Changing RGB color image to Grayscale image using Objective C

Question

I was developing a application that changes color image to gray image. However, some how the picture comes out wrong. I dont know what is wrong with the code. maybe the parameter that i put in is wrong please help.

 UIImage *c = [UIImage imageNamed:@"downRed.png"];
 CGImageRef cRef = CGImageRetain(c.CGImage);

 NSData* pixelData = (NSData*) CGDataProviderCopyData(CGImageGetDataProvider(cRef));

 size_t w = CGImageGetWidth(cRef);
 size_t h = CGImageGetHeight(cRef);

 unsigned char* pixelBytes = (unsigned char *)[pixelData bytes];

 unsigned char* greyPixelData = (unsigned char*) malloc(w*h);

 for (int y = 0; y < h; y++) {
 for(int x = 0; x < w; x++){

 int iter = 4*(w*y+x);
 int red = pixe lBytes[iter];
 int green = pixelBytes[iter+1];
 int blue = pixelBytes[iter+2];
 greyPixelData[w*y+x] = (unsigned char)(red*0.3 + green*0.59+ blue*0.11);
     int value = greyPixelData[w*y+x];

 }
 }

 CFDataRef imgData = CFDataCreate(NULL, greyPixelData, w*h);


 CGDataProviderRef imgDataProvider = CGDataProviderCreateWithCFData(imgData);


 size_t width = CGImageGetWidth(cRef);
 size_t height = CGImageGetHeight(cRef);
 size_t bitsPerComponent = 8;
 size_t bitsPerPixel = 8;
 size_t bytesPerRow = CGImageGetWidth(cRef);
 CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
 CGBitmapInfo info = kCGImageAlphaNone;
 CGFloat *decode = NULL;
 BOOL shouldInteroplate = NO;
 CGColorRenderingIntent intent = kCGRenderingIntentDefault;
  CGDataProviderRelease(imgDataProvider);

 CGImageRef throughCGImage = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpace, info, imgDataProvider, decode, shouldInteroplate, intent);


 UIImage* newImage = [UIImage imageWithCGImage:throughCGImage];

CGImageRelease(throughCGImage);
 newImageView.image = newImage;

Answer 1

If someone wants to gray scale images that contain transparent parts the solution offered by digipeople will have a black mask around it. Use this instead to leave the transparent as it is.

- (UIImage *)imageToGreyImage:(UIImage *)image {
    // Create image rectangle with current image width/height
    CGFloat actualWidth = image.size.width;
    CGFloat actualHeight = image.size.height;

    CGRect imageRect = CGRectMake(0, 0, actualWidth, actualHeight);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();

    CGContextRef context = CGBitmapContextCreate(nil, actualWidth, actualHeight, 8, 0, colorSpace, kCGImageAlphaNone);
    CGContextDrawImage(context, imageRect, [image CGImage]);

    CGImageRef grayImage = CGBitmapContextCreateImage(context);
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(context);

    context = CGBitmapContextCreate(nil, actualWidth, actualHeight, 8, 0, nil, kCGImageAlphaOnly);
    CGContextDrawImage(context, imageRect, [image CGImage]);
    CGImageRef mask = CGBitmapContextCreateImage(context);
    CGContextRelease(context);

    UIImage *grayScaleImage = [UIImage imageWithCGImage:CGImageCreateWithMask(grayImage, mask) scale:image.scale orientation:image.imageOrientation];
    CGImageRelease(grayImage);
    CGImageRelease(mask);

    // Return the new grayscale image
    return grayScaleImage;
}