Can I edit the pixels of the UIImage's property CGImage

Question

UIImage has a read-only property CGImage. I have to read its pixels to a memory block and edit them and then make a new UIImage to replace the old one. I want to know if th

Answer 1

For the more obtuse among us (read: future me) here's some working code based on Itay and Dave R's answers. It starts with a UIImage and ends with a modified UIImage:

    // load image
    UIImage *image      = [UIImage imageNamed:@"test.png"];
    CGImageRef imageRef = image.CGImage;
    NSData *data        = (NSData *)CGDataProviderCopyData(CGImageGetDataProvider(imageRef));
    char *pixels        = (char *)[data bytes];

    // this is where you manipulate the individual pixels
    // assumes a 4 byte pixel consisting of rgb and alpha
    // for PNGs without transparency use i+=3 and remove int a
    for(int i = 0; i < [data length]; i += 4)
    {
        int r = i;
        int g = i+1;
        int b = i+2;
        int a = i+3;

        pixels[r]   = 0; // eg. remove red
        pixels[g]   = pixels[g];
        pixels[b]   = pixels[b];
        pixels[a]   = pixels[a];
    }

    // create a new image from the modified pixel data
    size_t width                    = CGImageGetWidth(imageRef);
    size_t height                   = CGImageGetHeight(imageRef);
    size_t bitsPerComponent         = CGImageGetBitsPerComponent(imageRef);
    size_t bitsPerPixel             = CGImageGetBitsPerPixel(imageRef);
    size_t bytesPerRow              = CGImageGetBytesPerRow(imageRef);

    CGColorSpaceRef colorspace      = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo         = CGImageGetBitmapInfo(imageRef);
    CGDataProviderRef provider      = CGDataProviderCreateWithData(NULL, pixels, [data length], NULL);

    CGImageRef newImageRef = CGImageCreate (
                              width,
                              height,
                              bitsPerComponent,
                              bitsPerPixel,
                              bytesPerRow,
                              colorspace,
                              bitmapInfo,
                              provider,
                              NULL,
                              false,
                              kCGRenderingIntentDefault
                              );
    // the modified image
    UIImage *newImage   = [UIImage imageWithCGImage:newImageRef];

    // cleanup
    free(pixels);
    CGImageRelease(imageRef);
    CGColorSpaceRelease(colorspace);
    CGDataProviderRelease(provider);
    CGImageRelease(newImageRef);