保存动画 Gif 时 iOS 颜色不正确

Question

我遇到了这个非常奇怪的问题。我正在从 UIImages 创建动画 gif，大多数时候它们都是正确的。然而，当我开始使用更大尺寸的图像时，我的颜色开始消失。例如，如果我使用不超过 10 种颜色的 4 帧 32 x 32 像素图像就没问题。如果我将同一图像缩放到 832 x 832，我会失去粉红色，棕色变成绿色。

@1x 32 x 32

@10x 320 x 320

@26x 832 x 832

这是我用来创建 gif 的代码...

var kFrameCount = 0

for smdLayer in drawingToUse!.layers{
    if !smdLayer.hidden {
        kFrameCount += 1
    }
}

let loopingProperty = [String(kCGImagePropertyGIFLoopCount): 0]
let fileProperties: [String: AnyObject] = [String(kCGImagePropertyGIFDictionary): loopingProperty as AnyObject];

let frameProperty = [String(kCGImagePropertyGIFDelayTime):  Float(speedLabel.text!)!]
let frameProperties: [String: AnyObject] = [String(kCGImagePropertyGIFDictionary): frameProperty as AnyObject];

let documentsDirectoryPath = "file://\(NSTemporaryDirectory())"

if let documentsDirectoryURL = URL(string: documentsDirectoryPath){

    let fileURL = documentsDirectoryURL.appendingPathComponent("\(drawing.name)\(getScaleString()).gif")
    let destination = CGImageDestinationCreateWithURL(fileURL as CFURL, kUTTypeGIF, kFrameCount, nil)!

    CGImageDestinationSetProperties(destination, fileProperties as CFDictionary);

    for smdLayer in drawingToUse!.layers{

        if !smdLayer.hidden{

            let image = UIImage(smdLayer: smdLayer, alphaBlend: useAlphaLayers, backgroundColor: backgroundColorButton.backgroundColor!, scale: scale)
            CGImageDestinationAddImage(destination, image.cgImage!, frameProperties as CFDictionary)
        }
    }

    if (!CGImageDestinationFinalize(destination)) {
        print("failed to finalize image destination")
    }        
}

我在调用 CGImageDestinationAddImage(destination, image.cgImage!, frameProperties as CFDictionary) 之前设置了一个断点，图像非常好，颜色正确。我希望有人知道我错过了什么。

更新

这是一个示例项目。请注意，虽然它在预览中没有动画，但它正在保存动画 gif 并且我在控制台中注销了图像的位置。

https://www.dropbox.com/s/pb52awaj8w3amyz/gifTest.zip?dl=0

Answer 1

似乎关闭全局颜色图可以解决问题:

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

请注意，与 PNG 不同，GIF 只能使用 256 色贴图，没有透明度。对于动画 GIF，可以有全局或每帧颜色图。

不幸的是，Core Graphics 不允许我们直接使用颜色图，因此在编码 GIF 时会有一些自动颜色转换。

看来关掉全局色图就可以了。此外，使用 kCGImagePropertyGIFImageColorMap 为每一帧显式设置颜色映射也可能有效。

由于这似乎不能可靠地工作，让我们为每一帧创建我们自己的颜色图:

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<Color>()

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

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

现在让我们更新我们的方法:

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

...

    var colorMap = ColorMap()
    let pixelData = imageRef.dataProvider!.data
    let rawData: UnsafePointer<UInt8> = 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))

和

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);
    }

当然，这只有在颜色数少于 256 时才有效。我真的会推荐一个可以正确处理颜色转换的自定义 GIF 库。