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

Question

我有一个 Cocoa 应用程序，旨在从 USB 显微镜捕获静态图像，然后对它们进行一些后处理，然后将它们保存到图像文件。目前，我试图从传递给我的 completionHandler 块的 CMSampleBufferRef 获取到 NSImage 或我可以操作的其他表示形式并使用熟悉的 Cocoa API 进行保存。

我在 AVFoundation 文档中找到了函数 imageFromSampleBuffer() ，该函数旨在将 CMSampleBuffer 转换为 UIImage （叹气），并对其进行了适当的修改返回一个NSImage。但在这种情况下它不起作用，因为对 CMSampleBufferGetImageBuffer() 的调用返回 nil。

以下日志显示了传递到我的完成块的 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，但我如何获取它？ （最好保留相关的元数据......）

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];