UIColour contrast and luminance

contrast.swift

/// Rip off of the JavaScript implementation of:
/// https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js
extension UIColor {
    
    typealias Contrast = (ratio: CGFloat, error: CGFloat, min: CGFloat, max: CGFloat)
    
    /// Range: 1 ... 21
    func contrast(_ other: UIColor) -> Contrast {
        var other = other
        
        // Formula: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
        guard alpha < 1 else {
            if other.alpha < 1 {
                other = other.overlayOn(self)
            }
            
            let l1 = self.luminance + 0.05
            let l2 = other.luminance + 0.05
            var ratio = l1/l2
            
            if l2 > l1 {
                ratio = 1 / ratio
            }
            
            return (
                ratio: ratio,
                error: 0,
                min: ratio,
                max: ratio
            )
        }
        
        // If we’re here, it means we have a semi-transparent background
        // The text color may or may not be semi-transparent, but that doesn't matter
        let onBlack = self.overlayOn(.black)
        let onWhite = self.overlayOn(.white)
        let contrastOnBlack = onBlack.contrast(other).ratio
        let contrastOnWhite = onWhite.contrast(other).ratio
        
        let minContrast: CGFloat
        if onBlack.luminance > other.luminance {
            minContrast = contrastOnBlack
        } else if onWhite.luminance < other.luminance {
            minContrast = contrastOnWhite
        } else {
            minContrast = 1
        }
        
        let maxContrast = Swift.max(contrastOnBlack, contrastOnWhite)
        
        return (
            ratio: (minContrast + maxContrast) / 2.0,
            error: (maxContrast - minContrast) / 2.0,
            min: minContrast,
            max: maxContrast
        )
    }
    
    /// Computed property that returns the RGBA components of this UIColor instance.
    var rgbaComponents: (red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat) {
        var redRational: CGFloat = 0.0
        var greenRational: CGFloat = 0.0
        var blueRational: CGFloat = 0.0
        var alphaRational: CGFloat = 0.0
        
        getRed(&redRational, green: &greenRational, blue: &blueRational, alpha: &alphaRational)
                
        return (redRational, greenRational, blueRational, alphaRational)
    }
    
    /// Range: 0 ... 1
    var luminance: CGFloat {
        func magic(_ v: CGFloat) -> CGFloat {
            assert(v <= 1.0, "Invalid number")
            return v <= 0.03928
                ? v / 12.92
                : pow((v + 0.055) / 1.055, 2.4)
        }
        let components = rgbaComponents
        return magic(components.red) * 0.2126
            + magic(components.green) * 0.7152
            + magic(components.blue) * 0.0722
    }
    
    var alpha: CGFloat { self.cgColor.alpha }
        
    func overlayOn(_ other: UIColor) -> UIColor {
        guard alpha < 1 else { return self }
        let this = self.rgbaComponents
        let other = other.rgbaComponents
        return UIColor(
            red: (this.red * this.alpha) + (other.red * other.alpha * (1 - this.alpha)),
            green: (this.green * this.alpha) + (other.green * other.alpha * (1 - this.alpha)),
            blue: (this.blue * this.alpha) + (other.blue * other.alpha * (1 - this.alpha)),
            alpha: this.alpha + (other.alpha * (1 - this.alpha))
        )
    }
}

// Example
UIColor(red: 1, green: 1, blue: 1, alpha: 1).contrast(UIColor(red: 1, green: 1, blue: 0, alpha: 1)) // 1.074 for yellow
UIColor(red: 1, green: 1, blue: 1, alpha: 1).contrast(UIColor(red: 0, green: 0, blue: 1, alpha: 1)) // 8.592 for blue
// Reversed
UIColor(red: 1, green: 1, blue: 0, alpha: 1).contrast(UIColor(red: 1, green: 1, blue: 1, alpha: 1)) // 1.074 for yellow
UIColor(red: 0, green: 0, blue: 1, alpha: 1).contrast(UIColor(red: 1, green: 1, blue: 1, alpha: 1)) // 8.592 for blue
// Alpha
UIColor(red: 0, green: 0, blue: 1, alpha: 1).contrast(UIColor(red: 1, green: 1, blue: 1, alpha: 0.5)) // 2.625 for blue