本文档描述使用AVAssetWriterInputPixelBufferAdaptor简化Metal渲染结果从BGRA到yuv420p的转换，适用于写成MP4或MOV容器。以前有人用类似的办法暴力实现iOS 8以下的H.264硬解，代价是消耗用户的闪存写次数。

文档结构：

1. 初始化AVAssetWriterInput
2. 初始化AVAssetWriterInputPixelBufferAdaptor
3. CVPixelBuffer写入AVAssetWriterInputPixelBufferAdaptor
4. 讨论：指定BT. 709 YUV转RGB矩阵

Metal Camera开发4：渲染到CVPixelBuffer实现了CVPixelBuffer存储渲染结果，对于摄像头开发，通常希望将渲染结果编码成H.264或H.265（iOS 11支持），iOS平台的H.264编码器原生支持yuv420sp VideoRange及FullRange，而Metal渲染到CVPixelBuffer时像素格式为BGRA，显然需要进行像素格式转换，比如使用libyuv、Metal 着色器实现RGBA转yuv或GLSL实现。对于将数据写到本地的需求，AVFoundation提供了AVAssetWriterInputPixelBufferAdaptor简化RGB转yuv的步骤。GPUImage也使用了这种方式实现OpenGL ES的渲染结果编码成H.264。下面描述它的编程过程。

AVAssetWriterInputPixelBufferAdaptor必须配合AVAssetWriterInput使用，不然没法调用构造函数（手动斜眼.jpg）。AVAssetWriterInputPixelBufferAdaptor类的头文件描述了它的功能：

Pixel buffers not in a natively supported format will be converted internally prior to encoding when possible.

对于编码Metal的渲染结果，初始化AVAssetWriterInputPixelBufferAdaptor指定sourcePixelBufferAttributes使用kCVPixelBufferPixelFormatTypeKey为kCVPixelFormatType_32BGRA，之后的像素格式转换都由AVFoundation内部实现，省去了手动实现BGRA转yuv。当编码CoreImage的渲染结果，kCVPixelBufferPixelFormatTypeKey对应的值为kCVPixelFormatType_32ARGB。总之，需要和数据源对应起来，否则视频会表现出异常颜色。接下来逐一描述编程步骤。

1. 初始化AVAssetWriterInput

let videoCompressionProperties = [
    AVVideoAverageBitRateKey: 1080 * 1920 * 15.15
]

let videoSettings: [String : Any] = [
        AVVideoCodecKey: AVVideoCodecH264,
        AVVideoWidthKey: 1080,
        AVVideoHeightKey: 1920,
        AVVideoCompressionPropertiesKey: videoCompressionProperties
]

self.videoWriterInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: videoSettings)

AVAssetWriterInput还需添加到AVAssetWriter，网上参考代码非常丰富，在此不一一描述。

2. 初始化AVAssetWriterInputPixelBufferAdaptor

var videoWriterInputPixelBufferAdaptor: AVAssetWriterInputPixelBufferAdaptor?
//-------
let sourcePixelBufferAttributes = [
        kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA,
        kCVPixelBufferWidthKey as String: 1080,
        kCVPixelBufferHeightKey as String: 1920
]

self.videoWriterInputPixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(
        assetWriterInput: self.videoWriterInput!,
        sourcePixelBufferAttributes: sourcePixelBufferAttributes
)

AVAssetWriterInputPixelBufferAdaptor是否应该在AVAssetWriterInput添加到AVAssetWriter前初始化？经实验，这一操作的顺序不影响程序的正常运行，在AVAssetWriter.startWriting()前初始化即可。

3. CVPixelBuffer写入AVAssetWriterInputPixelBufferAdaptor

self.videoWriterInputPixelBufferAdaptor!.assetWriterInput.isReadyForMoreMediaData {
    let whetherPixelBufferAppendedtoAdaptor = self.videoWriterInputPixelBufferAdaptor!.append(renderPixelBuffer!, withPresentationTime: presentationTime)
    if whetherPixelBufferAppendedtoAdaptor {
        print("adaptor appended CVPixelBuffer successfully")
    } else {
        print("adaptor appended CVPixelBuffer  failed")
    }
}

renderPixelBuffer为Metal渲染的目标CVPixelBuffer，即它存储了最终的渲染结果，细节可参考Metal Camera开发4：渲染到CVPixelBuffer。

4. 讨论：指定BT. 709 YUV转RGB矩阵

GPUImageMovieWriter类的createDataFBO方法有这么一段注释：

/* AVAssetWriter will use BT.601 conversion matrix for RGB to YCbCr conversion
 * regardless of the kCVImageBufferYCbCrMatrixKey value.
 * Tagging the resulting video file as BT.601, is the best option right now.
 * Creating a proper BT.709 video is not possible at the moment.
 */

在iOS 10的测试过程中，发现已支持kCVImageBufferYCbCrMatrixKey设置为kCVImageBufferTransferFunction_ITU_R_709_2。参考代码如下。

CVBufferSetAttachment(pixelBuffer!, kCVImageBufferColorPrimariesKey, kCVImageBufferColorPrimaries_ITU_R_709_2, CVAttachmentMode.shouldPropagate)
CVBufferSetAttachment(pixelBuffer!, kCVImageBufferYCbCrMatrixKey, kCVImageBufferTransferFunction_ITU_R_709_2, CVAttachmentMode.shouldPropagate)
CVBufferSetAttachment(pixelBuffer!, kCVImageBufferTransferFunctionKey, kCVImageBufferTransferFunction_ITU_R_709_2, CVAttachmentMode.shouldPropagate)

FFmpeg读取生成的视频信息如下所示。

AVAssetWriter已支持ITU_R_709