如何从从 -captureStillImageAsynchronouslyFromConnection:completionHandler: 获得的 CMSampleBuffer 中获取 NSImage？

Question

我有一个 Cocoa 应用程序，旨在从 USB 显微镜捕获静止图像，然后在将它们保存到图像文件之前对其进行一些后期处理。目前，我被困在试图从传递给我的completionHandler 块的CMSampleBufferRef 到NSImage 或其他一些我可以使用熟悉的Cocoa API 操作和保存的表示。

我在 AVFoundation 文档中找到了函数 imageFromSampleBuffer()，它声称将 CMSampleBuffer 转换为 UIImage（叹气），并对其进行了适当的修改以返回 NSImage。但在这种情况下它不起作用，因为对CMSampleBufferGetImageBuffer() 的调用返回nil。

这是一个日志，显示传递给我的完成块的CMSampleBuffer：

2012-01-21 19:38:36.293 LabCam[1402:cb0f] CMSampleBuffer 0x100335390 retainCount: 1 allocator: 0x7fff8c78620c
     invalid = NO
     dataReady = YES
     makeDataReadyCallback = 0x0
     makeDataReadyRefcon = 0x0
     buffer-level attachments:
          com.apple.cmio.buffer_attachment.discontinuity_flags(P) = 0
          com.apple.cmio.buffer_attachment.hosttime(P) = 79631546824089
          com.apple.cmio.buffer_attachment.sequence_number(P) = 42
     formatDescription = <CMVideoFormatDescription 0x100335220 [0x7fff782fff40]> {
     mediaType:'vide' 
     mediaSubType:'jpeg' 
     mediaSpecific: {
          codecType: 'jpeg'          dimensions: 640 x 480 
     } 
     extensions: {<CFBasicHash 0x100335160 [0x7fff782fff40]>{type = immutable dict, count = 5,
entries =>
     1 : <CFString 0x7fff773dff48 [0x7fff782fff40]>{contents = "Version"} = <CFNumber 0x183 [0x7fff782fff40]>{value = +1, type = kCFNumberSInt32Type}
     2 : <CFString 0x7fff773dff68 [0x7fff782fff40]>{contents = "RevisionLevel"} = <CFNumber 0x183 [0x7fff782fff40]>{value = +1, type = kCFNumberSInt32Type}
     3 : <CFString 0x7fff7781ab08 [0x7fff782fff40]>{contents = "CVFieldCount"} = <CFNumber 0x183 [0x7fff782fff40]>{value = +1, type = kCFNumberSInt32Type}
     4 : <CFString 0x7fff773dfdc8 [0x7fff782fff40]>{contents = "FormatName"} = <CFString 0x7fff76d35fb0 [0x7fff782fff40]>{contents = Photo - JPEG"}
     5 : <CFString 0x7fff773dff88 [0x7fff782fff40]>{contents = "Vendor"} = <CFString 0x7fff773dffa8 [0x7fff782fff40]>{contents = "appl"}
}
}
}
     sbufToTrackReadiness = 0x0
     numSamples = 1
     sampleTimingArray[1] = {
          {PTS = {2388943236/30000 = 79631.441, rounded}, DTS = {INVALID}, duration = {3698/30000 = 0.123}},
     }
     sampleSizeArray[1] = {
          sampleSize = 55911,
     }
     dataBuffer = 0x100335300

它显然包含 JPEG 数据，但我该如何处理呢？ （最好保留相关的元数据以备不时之需……）

Answer 1

我最终在另一个代码示例的帮助下解决了这个问题。 CMSampleBufferGetImageBuffer 仅返回相机可用的未压缩本机图像格式的有效结果。因此，为了让我的程序正常工作，我必须将 AVCaptureStillImageOutput 实例配置为使用 k32BGRAPixelFormat 而不是其默认 (JPEG) 压缩格式。

session = [[AVCaptureSession alloc] init];
session.sessionPreset = AVCaptureSessionPresetPhoto;
imageOutput = [[AVCaptureStillImageOutput alloc] init];
// Configure imageOutput for BGRA pixel format [#2].
NSNumber * pixelFormat = [NSNumber numberWithInt:k32BGRAPixelFormat];
[imageOutput setOutputSettings:[NSDictionary dictionaryWithObject:pixelFormat
                                                           forKey:(id)kCVPixelBufferPixelFormatTypeKey]];
[session addOutput:imageOutput];