The Color Cube Image Creator creates the specially formatted images needed to create data for the CIColorCube filter.

ColorCubeImageCreator-swift2.swift

//
//  ColorCubeImageCreator.swift
//  ColorCubeImageCreator
//
//  Created by Joshua Sullivan on 4/25/16.
//  Copyright © 2016 Joshua Sullivan. All rights reserved.
//

import UIKit

public final class ColorCubeImageCreator {
    
    enum Dimension: Int {
        /// A very small color cube. May exhibit posterization.
        case four = 4
        
        /// This size is good enough for many applications using noisy or lower-quality input.
        case sixteen = 16
        
        /// Higher quality. There is rarely a need to go beyond this setting.
        case sixtyFour = 64
        
        /// Excessive quality. Image is 4096x4096 and almost 35MB.
        case twoHundredFiftySix = 256
    }
    
    /// Creates a reference color cube image for the indicated size and writes it to disk.
    /// It is necessary to write to disk as part of this method in order to avoid either
    /// leaking the image buffer or having it garbage collected before the file can be written.
    ///
    /// - Parameter size: A value from the `ColorCubeImageCreator.Dimension` enum.
    /// - Parameter saveLocation: An optional URL for where to store the created image.
    ///    If left `nil` the method will place the image in the Documents folder in the app sandbox.
    /// - Returns: A Bool indicating the success or failure of the operation.
    static func createColorCube(size: Dimension, saveLocation: NSURL?) -> Bool {
        let cubeSize = size.rawValue
        
        /// The total number of pixels we'll need to represnt all points in the cube.
        let pixels = cubeSize * cubeSize * cubeSize
        
        /// The square dimensions of the image that will store exactly enough pixels.
        /// This is why I limit the available dimensions. The other powers of 2 have
        /// non-integral square roots.
        let imageSize = Int(sqrt(Double(pixels)))
        
        /// We're only encoding RGB. No alpha.
        let channels = 3
        
        /// A UInt8 is pretty obviously single-byte.
        let bytesPerChannel = 1
        
        /// The total number of bytes we will need to encode all of the data.
        let memorySize = pixels * channels * bytesPerChannel
        
        /// Pre-calculate the square of the cube size.
        let cubeSizeSquared = cubeSize * cubeSize
        
        /// This is the storage we'll be writing our RGB values to.
        let imageBuffer = UnsafeMutablePointer<UInt8>.alloc(memorySize)
        
        // For the 2 smallest sizes, the dealloc command can have the effect of destroying the bytes
        // backing the image before it can be written to disk, resulting in a corrupted image. That 
        // is why we save the image as part of this block.
        defer { imageBuffer.dealloc(memorySize) }
        
        /// The amount to vary the red, green, and blue channel values by at each step of the calculation.
        let colorStep = 255.0 / Float(cubeSize - 1)
        
        /// The offset in the imageBuffer we're working on.
        var offset = 0
        for i in 0..<pixels {
            offset = i * channels
            
            // Red value
            imageBuffer[offset + 0] = UInt8(round(Float(i % cubeSize) * colorStep))
            
            // Green value
            imageBuffer[offset + 1] = UInt8(round(Float((i / cubeSize) % cubeSize) * colorStep))
            
            // Blue value
            imageBuffer[offset + 2] = UInt8(round(Float(i / cubeSizeSquared) * colorStep))
        }
        
        /// Data provide created with the calculated values in the imageBuffer.
        let dataProvider = CGDataProviderCreateWithData(nil, imageBuffer, memorySize, nil)
        
        // The following are a bunch of setup values for the CGImageCreate() function.
        let bitsPerComponent = 8 * bytesPerChannel
        let bitsPerPixel = channels * bitsPerComponent
        let bytesPerRow = imageSize * channels
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let alphaInfo = CGImageAlphaInfo.None.rawValue
        let bitmapInfo: CGBitmapInfo = [.ByteOrderDefault, CGBitmapInfo(rawValue: alphaInfo)]
        let renderingIntent: CGColorRenderingIntent = .RenderingIntentDefault
        
        // Attempt to process the buffer of bytes into an image.
        guard let imageRef = CGImageCreate(imageSize, imageSize, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpace, bitmapInfo, dataProvider, nil, false, renderingIntent) else {
            assertionFailure("Unable to create CGImage.")
            return false
        }
        
        /// The UIImage created from the buffer data.
        let cubeImage = UIImage(CGImage: imageRef)
        
        /// The PNG representation of the `cubeImage`.
        let imageData = UIImagePNGRepresentation(cubeImage)
        
        // Check that we can establish a URL for saving.
        guard let imageURL = saveLocation ?? defaultLocationForSize(size) else {
            assertionFailure("Can't access save location.")
            return false
        }
        // Try to write the image to disk.
        do {
            try imageData?.writeToURL(imageURL, options: [])
            print("Image successfully written to:", imageURL.absoluteString)
            return true
        } catch let error {
            print("Failed to write image:", error)
            return false
        }
    }
    
    /// Generates default URLs for the various Dimension values.
    private static func defaultLocationForSize(size: Dimension) -> NSURL? {
        let imageName = "ColorCubeReferenceImage\(size.rawValue).png"
        guard let docsDir = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first else {
            assertionFailure("Can't get documents directory!")
            return nil
        }
        let imageURL = docsDir.URLByAppendingPathComponent(imageName)
        return imageURL
    }
}

ColorCubeImageCreator-swift3.swift

//
//  ColorCubeImageCreator.swift
//  ColorCubeImageCreator
//
//  Created by Joshua Sullivan on 10/01/16.
//  Copyright © 2016 Joshua Sullivan. All rights reserved.
//

import UIKit

public final class ColorCubeImageCreator {
    
    enum Dimension: Int {
        /// A very small color cube. May exhibit posterization.
        case four = 4
        
