Swift: distort a UIView to fit arbitrary corners

UIView+distort.swift

import UIKit

extension UIView {
    /// Updates `anchorPoint`, `frame`, and `layer.transform` to fit arbitrary corners.
    /// Based on https://stackoverflow.com/questions/9470493/transforming-a-rectangle-image-into-a-quadrilateral-using-a-catransform3d/39981054#39981054
    func distort(topLeft tl: CGPoint, topRight tr: CGPoint, bottomLeft bl: CGPoint, bottomRight br: CGPoint) {
        // Frame ----------------------------------------------------------- /
        let xmin = min(tr.x, tl.x, bl.x, br.x)
        let ymin = min(tr.y, tl.y, bl.y, br.y)
        let xmax = max(tr.x, tl.x, bl.x, br.x)
        let ymax = max(tr.y, tl.y, bl.y, br.y)
        self.layer.anchorPoint = .zero
        self.layer.transform = CATransform3DIdentity
        self.frame = CGRect(
            x: xmin,
            y: ymin,
            width: xmax - xmin,
            height: ymax - ymin
        )
        
        // Transform ------------------------------------------------------- //
        let rect = self.bounds
        let XX = rect.origin.x
        let YY = rect.origin.y
        let WW = rect.size.width
        let HH = rect.size.height
        
        let x1a = tl.x - xmin
        let y1a = tl.y - ymin
        let x2a = tr.x - xmin
        let y2a = tr.y - ymin
        let x3a = bl.x - xmin
        let y3a = bl.y - ymin
        let x4a = br.x - xmin
        let y4a = br.y - ymin
        
        let y21 = y2a - y1a
        let y32 = y3a - y2a
        let y43 = y4a - y3a
        let y14 = y1a - y4a
        let y31 = y3a - y1a
        let y42 = y4a - y2a
        
        let a = -HH * (x2a*x3a*y14 + x2a*x4a*y31 - x1a*x4a*y32 + x1a*x3a*y42)
        let b = WW * (x2a*x3a*y14 + x3a*x4a*y21 + x1a*x4a*y32 + x1a*x2a*y43)
        
        let c0 = -HH * WW * x1a * (x4a*y32 - x3a*y42 + x2a*y43)
        let cx = HH * XX * (x2a*x3a*y14 + x2a*x4a*y31 - x1a*x4a*y32 + x1a*x3a*y42)
        let cy = -WW * YY * (x2a*x3a*y14 + x3a*x4a*y21 + x1a*x4a*y32 + x1a*x2a*y43)
        let c = c0 + cx + cy
        let d = HH * (-x4a*y21*y3a + x2a*y1a*y43 - x1a*y2a*y43 - x3a*y1a*y4a + x3a*y2a*y4a)
        let e = WW * (x4a*y2a*y31 - x3a*y1a*y42 - x2a*y31*y4a + x1a*y3a*y42)
        
        let f0 = -WW * HH * (x4a * y1a * y32 - x3a * y1a * y42 + x2a * y1a * y43)
        let fx = HH * XX * (x4a * y21 * y3a - x2a * y1a * y43 - x3a * y21 * y4a + x1a * y2a * y43)
        let fy = -WW * YY * (x4a * y2a * y31 - x3a * y1a * y42 - x2a * y31 * y4a + x1a * y3a * y42)
        let f = f0 + fx + fy
        let g = HH * (x3a * y21 - x4a * y21 + (-x1a + x2a) * y43)
        let h = WW * (-x2a * y31 + x4a * y31 + (x1a - x3a) * y42)
        
        let iy = WW * YY * (x2a * y31 - x4a * y31 - x1a * y42 + x3a * y42)
        let ix = HH * XX * (x4a * y21 - x3a * y21 + x1a * y43 - x2a * y43)
        let i0 = HH * WW * (x3a * y42 - x4a * y32 - x2a * y43)
        var i = i0 + ix + iy
        let kEpsilon: CGFloat = 0.0001
        if abs(i) < kEpsilon {
            i = kEpsilon * (i > 0 ? 1 : -1)
        }
        
        self.layer.transform = CATransform3D(
            m11: a/i, m12: d/i, m13: 0, m14: g/i,
            m21: b/i, m22: e/i, m23: 0, m24: h/i,
            m31: 0, m32: 0, m33: 1, m34: 0,
            m41: c/i, m42: f/i, m43: 0, m44: 1.0
        )
    }
    
    /// Resets the transformation.
    func undistort(frame: CGRect) {
        self.layer.transform = CATransform3DIdentity
        self.frame = frame
    }
}