如何選擇用於AVAssetReader的像素格式類型（kCVPixelBufferPixelFormatTypeKey）？

我們使用AVAssetReader和AVAssetWriter的方式有點像Video Encoding using AVAssetWriter - CRASHES中提到的風格，基本上是從照片庫/資源庫中讀取視頻，然後以不同的比特率寫入視頻以減小其大小（最終網絡上傳）。如何選擇用於AVAssetReader的像素格式類型（kCVPixelBufferPixelFormatTypeKey）？

的竅門能讓這對我們的工作是對AVAssetReaderTrackOutput指定的outputSettings一個kCVPixelBufferPixelFormatTypeKey鍵和值，像這樣：

NSDictionary *outputSettings = [NSDictionary 
    dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] 
        forKey:(id)kCVPixelBufferPixelFormatTypeKey];  
readerTrackOutput = 
    [[AVAssetReaderTrackOutput alloc] initWithTrack:src_track 
            outputSettings:outputSettings];

所以基本上我們用kCVPixelFormatType_32BGRA價值爲kCVPixelBufferPixelFormatTypeKey關鍵。

但顯然有很多可能的像素格式類型，我們可以從中選擇。運行在Technical Q&A QA1501: Core Video - Available Pixel Formats在iOS 6.0.1 iPhone設備上標註的代碼，下面是它顯示支持像素格式類型的列表：

Core Video Supported Pixel Format Types: 
Core Video Pixel Format Type: 32 
Core Video Pixel Format Type: 24 
Core Video Pixel Format Type: 16 
Core Video Pixel Format Type (FourCC): L565 
Core Video Pixel Format Type (FourCC): 2vuy 
Core Video Pixel Format Type (FourCC): yuvs 
Core Video Pixel Format Type (FourCC): yuvf 
Core Video Pixel Format Type: 40 
Core Video Pixel Format Type (FourCC): L008 
Core Video Pixel Format Type (FourCC): 2C08 
Core Video Pixel Format Type (FourCC): r408 
Core Video Pixel Format Type (FourCC): v408 
Core Video Pixel Format Type (FourCC): y408 
Core Video Pixel Format Type (FourCC): y416 
Core Video Pixel Format Type (FourCC): BGRA 
Core Video Pixel Format Type (FourCC): b64a 
Core Video Pixel Format Type (FourCC): b48r 
Core Video Pixel Format Type (FourCC): b32a 
Core Video Pixel Format Type (FourCC): b16g 
Core Video Pixel Format Type (FourCC): R10k 
Core Video Pixel Format Type (FourCC): v308 
Core Video Pixel Format Type (FourCC): v216 
Core Video Pixel Format Type (FourCC): v210 
Core Video Pixel Format Type (FourCC): v410 
Core Video Pixel Format Type (FourCC): r4fl 
Core Video Pixel Format Type (FourCC): grb4 
Core Video Pixel Format Type (FourCC): rgg4 
Core Video Pixel Format Type (FourCC): bgg4 
Core Video Pixel Format Type (FourCC): gbr4 
Core Video Pixel Format Type (FourCC): 420v 
Core Video Pixel Format Type (FourCC): 420f 
Core Video Pixel Format Type (FourCC): 411v 
Core Video Pixel Format Type (FourCC): 411f 
Core Video Pixel Format Type (FourCC): 422v 
Core Video Pixel Format Type (FourCC): 422f 
Core Video Pixel Format Type (FourCC): 444v 
Core Video Pixel Format Type (FourCC): 444f 
Core Video Pixel Format Type (FourCC): y420 
Core Video Pixel Format Type (FourCC): f420 
Core Video Pixel Format Type (FourCC): a2vy 
Core Video Pixel Format Type (FourCC): L00h 
Core Video Pixel Format Type (FourCC): L00f 
Core Video Pixel Format Type (FourCC): 2C0h 
Core Video Pixel Format Type (FourCC): 2C0f 
Core Video Pixel Format Type (FourCC): RGhA 
Core Video Pixel Format Type (FourCC): RGfA

即使kCVPixelFormatType_32BGRA工作（至少表面上）對我們來說，我們是好奇，是否有比上述列表更好的選擇。我們應該如何去選擇正確的像素格式類型給我們？

來源

2013-01-07 Chris Markle

如果有用於解碼的原始視頻幀硬件兼容的紋理格式，你會認爲獲得該格式會比轉換成RGB例如更快。這可以交給GL和用於轉換爲屏幕色彩空間的片段着色器。 – MrMaxP