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

Swift 4

extension UIImage {
    
    func cropAlpha() -> UIImage {
        
        let cgImage = self.cgImage!;
        
        let width = cgImage.width
        let height = cgImage.height
        
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let bytesPerPixel:Int = 4
        let bytesPerRow = bytesPerPixel * width
        let bitsPerComponent = 8
        let bitmapInfo: UInt32 = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Big.rawValue
        
        guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo),
            let ptr = context.data?.assumingMemoryBound(to: UInt8.self) else {
                return self
        }
        
        context.draw(self.cgImage!, in: CGRect(x: 0, y: 0, width: width, height: height))
        
        var minX = width
        var minY = height
        var maxX: Int = 0
        var maxY: Int = 0
        
        for x in 1 ..< width {
            for y in 1 ..< height {
                
                let i = bytesPerRow * Int(y) + bytesPerPixel * Int(x)
                let a = CGFloat(ptr[i + 3]) / 255.0
                
                if(a>0) {
                    if (x < minX) { minX = x };
                    if (x > maxX) { maxX = x };
                    if (y < minY) { minY = y};
                    if (y > maxY) { maxY = y};
                }
            }
        }
        
        let rect = CGRect(x: CGFloat(minX),y: CGFloat(minY), width: CGFloat(maxX-minX), height: CGFloat(maxY-minY))
        let imageScale:CGFloat = self.scale
        let croppedImage =  self.cgImage!.cropping(to: rect)!
        let ret = UIImage(cgImage: croppedImage, scale: imageScale, orientation: self.imageOrientation)
        
        return ret;
    }
    
}