通过 vImage 提取图像通道有副作用

Question

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

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)")
    }
}

这是我得到的：

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色（即所有像素值 = 255？），我怎么会得到像素值 0？

还要注意，如果我使用 Xcode 的模拟器运行上面的代码，我会得到不同数量的 Pixel Val 0，但 capacity 的总数仍然是 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Finding the Sharpest Image in a Sequence of Captured Images 中的 Create Floating Point Pixels to Use in vDSP 以了解行字节如何超出图像的边界框）。

如果您使用来自Specifying Histograms with vImage 的代码，您应该会得到您期望的值：

     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)")
            }
        }