Compute the histogram of an image using vImageHistogramCalculation in swift - ios

Question

I'm trying to compute the histogram of an image using Accelerate vImageHistogramCalculation_ARGBFFFF function, but I'm getting a vImage_Error of type kvImageNullPointerArgument (error code is -21772).
This is the exact same question, but I'm working in Swift: Compute the histogram of an image using vImageHistogramCalculation
// Get CGImage from UIImage
var image:UIImage = UIImage(named: "happiness1")!
var img:CGImageRef = image.CGImage
// Create vImage_Buffer with data from CGImageRef
var inProvider:CGDataProviderRef = CGImageGetDataProvider(img)
var inBitmapData:CFDataRef = CGDataProviderCopyData(inProvider)
// The next three lines set up the inBuffer object
var height:vImagePixelCount = CGImageGetHeight(img)
var width:vImagePixelCount = CGImageGetWidth(img)
var rowBytes:UInt = CGImageGetBytesPerRow(img)
var data:UnsafePointer<Void> = UnsafePointer<Void>(CFDataGetBytePtr(inBitmapData))
// Setup inBuffer
var inBuffer = vImage_Buffer(data: &data, height: height, width: width, rowBytes: rowBytes)
var histogram_entries:UInt32 = 4
var minVal:Pixel_F = 0
var maxVal:Pixel_F = 255
//let flags:vImage_Flags = kvImageNoFlags = 0
var histogram = UnsafeMutablePointer<UnsafeMutablePointer<vImagePixelCount>>()
var error:vImage_Error = vImageHistogramCalculation_ARGBFFFF(&inBuffer, histogram, histogram_entries, minVal, maxVal, 0)
println(error)
The problem is in the histogram variable, I need to recreate something like this:
// create an array of four histograms with eight entries each.
vImagePixelCount histogram[4][8] = {{0}};
// vImageHistogramCalculation requires an array of pointers to the histograms.
vImagePixelCount *histogramPointers[4] = { &histogram[0][0], &histogram[1][0], &histogram[2][0], &histogram[3][0] };
vImage_Error error = vImageHistogramCalculation_ARGBFFFF(&inBuffer, histogramPointers, 8, 0, 255, kvImageNoFlags);
// You can now access bin j of the histogram for channel i as histogram[i][j].
// The storage for the histogram will be cleaned up when execution leaves the
// current lexical block.
Suggestion?

Answer 1

I've implemented vImageHistogramCalculation_ARGB8888 as an extension to UIImage in Swift with the following:
func SIHistogramCalculation() -> (alpha: [UInt], red: [UInt], green: [UInt], blue: [UInt]) {
let imageRef = self.CGImage
let inProvider = CGImageGetDataProvider(imageRef)
let inBitmapData = CGDataProviderCopyData(inProvider)
var inBuffer = vImage_Buffer(data: UnsafeMutablePointer(CFDataGetBytePtr(inBitmapData)), height: UInt(CGImageGetHeight(imageRef)), width: UInt(CGImageGetWidth(imageRef)), rowBytes: CGImageGetBytesPerRow(imageRef))
var alpha = [UInt](count: 256, repeatedValue: 0)
var red = [UInt](count: 256, repeatedValue: 0)
var green = [UInt](count: 256, repeatedValue: 0)
var blue = [UInt](count: 256, repeatedValue: 0)
var alphaPtr = UnsafeMutablePointer<vImagePixelCount>(alpha)
var redPtr = UnsafeMutablePointer<vImagePixelCount>(red)
var greenPtr = UnsafeMutablePointer<vImagePixelCount>(green)
var bluePtr = UnsafeMutablePointer<vImagePixelCount> (blue)
var rgba = [redPtr, greenPtr, bluePtr, alphaPtr]
var histogram = UnsafeMutablePointer<UnsafeMutablePointer<vImagePixelCount>>(rgba)
var error = vImageHistogramCalculation_ARGB8888(&inBuffer, histogram, UInt32(kvImageNoFlags))
return (alpha, red, green, blue)
}
(Taken from https://github.com/FlexMonkey/ShinpuruImage)

Answer 2

For Swift 5, you need to explicitly let the compiler know that your pointers are optional. Change your UnsafeMutablePointer declarations to the following:
Swift 5 version:
let redPtr = red.withUnsafeMutableBufferPointer { $0.baseAddress }
let greenPtr = green.withUnsafeMutableBufferPointer { $0.baseAddress }
let bluePtr = blue.withUnsafeMutableBufferPointer { $0.baseAddress }
let alphaPtr = alphaChannel.withUnsafeMutableBufferPointer { $0.baseAddress }
let histogram = UnsafeMutablePointer<UnsafeMutablePointer<vImagePixelCount>?>.allocate(capacity: 4)
histogram[0] = redPtr
histogram[1] = greenPtr
histogram[2] = bluePtr
histogram[3] = alphaPtr
let error:vImage_Error = vImageHistogramCalculation_ARGB8888(&inBuffer, histogram, UInt32(kvImageNoFlags))
Swift 4 version:
let redPtr: UnsafeMutablePointer<vImagePixelCount>? = UnsafeMutablePointer(mutating: red)
let greenPtr: UnsafeMutablePointer<vImagePixelCount>? = UnsafeMutablePointer(mutating:green)
let bluePtr: UnsafeMutablePointer<vImagePixelCount>? = UnsafeMutablePointer(mutating:blue)
let alphaPtr: UnsafeMutablePointer<vImagePixelCount>? = UnsafeMutablePointer(mutating:alpha)

Answer 3

Today, I write the code to analyze photo's RGB histogram. it's working now.
func getHistogram(_ image: UIImage) -> (alpha: [UInt], red: [UInt], green: [UInt], blue: [UInt]) {
guard
let cgImage = image.cgImage,
var imageBuffer = try? vImage_Buffer(cgImage: cgImage)
else {
return nil
}
defer {
imageBuffer.free()
}
var redArray: [vImagePixelCount] = Array(repeating: 0, count: 256)
var greenArray: [vImagePixelCount] = Array(repeating: 0, count: 256)
var blueArray: [vImagePixelCount] = Array(repeating: 0, count: 256)
var alphaArray: [vImagePixelCount] = Array(repeating: 0, count: 256)
var error: vImage_Error = kvImageNoError
redArray.withUnsafeMutableBufferPointer { rPointer in
greenArray.withUnsafeMutableBufferPointer { gPointer in
blueArray.withUnsafeMutableBufferPointer { bPointer in
alphaArray.withUnsafeMutableBufferPointer { aPointer in
var histogram = [ rPointer.baseAddress, gPointer.baseAddress, bPointer.baseAddress, aPointer.baseAddress ]
histogram.withUnsafeMutableBufferPointer { hPointer in
if let hBaseAddress = hPointer.baseAddress {
error = vImageHistogramCalculation_ARGB8888(&imageBuffer, hBaseAddress, vNoFlags)
}
}
}
}
}
}
guard error == kvImageNoError else {
printVImageError(error: error)
return nil
}
return (alphaArray, redArray, greenArray, blueArrat)
}