Question

We are using the following code to generate GIF file from a set of JPEG images, for the setting on doing lossless compression, it doesn’t seem to generate a smaller sized file at all. Are we doing the right thing here?

CGImageDestinationRef imageDestination = CGImageDestinationCreateWithURL((CFURLRef)pathUrl, CFSTR("com.compuserve.gif"), images.count, NULL);

// image/frame level properties
NSDictionary *imageProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                                 [NSNumber numberWithFloat:delayTime], (NSString *)kCGImagePropertyGIFDelayTime,
                                 nil];
NSDictionary *properties = [NSDictionary dictionaryWithObjectsAndKeys:
                            imageProperties, (NSString *)kCGImagePropertyGIFDictionary, 
                            nil];

for (UIImage *image in [images objectEnumerator]) {
    CGImageDestinationAddImage(imageDestination, image.CGImage, (CFDictionaryRef)properties);
}

// gif level properties
NSDictionary *gifProperties = [NSDictionary dictionaryWithObjectsAndKeys:
                               [NSNumber numberWithInt:0], (NSString *)kCGImagePropertyGIFLoopCount,
                               [NSNumber numberWithInt:1.0], kCGImageDestinationLossyCompressionQuality,
                               nil];
properties = [NSDictionary dictionaryWithObjectsAndKeys:
              gifProperties, (NSString *)kCGImagePropertyGIFDictionary,
              nil];
CGImageDestinationSetProperties(imageDestination, (CFDictionaryRef)properties);
CGImageDestinationFinalize(imageDestination);
CFRelease(imageDestination);

Answer 1

The GIF image format IS lossless compression. However you are compressing a (lossy) compressed format. File size may go up.