CoreGraphics - 无法将 UnsafeMutablePointer<UInt32> 替换为 UnsafeMutablePointer<UInt8>

Question

我刚开始搞图像处理，遇到了几个非常奇怪的问题，或者至少我认为是这样。我假设我犯了一些非常愚蠢的错误。

我打算 post 关于这个的另一个问题，但是，使用下面的代码有时我也会得到随机噪声而不是用户抽取数字的像素表示。如果有人能告诉我为什么会发生这种情况，我将不胜感激。我很难找出原因，因为我阅读的所有内容都表明这段代码应该有效。

如果有人需要更多信息，请告诉我！提前感谢您的帮助！

目标：
首先，获取用户在屏幕上绘制的数字。然后，将图像大小调整为 28 x 28。接下来，将图像转换为灰度并获得归一化像素值数组。最后，将归一化的灰度像素值输入机器学习算法。

[注意：在下图中，点代表 0 值，1 代表值 > 0。]

下面代码的输出效果很好。如果用户画了一个“3”，我通常会得到如下结果：

问题：

如果我将 UnsafeMutablePointer 和 Buffer 的大小更改为 UInt8，我会得到看起来像随机噪声的东西。或者，如果我用 [UInt32](repeating: 0, count: totalBytes) 甚至 [UInt8](repeating: 0, count: totalBytes) 替换 UnsafeMutablePointer 和 Buffer，每个像素最终都为 0，我真的不明白。

如果我将 UnsafeMutablePointer 和 Buffer 的大小更改为 UInt8，则像素输出如下：

获取灰度像素代码：

public extension UIImage
{
    private func grayScalePixels() -> UnsafeMutableBufferPointer<UInt32>?
    {
        guard let cgImage = self.cgImage else { return nil }

        let bitsPerComponent = 8
        let width = cgImage.width
        let height = cgImage.height
        let totalBytes = (width * height)
        let colorSpace = CGColorSpaceCreateDeviceGray()
        let data = UnsafeMutablePointer<UInt32>.allocate(capacity: totalBytes)
        defer { data.deallocate(capacity: totalBytes) }

        guard let imageContext = CGContext(data: data, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: width, space: colorSpace, bitmapInfo: 0) else { return nil }
        imageContext.draw(cgImage, in: CGRect(origin: CGPoint.zero, size: CGSize(width: width, height: height)))

        return UnsafeMutableBufferPointer<UInt32>(start: data, count: totalBytes)
    }

    public func normalizedGrayScalePixels() -> [CGFloat]?
    {
        guard let cgImage = self.cgImage else { return nil }
        guard let pixels = self.grayScalePixels() else { return nil }

        let width = cgImage.width
        let height = cgImage.height
        var result = [CGFloat]()

        for y in 0..<height
        {
            for x in 0..<width
            {
                let index = ((width * y) + x)
                let pixel = (CGFloat(pixels[index]) / 255.0)
                result.append(pixel)
            }
        }

        return result
    }
}

抽号代码：

    func drawLineFrom(fromPoint: CGPoint, toPoint: CGPoint)
    {
        UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, false, 1)

        self.tempImageView.image?.draw(at: CGPoint.zero)

        let context = UIGraphicsGetCurrentContext()
        context?.move(to: fromPoint)
        context?.addLine(to: toPoint)
        context?.setLineCap(.round)
        context?.setLineWidth(self.brushWidth)
        context?.setStrokeColor(gray: 0, alpha: 1)
        context?.strokePath()

        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext()
        self.tempImageView.alpha = self.opacity

        UIGraphicsEndImageContext()
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?)
    {
        self.swiped = false

        if let touch = touches.first {
            self.lastPoint = touch.location(in: self.view)
        }
    }

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?)
    {
        self.swiped = true

        if let touch = touches.first
        {
            let currentPoint = touch.location(in: self.view)
            self.drawLineFrom(fromPoint: self.lastPoint, toPoint: currentPoint)

            self.lastPoint = currentPoint
        }
    }

    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?)
    {
        if !swiped {
            self.drawLineFrom(fromPoint: self.lastPoint, toPoint: self.lastPoint)
        }

        self.predictionLabel.text = "Predication: \(self.predict())"

        self.tempImageView.image = nil
    }

预测号码的代码：

    private func printNumber(rowSize: Int, inputs: Vector)
    {
        for (index, pixel) in inputs.enumerated()
        {
            if index % rowSize == 0 { print() }

            if (pixel > 0) {
                print("1", terminator: " ")
            }
            else { print(".", terminator: " ") }
        }

        print()
    }

    private func predict() -> Scalar
    {
        let resizedImaege = self.tempImageView.image!.resizedImage(CGSize(width: 28, height: 28), interpolationQuality: .high)
        let inputs = resizedImaege!.normalizedGrayScalePixels()!.flatMap({ Scalar([=14=]) })
        self.feedforwardResult = self.neuralNetwork!.feedForward(inputs: inputs)

        self.printNumber(rowSize: 28, inputs: inputs)

        let max = self.feedforwardResult!.activations.last!.max()!
        let prediction = self.feedforwardResult!.activations.last!.index(of: max)!
        return Scalar(prediction)
    }

