How do I autocrop a UIImage?

Question

I have a UIImage which contains a shape; the rest is transparent. I\'d like to get another UIImage by cropping out as much of the transparent part as possible, still retain

Answer 1

Improved from @Danny182's answer, i also added white-space (any pixels brighter than 0xe0e0e0) trimming for my own need.

Usage:

let newimage = UIImage(named: "XXX")!.trim()

---

import UIKit

extension UIImage {

    func trim() -> UIImage {
        let newRect = self.cropRect
        if let imageRef = self.cgImage!.cropping(to: newRect) {
            return UIImage(cgImage: imageRef)
        }
        return self
    }

    var cropRect: CGRect {
        let cgImage = self.cgImage
        let context = createARGBBitmapContextFromImage(inImage: cgImage!)
        if context == nil {
            return CGRect.zero
        }

        let height = CGFloat(cgImage!.height)
        let width = CGFloat(cgImage!.width)

        let rect = CGRect(x: 0, y: 0, width: width, height: height)
        context?.draw(cgImage!, in: rect)

        //let data = UnsafePointer(CGBitmapContextGetData(context))
        guard let data = context?.data?.assumingMemoryBound(to: UInt8.self) else {
            return CGRect.zero
        }

        var lowX = width
        var lowY = height
        var highX: CGFloat = 0
        var highY: CGFloat = 0

        let heightInt = Int(height)
        let widthInt = Int(width)
        //Filter through data and look for non-transparent pixels.
        for y in (0 ..< heightInt) {
            let y = CGFloat(y)
            for x in (0 ..< widthInt) {
                let x = CGFloat(x)
                let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */

                if data[Int(pixelIndex)] == 0  { continue } // crop transparent

                if data[Int(pixelIndex+1)] > 0xE0 && data[Int(pixelIndex+2)] > 0xE0 && data[Int(pixelIndex+3)] > 0xE0 { continue } // crop white

                if (x < lowX) {
                    lowX = x
                }
                if (x > highX) {
                    highX = x
                }
                if (y < lowY) {
                    lowY = y
                }
                if (y > highY) {
                    highY = y
                }

            }
        }

        return CGRect(x: lowX, y: lowY, width: highX - lowX, height: highY - lowY)
    }

    func createARGBBitmapContextFromImage(inImage: CGImage) -> CGContext? {

        let width = inImage.width
        let height = inImage.height

        let bitmapBytesPerRow = width * 4
        let bitmapByteCount = bitmapBytesPerRow * height

        let colorSpace = CGColorSpaceCreateDeviceRGB()

        let bitmapData = malloc(bitmapByteCount)
        if bitmapData == nil {
            return nil
        }

        let context = CGContext (data: bitmapData,
                                 width: width,
                                 height: height,
                                 bitsPerComponent: 8,      // bits per component
            bytesPerRow: bitmapBytesPerRow,
            space: colorSpace,
            bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue)

        return context
    }
}