无法通过CIAreaHistogram从视频输出中产生合理的结果

Question

我正在尝试创建相机输出的直方图。我创建了一个AVCaptureVideoDataOutputSampleBufferDelegate类来处理来自摄像机的视频。下面是我处理视频样本的captureOutput（）函数。

// AVCaptureVideoDataOutputSampleBufferDelegate
func captureOutput(_ captureOutput: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
    if let buffer = CMSampleBufferGetImageBuffer(sampleBuffer) {
        processUsingCIImage(pixelBuffer: buffer)
    }
}

func processUsingCIImage(pixelBuffer: CVPixelBuffer) {
    let image = CIImage(cvPixelBuffer: pixelBuffer)
    let params : [String : Any] = [kCIInputImageKey: image,
                  kCIInputExtentKey: CIVector.init(cgRect: image.extent),
                  "inputCount":16,
                  "inputScale":200]
    let filter = CIFilter(name:"CIAreaHistogram", withInputParameters:params)
    if let outputImage = filter?.outputImage {

        let width = Int(outputImage.extent.width)
        let height = Int(outputImage.extent.height)
        var data = Data(capacity:width*height*4)

        let context = CIContext()
        data.withUnsafeMutableBytes { (data: UnsafeMutablePointer<UInt8>) in
            context.render(outputImage, 
                           toBitmap: data, 
                           rowBytes:width*4, 
                           bounds:outputImage.extent, 
                           format:kCIFormatRGBA8, 
                           colorSpace:nil)
            for i in 0..<width {
                print("R:\(data[i*4]) G:\(data[i*4+1]) B:\(data[i*4+2])")
            }
        }
    }
}

相机预览图像总是输出

并不重要

R:0 G:0 B:0

对于直方图的所有值，除了最后一个看起来依赖于图像的值。我在这做错了什么？