UIImage masking gives black background on the iPhone (and not in simulator)

I've got mask images w/o alpha channel, pure black and white with a black polygone on the center sourrounded by white.

I've got png's image that I'd like to mask. the png have got an alpha channel. using the method here below : ok on simulator, but background is black on device ... any idea to fix that ?

- (UIImage*) maskImage:(UIImage *)image withMask:(UIImage *)maskImage {

CGImageRef maskRef = maskImage.CGImage; 

CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                    CGImageGetHeight(maskRef),
                                    CGImageGetBitsPerComponent(maskRef),
                                    CGImageGetBitsPerPixel(maskRef),
                                    C开发者_运维技巧GImageGetBytesPerRow(maskRef),
                                    CGImageGetDataProvider(maskRef), NULL, false);

CGImageRef masked = CGImageCreateWithMask([image CGImage], mask);

//CGImageRelease(mask);
//CGImageRelease(maskRef);
return [UIImage imageWithCGImage:masked];

}

Problem Solved : solution is : the image to be masked MUST be created with an alpha channel. The Alpha channel may not be created from the code.

Same here with the following code

- (UIImage*)maskImage:(UIImage *)image withMask:(UIImage *)maskImage {
CGImageRef maskRef = maskImage.CGImage;
CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                    CGImageGetHeight(maskRef),
                                    CGImageGetBitsPerComponent(maskRef),
                                    CGImageGetBitsPerPixel(maskRef),
                                    CGImageGetBytesPerRow(maskRef),
                                    CGImageGetDataProvider(maskRef), NULL, false);

CGImageRef sourceImage = [image CGImage];
CGImageRef imageWithAlpha = sourceImage;
//add alpha channel for images that don't have one (ie GIF, JPEG, etc...)
//this however has a computational cost
if (CGImageGetAlphaInfo(sourceImage) == kCGImageAlphaNone) { 
    imageWithAlpha = CopyImageAndAddAlphaChannel(sourceImage);
}

CGImageRef masked = CGImageCreateWithMask(imageWithAlpha, mask);
//const float colorMasking[6] = { 100,100, 100, 250,250, 250};
//CGImageRef maskedImageRef = CGImageCreateWithMaskingColors(temp.CGImage, colorMasking); 
CGImageRelease(mask);

//release imageWithAlpha if it was created by CopyImageAndAddAlphaChannel
if (sourceImage != imageWithAlpha) {
    CGImageRelease(imageWithAlpha);
}

UIImage* retImage = [UIImage imageWithCGImage:masked];
CGImageRelease(masked);

return retImage;

}

CGImageRef CopyImageAndAddAlphaChannel(CGImageRef sourceImage) {
CGImageRef retVal = NULL;

size_t width = CGImageGetWidth(sourceImage);
size_t height = CGImageGetHeight(sourceImage);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

CGContextRef offscreenContext = CGBitmapContextCreate(NULL, width, height, 
                                                      8, 0, colorSpace, kCGImageAlphaPremultipliedFirst);

if (offscreenContext != NULL) {
    CGContextDrawImage(offscreenContext, CGRectMake(0, 0, width, height), sourceImage);

    retVal = CGBitmapContextCreateImage(offscreenContext);
    CGContextRelease(offscreenContext);
}

CGColorSpaceRelease(colorSpace);

return retVal;

}

Works perfectly on simulator but not on iPhone device : all images are masked but with black background.