Core Graphics & GIF Color Table

Question

I am trying to limit the number of colors of an animated GIF (created from an array of CGImageRef).

However, I am have difficulty actually setting the custom color table. Does anyone know how to do this with Core Graphics?

I know of kCGImagePropertyGIFImageColorMap. Below is some test code (borrowing heavily from this github gist -- since it's the only instance of kCGImagePropertyGIFImageColorMap Google could find).

NSString *path = [@"~/Desktop/test.png" stringByExpandingTildeInPath];

CGDataProviderRef imgDataProvider = CGDataProviderCreateWithCFData((CFDataRef)[NSData dataWithContentsOfFile:path]);
CGImageRef image = CGImageCreateWithPNGDataProvider(imgDataProvider, NULL, true, kCGRenderingIntentDefault);

const uint8_t colorTable[ 8 ] = { 0, 0, 0, 0, 255, 255, 255 , 255};
NSData* colorTableData = [ NSData dataWithBytes: colorTable length: 8 ];
NSMutableDictionary *gifProps = [NSMutableDictionary dictionary];

[gifProps setObject:colorTableData forKey:(NSString *)kCGImagePropertyGIFImageColorMap];
[gifProps setObject:[NSNumber numberWithBool:NO] forKey:(NSString *)kCGImagePropertyGIFHasGlobalColorMap];

NSDictionary* imgProps = [ NSDictionary dictionaryWithObject: gifProps 
                                                      forKey: (NSString*) kCGImagePropertyGIFDictionary ];

NSURL* destUrl = [NSURL fileURLWithPath:[@"~/Desktop/test.gif" stringByExpandingTildeInPath]];

CGImageDestinationRef dst = CGImageDestinationCreateWithURL( (CFURLRef) destUrl, kUTTypeGIF, 1, NULL );


CGImageDestinationAddImage( dst, image, imgProps );
CGImageDestinationSetProperties(dst, (CFDictionaryRef) imgProps);
CGImageDestinationFinalize( dst );
CFRelease( dst );

This, however, does not produce a black & white image.

Furthermore, I've tried opening a GIF to find the color table information, but that's providing little help.

CGImageSourceRef imageSourceRef = CGImageSourceCreateWithURL((CFURLRef) destUrl, NULL);
NSDictionary *dict = (NSDictionary *)CGImageSourceCopyPropertiesAtIndex(imageSourceRef,0, NULL);
CGImageRef img = CGImageSourceCreateImageAtIndex(imageSourceRef, 0, NULL);

printf("Color space model: %d, indexed=%d, rgb = %d\n",  CGColorSpaceGetModel(CGImageGetColorSpace(img)), kCGColorSpaceModelIndexed,kCGColorSpaceModelRGB);
NSLog(@"%@", dict);

It says the color space is RGB for GIFs. Yet, if I try that code with an indexed PNG, it says the color space is indexed.

Furthermore, all the GIFs I've tried have a image dictionary that looks roughly like the following:

{
ColorModel = RGB;
Depth = 8;
HasAlpha = 1;
PixelHeight = 176;
PixelWidth = 314;
"{GIF}" =     {
    DelayTime = "0.1";
    UnclampedDelayTime = "0.04";
};
}

(If I use CGImageSourceCopyProperties(...), it mentions a global color map, but again no color table is provided.)

Answer 1

I didn't run your code, but from reading it I saw one mistake: The color space in gif is rgb, so your color map should have numcolors*3 items (in stead of *4). IOS doesn't deal gracefully with color maps whose size is not a multiple of 3...

In other words:

const uint8_t colorTable[ 6 ] = { 0, 0, 0, 255, 255 , 255};
NSData* colorTableData = [ NSData dataWithBytes: colorTable length:6];

should do the job.