ios - 如何选择用于 AVAssetReader 的像素格式类型 (kCVPixelBufferPixelFormatTypeKey)？

Question

我们正在使用 AVAssetReader 和 AVAssetWriter 的样式，如 Video Encoding using AVAssetWriter - CRASHES 中所述基本上是读取我们从照片库/ Assets 库中获得的视频，然后以不同的比特率写入它以减小其大小(用于最终的网络上传)。

让这个为我们工作的技巧是在 AVAssetReaderTrackOutput 的 outputSettings 中指定一个 kCVPixelBufferPixelFormatTypeKey 键和值，像这样:

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

所以基本上我们为 kCVPixelBufferPixelFormatTypeKey 键使用了 kCVPixelFormatType_32BGRA 值。

但显然有许多可能的像素格式类型可供我们选择。运行 Technical Q&A QA1501: Core Video - Available Pixel Formats 中记录的代码在 iOS 6.0.1 iPhone 设备上，这是它显示的支持像素格式类型的列表:

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

尽管 kCVPixelFormatType_32BGRA 对我们有效(至少表面上如此)，但我们很好奇是否有比上述列表更好的选择。我们应该如何为我们挑选正确的像素格式类型？

Answer 1

您可以尝试不同的设置并比较性能。以不同的像素格式向编码器提供缓冲区可能会导致编码性能的差异。硬件加速编码器通常使用“420v”。

另请参阅，Query for optimal pixel format when capturing video on iOS?