在Swift 3中绘制CGImage的直方图

Question

在将库从Swift 2转换为Swift 3版本时，我遇到了vImageHistogramCalculation_ARGB8888方法的问题。问题是该方法仅接受“histogram”参数作为

UnsafeMutablePointer<UnsafeMutablePointer<T>?> 

但是Swift 3的构建方式

let histogram = UnsafeMutablePointer<UnsafeMutablePointer<vImagePixelCount>>(mutating: rgba)  

返回未包装的值，所以我无法将其强制转换为正确的类型。

编译器错误是：

：无法使用类型'（mutating：[UnsafeMutablePointer]）'的参数列表调用类型为“ UnsafeMutablePointer？>”的初始化程序

你有一些想法吗？我尝试在直方图常量中添加“？”但是然后我收到错误：

'init'不可用：使用'withMemoryRebound（to：capacity:_）'临时将内存视为另一种兼容布局的类型。

有一些编译器建议，但我不知道如何使用它。

- Mr. A

Answer 1

转换为UnsafeMutablePointer<vImagePixelCount>?:

let alphaPtr = UnsafeMutablePointer<vImagePixelCount>(mutating: alpha) as UnsafeMutablePointer<vImagePixelCount>?

完整的编译示例：

var inBuffer = vImage_Buffer() // set your buffer here

let alpha = [UInt](repeating: 0, count: 256)
let red = [UInt](repeating: 0, count: 256)
let green = [UInt](repeating: 0, count: 256)
let blue = [UInt](repeating: 0, count: 256)

let alphaPtr = UnsafeMutablePointer<vImagePixelCount>(mutating: alpha) as UnsafeMutablePointer<vImagePixelCount>?
let redPtr = UnsafeMutablePointer<vImagePixelCount>(mutating: red) as UnsafeMutablePointer<vImagePixelCount>?
let greenPtr = UnsafeMutablePointer<vImagePixelCount>(mutating: green) as UnsafeMutablePointer<vImagePixelCount>?
let bluePtr = UnsafeMutablePointer<vImagePixelCount>(mutating: blue) as UnsafeMutablePointer<vImagePixelCount>?

let rgba = [redPtr, greenPtr, bluePtr, alphaPtr]

let histogram = UnsafeMutablePointer<UnsafeMutablePointer<vImagePixelCount>?>(mutating: rgba)
let error = vImageHistogramCalculation_ARGB8888(&inBuffer, histogram, UInt32(kvImageNoFlags))

// alpha, red, green, blue are set now

- shallowThought

---

那么，对于rgba，我需要调用vImageHistogramCalculation_ARGB8888四次，对吗？每个值都需要？无论如何...不起作用:/ 不能像您的示例中那样构建直方图常量。你试过了吗？ - Mr. A

更新的答案。 - shallowThought

这段代码在Swift 5中会产生“悬空指针”警告。 - Valeriy Van

这是苹果公司对于问题的答复：https://developer.apple.com/documentation/accelerate/specifying_histograms_with_vimage - Valeriy Van

@ValeriyVan，感谢您提供的链接！我使用该链接创建了shallowThought代码的Swift 5版本。我正在使用的函数明确将零、一、二、三命名为红色、绿色、蓝色和alpha。 - Rethunk

Answer 2

Swift 5

shallowThought的代码可以工作，但对于Swift 5会生成警告：

Initialization of 'UnsafeMutablePointer<vImagePixelCount>' 
(aka 'UnsafeMutablePointer<UInt>') results in a dangling pointer

为了跟进Valerly Van对shallowThought所接受答案的评论，这里提供了Swift 5代码，可以消除警告。以下代码是从两个来源复制、粘贴和拼凑而成。

苹果公司的示例，“使用vImage指定直方图” https://developer.apple.com/documentation/accelerate/specifying_histograms_with_vimage

一个关于生成和使用直方图的SO帖子，Swift 2.2 - Count Black Pixels in UIImage

func histogram(image: UIImage) -> (red: [UInt], green: [UInt], blue: [UInt], alpha: [UInt]) {
let img: CGImage = CIImage(image: image)!.cgImage!

let imgProvider: CGDataProvider = img.dataProvider!
let imgBitmapData: CFData = imgProvider.data!
var imgBuffer = vImage_Buffer(
    data: UnsafeMutableRawPointer(mutating: CFDataGetBytePtr(imgBitmapData)),
    height: vImagePixelCount(img.height),
    width: vImagePixelCount(img.width),
    rowBytes: img.bytesPerRow)

// bins: zero = red, green = one, blue = two, alpha = three
var binZero = [vImagePixelCount](repeating: 0, count: 256)
var binOne = [vImagePixelCount](repeating: 0, count: 256)
var binTwo = [vImagePixelCount](repeating: 0, count: 256)
var binThree = [vImagePixelCount](repeating: 0, count: 256)

binZero.withUnsafeMutableBufferPointer { zeroPtr in
    binOne.withUnsafeMutableBufferPointer { onePtr in
        binTwo.withUnsafeMutableBufferPointer { twoPtr in
            binThree.withUnsafeMutableBufferPointer { threePtr in
                
                var histogramBins = [zeroPtr.baseAddress, onePtr.baseAddress,
                                     twoPtr.baseAddress, threePtr.baseAddress]
                
                histogramBins.withUnsafeMutableBufferPointer {
                    histogramBinsPtr in
                    let error = vImageHistogramCalculation_ARGB8888(
                        &imgBuffer,
                        histogramBinsPtr.baseAddress!,
                        vImage_Flags(kvImageNoFlags))
                    
                    guard error == kvImageNoError else {
                        fatalError("Error calculating histogram: \(error)")
                    }
                }
            }
        }
    }
}

return (binZero, binOne, binTwo, binThree)
}

将（零、一、二、三）与（红、绿、蓝、透明度）匹配正好适用于我当前而又有限的用例。

- Rethunk