        /// This size is good enough for many applications using noisy or lower-quality input.
        case sixteen = 16
        
        /// Higher quality. There is rarely a need to go beyond this setting.
        case sixtyFour = 64
        
        /// Excessive quality. Image is 4096x4096 and almost 35MB.
        case twoHundredFiftySix = 256
    }
    
    /// Creates a reference color cube image for the indicated size and writes it to disk.
    /// It is necessary to write to disk as part of this method in order to avoid either
    /// leaking the image buffer or having it garbage collected before the file can be written.
    ///
    /// - Parameter size: A value from the `ColorCubeImageCreator.Dimension` enum.
    /// - Parameter saveLocation: An optional URL for where to store the created image.
    ///    If left `nil` the method will place the image in the Documents folder in the app sandbox.
    /// - Returns: A Bool indicating the success or failure of the operation.
    static func createColorCube(size: Dimension, saveLocation: URL?) -> Bool {
        let cubeSize = size.rawValue
        
        /// The total number of pixels we'll need to represnt all points in the cube.
        let pixels = cubeSize * cubeSize * cubeSize
        
        /// The square dimensions of the image that will store exactly enough pixels.
        /// This is why I limit the available dimensions. The other powers of 2 have
        /// non-integral square roots.
        let imageSize = Int(sqrt(Double(pixels)))
        
        /// We're only encoding RGB. No alpha.
        let channels = 3
        
        /// A UInt8 is pretty obviously single-byte.
        let bytesPerChannel = 1
        
        /// The total number of bytes we will need to encode all of the data.
        let memorySize = pixels * channels * bytesPerChannel
        
        /// Pre-calculate the square of the cube size.
        let cubeSizeSquared = cubeSize * cubeSize
        
        /// This is the storage we'll be writing our RGB values to.
        let imageBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: memorySize)
        
        // For the 2 smallest sizes, the dealloc command can have the effect of destroying the bytes
        // backing the image before it can be written to disk, resulting in a corrupted image. That
        // is why we save the image as part of this block.
        defer { imageBuffer.deallocate(capacity: memorySize) }
        
        /// The amount to vary the red, green, and blue channel values by at each step of the calculation.
        let colorStep = 255.0 / Float(cubeSize - 1)
        
        /// The offset in the imageBuffer we're working on.
        var offset = 0
        for i in 0..<pixels {
            offset = i * channels
            
            // Red value
            imageBuffer[offset + 0] = UInt8(round(Float(i % cubeSize) * colorStep))
            
            // Green value
            imageBuffer[offset + 1] = UInt8(round(Float((i / cubeSize) % cubeSize) * colorStep))
            
            // Blue value
            imageBuffer[offset + 2] = UInt8(round(Float(i / cubeSizeSquared) * colorStep))
        }
        
        /// Data provide created with the calculated values in the imageBuffer.
        
        let callback: CGDataProviderReleaseDataCallback = { _,_,_ in }
        guard let dataProvider = CGDataProvider(dataInfo: nil, data: imageBuffer, size: memorySize, releaseData: callback) else {
            preconditionFailure("Couldn't create CGDataProvider.")
        }
        
        // The following are a bunch of setup values for the CGImageCreate() function.
        let bitsPerComponent = 8 * bytesPerChannel
        let bitsPerPixel = channels * bitsPerComponent
        let bytesPerRow = imageSize * channels
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let alphaInfo = CGImageAlphaInfo.none.rawValue
        let bitmapInfo: CGBitmapInfo = [CGBitmapInfo(rawValue: alphaInfo)]
        let renderingIntent: CGColorRenderingIntent = .defaultIntent
        
        // Attempt to process the buffer of bytes into an image.
        guard let imageRef = CGImage(width: imageSize, height: imageSize, bitsPerComponent: bitsPerComponent, bitsPerPixel: bitsPerPixel, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo, provider: dataProvider, decode: nil, shouldInterpolate: false, intent: renderingIntent) else {
            assertionFailure("Unable to create CGImage.")
            return false
        }
        
        /// The UIImage created from the buffer data.
        let cubeImage = UIImage(cgImage: imageRef)
        
        /// The PNG representation of the `cubeImage`.
        let imageData = UIImagePNGRepresentation(cubeImage)
        
        // Check that we can establish a URL for saving.
        guard let imageURL = saveLocation ?? defaultLocationForSize(size: size) else {
            assertionFailure("Can't access save location.")
            return false
        }
        // Try to write the image to disk.
        do {
            try imageData?.write(to: imageURL, options: [])
            print("Image successfully written to:", imageURL.absoluteString)
            return true
        } catch let error {
            print("Failed to write image:", error)
            return false
        }
    }
    
    /// Generates default URLs for the various Dimension values.
    private static func defaultLocationForSize(size: Dimension) -> URL? {
        let imageName = "ColorCubeReferenceImage\(size.rawValue).png"
        guard let docsDir = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
            assertionFailure("Can't get documents directory!")
            return nil
        }
        let imageURL = docsDir.appendingPathComponent(imageName)
        return imageURL
    }
}

ViewController.swift

//
//  ViewController.swift
//  ColorCubeImageCreator
//
//  Created by Joshua Sullivan on 4/25/16.
//  Copyright © 2016 Joshua Sullivan. All rights reserved.
//

import UIKit

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let sizes: [ColorCubeImageCreator.Dimension] = [.four, .sixteen, .sixtyFour, .twoHundredFiftySix]
        let results = sizes.map {
            ColorCubeImageCreator.createColorCube($0, saveLocation: nil)
        }
        print("results:", results)
    }
}