iOS Colors Incorrect When Saving Animated Gif

Question

I am having this very strange issue. I am creating animated gifs from UIImages and most of the time they come out correct. However when I start to get into larger size images my

Answer 1

It seems that turning off the global color map fixes the problem:

let loopingProperty: [String: AnyObject] = [
    kCGImagePropertyGIFLoopCount as String: 0 as NSNumber,
    kCGImagePropertyGIFHasGlobalColorMap as String: false as NSNumber
]

Note that unlike PNGs, GIFs can use only a 256 color map, without transparency. For animated GIFs there can be either a global or a per-frame color map.

Unfortunately, Core Graphics does not allow us to work with color maps directly, therefore there is some automatic color conversion when the GIF is encoded.

It seems that turning off the global color map is all what is needed. Also setting up color map explicitly for every frame using kCGImagePropertyGIFImageColorMap would probably work too.

Since this seems not to work reliably, let's create our own color map for every frame:

struct Color : Hashable {
    let red: UInt8
    let green: UInt8
    let blue: UInt8

    var hashValue: Int {
        return Int(red) + Int(green) + Int(blue)
    }

    public static func ==(lhs: Color, rhs: Color) -> Bool {
        return [lhs.red, lhs.green, lhs.blue] == [rhs.red, rhs.green, rhs.blue]
    }
}

struct ColorMap {
    var colors = Set()

    var exported: Data {
        let data = Array(colors)
            .map { [$0.red, $0.green, $0.blue] }
            .joined()

        return Data(bytes: Array(data))
    }
}

Now let's update our methods:

func getScaledImages(_ scale: Int) -> [(CGImage, ColorMap)] {
    var sourceImages = [UIImage]()
    var result: [(CGImage, ColorMap)] = []

...

    var colorMap = ColorMap()
    let pixelData = imageRef.dataProvider!.data
    let rawData: UnsafePointer = CFDataGetBytePtr(pixelData)

    for y in 0 ..< imageRef.height{
        for _ in 0 ..< scale {
            for x in 0 ..< imageRef.width{
                 let offset = y * imageRef.width * 4 + x * 4

                 let color = Color(red: rawData[offset], green: rawData[offset + 1], blue: rawData[offset + 2])
                 colorMap.colors.insert(color)

                 for _ in 0 ..< scale {
                     pixelPointer[byteIndex] = rawData[offset]
                     pixelPointer[byteIndex+1] = rawData[offset+1]
                     pixelPointer[byteIndex+2] = rawData[offset+2]
                     pixelPointer[byteIndex+3] = rawData[offset+3]

                     byteIndex += 4
                }
            }
        }
    }

    let cgImage = context.makeImage()!
    result.append((cgImage, colorMap))

and

func createAnimatedGifFromImages(_ images: [(CGImage, ColorMap)]) -> URL {

...

    for (image, colorMap) in images {
        let frameProperties: [String: AnyObject] = [
            String(kCGImagePropertyGIFDelayTime): 0.2 as NSNumber,
            String(kCGImagePropertyGIFImageColorMap): colorMap.exported as NSData
        ]

        let properties: [String: AnyObject] = [
            String(kCGImagePropertyGIFDictionary): frameProperties as AnyObject
        ];

        CGImageDestinationAddImage(destination, image, properties as CFDictionary);
    }

Of course, this will work only if the number of colors is less than 256. I would really recommend a custom GIF library that can handle the color conversion correctly.