使用AVMutableVideoComposition后视频色彩失真，是否为色彩空间问题？

问题核心 #

使用AVMutableComposition+AVVideoCompositionCoreAnimationTool结合CALayer渲染导出视频时，CoreAnimation默认色彩空间与视频色彩空间不匹配，导致色彩错误转换，出现视频偏亮、纯色层变色（如纯红变为偏橙红）的问题。

关键修复步骤 #

1. 为所有CALayer指定sRGB色彩空间 #

给宿主层、视频层、形状层统一设置色彩空间与缩放比例，强制CoreAnimation在sRGB空间内渲染：

// 宿主层
let parentLayer = CALayer()
parentLayer.contentsScale = UIScreen.main.scale
parentLayer.colorSpace = CGColorSpace(name: CGColorSpace.extendedSRGB)

// 视频层（AVPlayerLayer）
let videoLayer = AVPlayerLayer(player: nil)
videoLayer.contentsScale = UIScreen.main.scale
videoLayer.colorSpace = CGColorSpace(name: CGColorSpace.extendedSRGB)

// 形状层
let shapeLayer = CAShapeLayer()
shapeLayer.fillColor = UIColor(red: 1.0, green: 0.0, blue: 0.0, alpha: 1.0).cgColor
shapeLayer.contentsScale = UIScreen.main.scale
shapeLayer.colorSpace = CGColorSpace(name: CGColorSpace.extendedSRGB)

2. 完善AVVideoComposition渲染配置 #

保留原有色彩空间参数的同时，匹配原视频的渲染尺寸与缩放比例：

let videoComposition = AVMutableVideoComposition()
videoComposition.renderSize = originalVideoSize
videoComposition.renderScale = UIScreen.main.scale
videoComposition.colorPrimaries = AVVideoColorPrimaries_ITU_R_709_2
videoComposition.colorYCbCrMatrix = AVVideoYCbCrMatrix_ITU_R_709_2
videoComposition.colorTransferFunction = AVVideoTransferFunction_ITU_R_709_2

3. 修正AVAssetWriter输出配置 #

选用支持sRGB的像素格式，保持色彩空间参数与视频一致：

let videoOutputSettings: [String: Any] = [
    AVVideoCodecKey: AVVideoCodecType.h264,
    AVVideoWidthKey: originalVideoSize.width,
    AVVideoHeightKey: originalVideoSize.height,
    AVVideoColorPropertiesKey: [
        AVVideoColorPrimariesKey: AVVideoColorPrimaries_ITU_R_709_2,
        AVVideoTransferFunctionKey: AVVideoTransferFunction_ITU_R_709_2,
        AVVideoYCbCrMatrixKey: AVVideoYCbCrMatrix_ITU_R_709_2
    ],
    AVVideoCompressionPropertiesKey: [
        AVVideoAverageBitRateKey: 10_000_000,
        AVVideoMaxKeyFrameIntervalKey: 30
    ]
]

原理说明 #

从单帧转换sRGB后色彩恢复正常的现象可以确定：CoreAnimation默认使用线性RGB色彩空间渲染，而视频采用ITU-R 709标准的YCbCr空间，两者间的转换未被正确处理。通过给所有CALayer指定sRGB色彩空间，强制CoreAnimation在sRGB空间完成渲染，再由AVFoundation正确转换到视频所需的YCbCr空间，即可解决色彩褪色问题。

直接导出原视频正常的结果，也验证了问题出在CoreAnimation渲染环节，而非AVAssetWriter本身。

内容的提问来源于stack exchange，提问作者tin