Implementing Ordered Dithering (24 bit RGB to 3 bit per channel RGB) - image-processing

Question

I'm writing an image editing programme, and I need functionality to dither any arbitrary 24-bit RGB image (I've taken care of loading it with CoreGraphics and such) to an image with 3 bit colour channels, then displaying it. I've set up my matrices and such, but I've not got any results from the code below besides a simple pattern that is applied to the image:
- (CGImageRef) ditherImageTo16Colours:(CGImageRef)image withDitheringMatrixType:(SQUBayerDitheringMatrix) matrix {
if(image == NULL) {
NSLog(#"Image is NULL!");
return NULL;
}
unsigned int imageWidth = CGImageGetWidth(image);
unsigned int imageHeight = CGImageGetHeight(image);
NSLog(#"Image size: %u x %u", imageWidth, imageHeight);
CGContextRef context = CGBitmapContextCreate(NULL,
imageWidth,
imageHeight,
8,
4 * (imageWidth),
CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB),
kCGImageAlphaNoneSkipLast);
CGContextDrawImage(context, CGRectMake(0, 0, imageWidth, imageHeight), image); // draw it
CGImageRelease(image); // get rid of the image, we don't want it anymore.
unsigned char *imageData = CGBitmapContextGetData(context);
unsigned char ditheringModulusType[0x04] = {0x02, 0x03, 0x04, 0x08};
unsigned char ditheringModulus = ditheringModulusType[matrix];
unsigned int red;
unsigned int green;
unsigned int blue;
uint32_t *memoryBuffer;
memoryBuffer = (uint32_t *) malloc((imageHeight * imageWidth) * 4);
unsigned int thresholds[0x03] = {256/8, 256/8, 256/8};
for(int y = 0; y < imageHeight; y++) {
for(int x = 0; x < imageWidth; x++) {
// fetch the colour components, add the dither value to them
red = (imageData[((y * imageWidth) * 4) + (x << 0x02)]);
green = (imageData[((y * imageWidth) * 4) + (x << 0x02) + 1]);
blue = (imageData[((y * imageWidth) * 4) + (x << 0x02) + 2]);
if(red > 36 && red < 238) {
red += SQUBayer117_matrix[x % ditheringModulus][y % ditheringModulus];
} if(green > 36 && green < 238) {
green += SQUBayer117_matrix[x % ditheringModulus][y % ditheringModulus];
} if(blue > 36 && blue < 238) {
blue += SQUBayer117_matrix[x % ditheringModulus][y % ditheringModulus];
}
// memoryBuffer[(y * imageWidth) + x] = (0xFF0000 + ((x >> 0x1) << 0x08) + (y >> 2));
memoryBuffer[(y * imageWidth) + x] = find_closest_palette_colour(((red & 0xFF) << 0x10) | ((green & 0xFF) << 0x08) | (blue & 0xFF));
}
}
//CGContextRelease(context);
context = CGBitmapContextCreate(memoryBuffer,
imageWidth,
imageHeight,
8,
4 * (imageWidth),
CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB),
kCGImageAlphaNoneSkipLast);
NSLog(#"Created context from buffer: %#", context);
CGImageRef result = CGBitmapContextCreateImage(context);
return result;
}
Note that find_closest_palette_colour doesn't do anything besides returning the original colour right now for testing.
I'm trying to implement the example pseudocode from Wikipedia, and I don't really get anything out of that right now.
Anyone got a clue on how to fix this up?

Answer 1

Use the code that I have provided here: https://stackoverflow.com/a/17900812/342646
This code converts the image to a single-channel gray-scale first. If you want the dithering to be done on a three-channel image, you can just split your image into three channels and call the function three times (once per channel).