Alpha Mask Extraction process Objective C implementation - objective-c

Question

Do you have an Objective C implementation equivalent to ImageMagick's command :
convert -alpha Extract -type optimize -strip -quality 60 +dither Source.png Alpha.jpg
I was not able to find any solution right now.
I m looking for an AlphaExtractor snippet that would extract the alpha from a png and save it to JPG Grayscale
The mask is created using the code snippet :
CGImageRef createMaskWithImage(CGImageRef image)
{
int maskWidth = CGImageGetWidth(image);
int maskHeight = CGImageGetHeight(image);
// round bytesPerRow to the nearest 16 bytes, for performance's sake
int bytesPerRow = (maskWidth + 15) & 0xfffffff0;
int bufferSize = bytesPerRow * maskHeight;
// we use CFData instead of malloc(), because the memory has to stick around
// for the lifetime of the mask. if we used malloc(), we'd have to
// tell the CGDataProvider how to dispose of the memory when done. using
// CFData is just easier and cleaner.
CFMutableDataRef dataBuffer = CFDataCreateMutable(kCFAllocatorDefault, 0);
CFDataSetLength(dataBuffer, bufferSize);
// the data will be 8 bits per pixel, no alpha
CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericGray);//CGColorSpaceCreateDeviceGray();
CGContextRef ctx = CGBitmapContextCreate(CFDataGetMutableBytePtr(dataBuffer),
maskWidth, maskHeight,
8, bytesPerRow, colorSpace, kCGImageAlphaNone);
// drawing into this context will draw into the dataBuffer.
CGContextDrawImage(ctx, CGRectMake(0, 0, maskWidth, maskHeight), image);
CGContextRelease(ctx);
// now make a mask from the data.
CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData(dataBuffer);
CGImageRef mask = CGImageMaskCreate(maskWidth, maskHeight, 8, 8, bytesPerRow,
dataProvider, NULL, FALSE);
CGDataProviderRelease(dataProvider);
CGColorSpaceRelease(colorSpace);
CFRelease(dataBuffer);
return mask;
}
and saved :
-(void)_saveJPEGImage:(CGImageRef)imageRef path:(NSString *)path {
NSURL *fileURL = [NSURL fileURLWithPath:path];
CFURLRef fileUrlRef=(CFURLRef)fileURL;
CFMutableDictionaryRef mSaveMetaAndOpts = CFDictionaryCreateMutable(nil, 0, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);
CFDictionarySetValue(mSaveMetaAndOpts, kCGImageDestinationLossyCompressionQuality, [NSNumber numberWithFloat:0.7]); // set the compression quality here
CFDictionarySetValue(mSaveMetaAndOpts, kCGImageDestinationBackgroundColor, kCGColorClear);
CGImageDestinationRef dr = CGImageDestinationCreateWithURL (fileUrlRef, kUTTypeJPEG , 1, NULL);
CGImageDestinationAddImage(dr, imageRef, mSaveMetaAndOpts);
CGImageDestinationFinalize(dr);
CFRelease(dr);
}

Answer 1

A really quick an dirty working solution :
Assuming we have a 32bytes raw data (if not the code need to be adapted)
1- we iterate through the bytes by +4 steps and alter the r,g,b components.
CGImageRef ref=CGImageCreateCopy([_imageView image]);
NSData *data = (NSData *)CGDataProviderCopyData(CGImageGetDataProvider(ref));
char *bytes = (char *)[data bytes];
int i;
for( i= 0; i < [data length]; i += 4)
{
int r = i;
int g = i+1;
int b = i+2;
int a = i+3;
bytes[r] = 0;
bytes[g] = 0;
bytes[b] = 0;
bytes[a] = bytes[a];
}
2- We create a new RGBA (32Bit) image reference with the "modified data" :
size_t width = CGImageGetWidth(ref);
size_t height = CGImageGetHeight(ref);
size_t bitsPerComponent = CGImageGetBitsPerComponent(ref);
size_t bitsPerPixel = CGImageGetBitsPerPixel(ref);
size_t bytesPerRow = CGImageGetBytesPerRow(ref);
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(ref);
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL,bytes, [data length], NULL);
CGImageRef newImageRef = CGImageCreate (
width,
height,
bitsPerComponent,
bitsPerPixel,
bytesPerRow,
colorspace,
bitmapInfo,
provider,
NULL,
false,
kCGRenderingIntentDefault
);
3- We save this new 32Bytes image reference to a jpeg file.
The generated JPG will be usable as a mask.
We could do it a cleaner way by creating an 8Bit context and writing the "alpha component" only.

Answer 2

I see two problems:
CGContextDrawImage(ctx, CGRectMake(0, 0, maskWidth, maskHeight), image);
doesn't extract the alpha, it just alpha-composites the image onto a black background.
If the image is black with transparency then an all-black image would be the expected output.
and:
CGImageRef mask = CGImageMaskCreate(maskWidth, maskHeight, 8, 8, bytesPerRow,
dataProvider, NULL, FALSE);
You're treating this mask you create like a real image. If you replace this line with
CGImageRef mask = CGImageCreate(maskWidth, maskHeight, 8, 8, bytesPerRow, colorSpace, 0,
dataProvider, NULL, FALSE, kCGRenderingIntentDefault);
Then you will get a greyscale version of your image (see problem 1)