通过vImage提取图像通道有副作用

Question

我正在尝试使用RGB API来提取vImageConvert_ARGB8888toPlanar8通道。假设下面的inputImage是来自：https://www.solidbackgrounds.com/images/1080x1920/1080x1920-white-solid-color-background.jpg的纯白图像

这是我的代码：

let inputImage = #imageLiteral(resourceName: "1080x1920-white-solid-color-background.jpg")

guard let sourceCGImage = inputImage.cgImage,
      let sourceImageFormat = vImage_CGImageFormat(cgImage: sourceCGImage),
      var sourceImageBuffer = try? vImage_Buffer(cgImage: sourceCGImage,
                                                 format: sourceImageFormat) else {
    fatalError()
}

let componentCount = sourceImageFormat.componentCount
var argbSourcePlanarBuffers: [vImage_Buffer] = (0..<componentCount).map { _ in
    let bitsPerPixel = UInt32(8)

    guard let buffer = try? vImage_Buffer(width: Int(sourceImageBuffer.width),
                                          height: Int(sourceImageBuffer.height),
                                          bitsPerPixel: bitsPerPixel) else {
        fatalError("Error creating planar buffers.")
    }

    return buffer
}

vImageConvert_ARGB8888toPlanar8(&sourceImageBuffer,
                                &argbSourcePlanarBuffers[0],  // R
                                &argbSourcePlanarBuffers[1],  // G
                                &argbSourcePlanarBuffers[2],  // B
                                &argbSourcePlanarBuffers[3],  // A
                                vImage_Flags(kvImageNoFlags))

print("Image H: \(sourceCGImage.height), W: \(sourceCGImage.width)")
let capacity = sourceCGImage.width * sourceCGImage.height
print("capacity: \(capacity)")

// Repeat the following code for other 3 channels
let channelDataPtrR = argbSourcePlanarBuffers[0].data.bindMemory(to: Pixel_8.self,
                                                                 capacity: capacity)
let channelRawDataR = UnsafeBufferPointer(start: channelDataPtrR, count: capacity)
let channelDataR = Array(channelRawDataR)
var histogramR = [Double](repeating: 0.0, count: 255 + 1)
channelDataR.map { histogramR[Int($0)] += 1.0 }
print("histogramR:")
for (i, px) in histogramR.enumerated() {
    if px > 0.0 {
        print("Pixel Val \(i): \(px)")
    }
}

我得到的是：

Image H: 1920, W: 1080
capacity: 2073600
histogramR:
Pixel Val 0: 129600.0
Pixel Val 255: 1944000.0
histogramG:
Pixel Val 0: 129600.0
Pixel Val 255: 1944000.0
histogramB:
Pixel Val 0: 129600.0
Pixel Val 255: 1944000.0
histogramA:
Pixel Val 0: 129600.0
Pixel Val 255: 1944000.0

问题：，如果图像应该是纯白的(即所有像素值= 255?)，为什么要得到像素值0？

还要注意的是，如果我使用Xcode的模拟器运行上面的代码，我将得到不同数量的Pixel Val 0，但是总的capacity仍然是2073600。

Answer 1

这可能是因为缓冲区的实际行字节比图像宽(请参阅在vDSP中在捕获的图像序列中寻找最清晰的图像中使用的创建浮点像素，以了解行字节如何扩展到图像的边界框之外)。

如果您使用来自用vImage指定直方图的代码，您应该得到您期望的值：

     var histogramBinZero = [vImagePixelCount](repeating: 0, count: 256)
        var histogramBinOne = [vImagePixelCount](repeating: 0, count: 256)
        var histogramBinTwo = [vImagePixelCount](repeating: 0, count: 256)
        var histogramBinThree = [vImagePixelCount](repeating: 0, count: 256)
        
        histogramBinZero.withUnsafeMutableBufferPointer { zeroPtr in
            histogramBinOne.withUnsafeMutableBufferPointer { onePtr in
                histogramBinTwo.withUnsafeMutableBufferPointer { twoPtr in
                    histogramBinThree.withUnsafeMutableBufferPointer { threePtr in
                        
                        var histogramBins = [zeroPtr.baseAddress, onePtr.baseAddress,
                                             twoPtr.baseAddress, threePtr.baseAddress]
                        
                        histogramBins.withUnsafeMutableBufferPointer { histogramBinsPtr in
                            let error = vImageHistogramCalculation_ARGB8888(&sourceImageBuffer,
                                                                            histogramBinsPtr.baseAddress!,
                                                                            vImage_Flags(kvImageNoFlags))
                            
                            guard error == kvImageNoError else {
                                fatalError("Error calculating histogram: \(error)")
                            }
                        }
                    }
                }
            }
        }
        
        for (i, px) in histogramBinTwo.enumerated() {
            if px > 0 {
                print("Pixel Val \(i): \(px)")
            }
        }