCGBlendModeHue,
kCGBlendModeSaturation,
kCGBlendModeColor,
kCGBlendModeLuminosity,
/* Available in Mac OS X 10.5 & later. R, S, and D are, respectively,
premultiplied result, source, and destination colors with alpha; Ra,
Sa, and Da are the alpha components of these colors.
The Porter-Duff "source over" mode is called `kCGBlendModeNormal':
R = S + D*(1 - Sa)
Note that the Porter-Duff "XOR" mode is only titularly related to the
classical bitmap XOR operation (which is unsupported by
CoreGraphics). */
kCGBlendModeClear, /* R = 0 */
kCGBlendModeCopy, /* R = S */
kCGBlendModeSourceIn, /* R = S*Da */
kCGBlendModeSourceOut, /* R = S*(1 - Da) */
kCGBlendModeSourceAtop, /* R = S*Da + D*(1 - Sa) */
kCGBlendModeDestinationOver, /* R = S*(1 - Da) + D */
kCGBlendModeDestinationIn, /* R = D*Sa */
kCGBlendModeDestinationOut, /* R = D*(1 - Sa) */
kCGBlendModeDestinationAtop, /* R = S*(1 - Da) + D*Sa */
kCGBlendModeXOR, /* R = S*(1 - Da) + D*(1 - Sa) */
kCGBlendModePlusDarker, /* R = MAX(0, (1 - D) + (1 - S)) */
kCGBlendModePlusLighter /* R = MIN(1, S + D) */
};
typedef enum CGBlendMode CGBlendMode; /* Available in Mac OS X 10.4 & later. */