Answer 1

你的代码中有一个非常糟糕的地方是这一行：

    defer { data.deallocate(capacity: totalBytes) }

data.deallocate(capacity: totalBytes) 在退出方法 grayScalePixels() 之前执行。因此，返回的 UnsafeMutableBufferPointer 的 baseAddress 指向一个已经解除分配的区域，这意味着您在访问该区域时不能期望任何可预测的结果。

如果你想使用UnsafeMutableBufferPointer，你需要在完成对它的所有访问后释放该区域（下面代码中的#1）：

private func grayScalePixels() -> UnsafeMutableBufferPointer<UInt8>? {
    guard let cgImage = self.cgImage else { return nil }
    
    let bitsPerComponent = 8
    let width = cgImage.width
    let height = cgImage.height
    let totalBytes = width * height
    let colorSpace = CGColorSpaceCreateDeviceGray()
    let data = UnsafeMutablePointer<UInt8>.allocate(capacity: totalBytes)
    data.initialize(to: UInt8.max, count: totalBytes)   //<- #4
    
    guard let imageContext = CGContext(data: data, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: width, space: colorSpace, bitmapInfo: 0) else { return nil }
    imageContext.draw(cgImage, in: CGRect(x: 0, y: 0, width: width, height: height))
    
    return UnsafeMutableBufferPointer(start: data, count: totalBytes)
}

public func normalizedGrayScalePixels() -> [CGFloat]? {
    guard let cgImage = self.cgImage else { return nil }
    guard let pixels = self.grayScalePixels() else { return nil }
    
    let width = cgImage.width
    let height = cgImage.height
    var result: [CGFloat] = []
    
    for y in 0..<height {
        for x in 0..<width {
            let index = width * y + x
            let pixel = CGFloat(pixels[index]) / CGFloat(UInt8.max)
            result.append(pixel)
        }
    }
    pixels.baseAddress!.deinitialize(count: pixels.count)   //<- #2
    pixels.baseAddress!.deallocate(capacity: pixels.count)  //<- #1
    
    return result
}

(#2) 在 Swift 的当前实现中 UInt8 可能不需要 deinitialize，但顺序：分配 - 初始化 - 取消初始化 - 解除分配是推荐的方式.

（其他一些行只是我的喜好，并不重要。）

否则，如果您想使用 Swift Array 而不是 UnsafeMutableBufferPointer，您可以这样写：

private func grayScalePixels() -> [UInt8]? {
    guard let cgImage = self.cgImage else { return nil }
    
    let bitsPerComponent = 8
    let width = cgImage.width
    let height = cgImage.height
    let totalBytes = width * height
    let colorSpace = CGColorSpaceCreateDeviceGray()
    var byteArray: [UInt8] = Array(repeating: UInt8.max, count: totalBytes) //<- #4
    let success = byteArray.withUnsafeMutableBufferPointer {(buffer)->Bool in
        guard let imageContext = CGContext(data: buffer.baseAddress!, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: width, space: colorSpace, bitmapInfo: 0) else { return false }
        imageContext.draw(cgImage, in: CGRect(x: 0, y: 0, width: width, height: height))
        return true;
    }
    return success ? byteArray : nil
}

public func normalizedGrayScalePixels() -> [CGFloat]? {
    guard let cgImage = self.cgImage else { return nil }
    guard let pixels = self.grayScalePixels() else { return nil }
    
    let width = cgImage.width
    let height = cgImage.height
    var result: [CGFloat] = []
    
    for y in 0..<height {
        for x in 0..<width {
            let index = width * y + x
            let pixel = CGFloat(pixels[index]) / CGFloat(UInt8.max)
            result.append(pixel)
        }
    }
    
    return result
}

您可能需要修改我上面的代码才能使它们与您的代码一起工作，因为我无法用您的 UInt32 版本的 grayScalePixels() 重现相同的结果。

编辑

我发现我的代码有问题。您的绘图代码画线为：

    context?.setStrokeColor(gray: 0, alpha: 1)

灰度0，黑色。在我的旧代码中，我将位图初始化为：

    data.initialize(to: 0, count: totalBytes)

或：

    var byteArray: [UInt8] = Array(repeating: 0, count: totalBytes)

于是，在黑上画黑，结果：全黑，8位灰度，全0。（我第一次写 initialize 可能不需要，但那是一个错误。带有 alpha 的图像将与初始位图内容混合绘制。）

我更新的代码（标有#4）用白色初始化位图（8 位灰度，255 == 0xFF == UInt8.max） .

而且您最好通过更新 printNumber(rowSize:inputs:):

来检测非白色像素

private func printNumber(rowSize: Int, inputs: Vector) {
    for (index, pixel) in inputs.enumerated() {
        if index % rowSize == 0 { print() }
        
        if pixel < 1.0 { //<- #4
            print("1", terminator: "")
        }
        else { print(".", terminator: "") }
    }
    
    print()
}

在归一化灰度为浮点数时，1.0 是白色的值，您最好将非白色显示为 1。（或者，找到另一个更好的阈值。）

CoreGraphics - 无法将 UnsafeMutablePointer<UInt32> 替换为 UnsafeMutablePointer<UInt8>

CoreGraphics - Can't replace UnsafeMutablePointer<UInt32> with UnsafeMutablePointer<UInt8>

core-graphics

ios

swift

编辑