Change color of some specific pixels in UIImage

Question

I have a simple UIImageView with some image of person. Now I want to change color of some of the pixels based on their location or some frame value. How this can be done?

Any help...

Answer 1

For long-term implementatio you should take a look at Core Image Framework tutorial. For one-time case you can refer to already existing answer at iPhone : How to change color of particular pixel of a UIImage? I've found nice non-ARC solution that is working for changing picture color within entire frame, but you can try to adopt it to be applied only to certain pixel:

- (void) grayscale:(UIImage*) image {
    CGContextRef ctx; 
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = malloc(height * width * 4);
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                                 bitsPerComponent, bytesPerRow, colorSpace,
                                                 kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGColorSpaceRelease(colorSpace);

    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGContextRelease(context);

    // Now your rawData contains the image data in the RGBA8888 pixel format.
    int byteIndex = (bytesPerRow * 0) + 0 * bytesPerPixel;
    for (int ii = 0 ; ii < width * height ; ++ii)
    {
       // Get color values to construct a UIColor
          CGFloat red   = (rawData[byteIndex]     * 1.0) / 255.0;
        CGFloat green = (rawData[byteIndex + 1] * 1.0) / 255.0;
        CGFloat blue  = (rawData[byteIndex + 2] * 1.0) / 255.0;
        CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;

        rawData[byteIndex] = (char) (red);
        rawData[byteIndex+1] = (char) (green);
        rawData[byteIndex+2] = (char) (blue);

        byteIndex += 4;
    }

    ctx = CGBitmapContextCreate(rawData,  
                                CGImageGetWidth( imageRef ),  
                                CGImageGetHeight( imageRef ),  
                                8,  
                                CGImageGetBytesPerRow( imageRef ),  
                                CGImageGetColorSpace( imageRef ),  
                                kCGImageAlphaPremultipliedLast ); 

    imageRef = CGBitmapContextCreateImage (ctx);  
    UIImage* rawImage = [UIImage imageWithCGImage:imageRef];  

    CGContextRelease(ctx);  

    self.workingImage = rawImage;  
    [self.imageView setImage:self.workingImage];

    free(rawData);

}

Source: http://brandontreb.com/image-manipulation-retrieving-and-updating-pixel-values-for-a-uiimage