angleGradientImage.swift

angleGradientImage.swift

//: angleGradientImage - Ported to Swift from http://stackoverflow.com/a/33991838/136582

import UIKit
import CoreImage

public func angleGradientImage(size: CGSize, radius: Float?, scale: CGFloat?, colors: (start:UIColor, end:UIColor)?) -> UIImage? {
    let gradientRadius = (radius != nil) ? radius! : Float(min(size.width,size.height) - 2)
    let gradientScale = (scale != nil) ? scale! : UIScreen.mainScreen().scale
    let startColor : CIColor
    let endColor : CIColor
    if let _colors = colors {
        startColor = CIColor(color: _colors.start)
        endColor = CIColor(color: _colors.end)
    }
    else {
        startColor = CIColor(red: 0.5, green: 0.5, blue: 0.5)
        endColor = CIColor(color: UIColor.whiteColor())
    }
    UIGraphicsBeginImageContextWithOptions(size, false, gradientScale)

    guard let contextImage = UIGraphicsGetImageFromCurrentImageContext().CGImage else {
        return nil
    }

    let emptyImage = CIImage(CGImage: contextImage)

    let kernelCode = "kernel vec4 circularGradientKernel(__color startColor, __color endColor, vec2 center, float radius) { \n" +
        "vec2 point = destCoord() - center; \n" +
        "float rsq = point.x * point.x + point.y * point.y; \n" +
        "float theta = mod(atan(point.y, point.x), radians(360.0)); \n" +
        "return (rsq < radius*radius) ? mix(startColor, endColor, 0.5+0.5*cos(4.0*theta)) : vec4(0.0, 0.0, 0.0, 1.0); \n" +
        "}"

    guard let kernel = CIColorKernel(string: kernelCode) else {
        return nil
    }

    let extent = emptyImage.extent
    let point = CGPointMake(CGRectGetMidX(extent), CGRectGetMidY(extent))
    let vector = CIVector(CGPoint: point)

    guard let ciOutput = kernel.applyWithExtent(extent, arguments: [startColor,endColor,vector, gradientRadius]) else {
        return nil
    }
    let gradientImage = UIImage(CIImage: ciOutput, scale: gradientScale, orientation: UIImageOrientation.Up)

    UIGraphicsEndImageContext()

    return gradientImage
}

let gradientImage = angleGradientImage(CGSizeMake(256.0, 256.0), radius: nil, scale: nil, colors:nil)