Mr. A Mr. A - 24 days ago 15
iOS Question

Drawing histogram of CGImage in Swift 3

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.

Answer

Cast to `UnsafeMutablePointer?:

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