Drawing histogram of CGImage in Swift 3

问题

I have a problem with vImageHistogramCalculation_ARGB8888 method while trying to convert library from Swift 2 to Swift 3 version. The problem is that the method accepts "histogram" argument only as

UnsafeMutablePointer<UnsafeMutablePointer<T>?> 

but Swift 3 construction

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

return unwrapped value, so I can't cast it to properly type.

The compiler error is:

: Cannot invoke initializer for type 'UnsafeMutablePointer?>' with an argument list of type '(mutating: [UnsafeMutablePointer])'

Have you some ideas? I tried to add "?" to histogram constant but then I receive error:

'init' is unavailable: use 'withMemoryRebound(to:capacity:_)' to temporarily view memory as another layout-compatible type.

there is some compiler suggestion but I have no idea how can use it.

回答1:

Cast to UnsafeMutablePointer<vImagePixelCount>?:

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

Complete, compiling example:

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

来源：https://stackoverflow.com/questions/40562889/drawing-histogram-of-cgimage-in-swift-3