Detect cropping rectangle for UIImage with transparent background

Have you had a chance to see https://gist.github.com/spinogrizz/3549921 ?

it looks like it's exactly what you need.

just so it's not lost, a copy & paste from that page:

- (UIImage *) imageByTrimmingTransparentPixels {
    int rows = self.size.height;
    int cols = self.size.width;
    int bytesPerRow = cols*sizeof(uint8_t);

    if ( rows < 2 || cols < 2 ) {
        return self;
    }

    //allocate array to hold alpha channel
    uint8_t *bitmapData = calloc(rows*cols, sizeof(uint8_t));

    //create alpha-only bitmap context
    CGContextRef contextRef = CGBitmapContextCreate(bitmapData, cols, rows, 8, bytesPerRow, NULL, kCGImageAlphaOnly);

    //draw our image on that context
    CGImageRef cgImage = self.CGImage;
    CGRect rect = CGRectMake(0, 0, cols, rows);
    CGContextDrawImage(contextRef, rect, cgImage);

    //summ all non-transparent pixels in every row and every column
    uint16_t *rowSum = calloc(rows, sizeof(uint16_t));
    uint16_t *colSum = calloc(cols, sizeof(uint16_t));

    //enumerate through all pixels
    for ( int row = 0; row < rows; row++) {
        for ( int col = 0; col < cols; col++)
        {
            if ( bitmapData[row*bytesPerRow + col] ) { //found non-transparent pixel
                rowSum[row]++;
                colSum[col]++;
            }
        }
    }

    //initialize crop insets and enumerate cols/rows arrays until we find non-empty columns or row
    UIEdgeInsets crop = UIEdgeInsetsMake(0, 0, 0, 0);

    for ( int i = 0; i<rows; i++ ) {        //top
        if ( rowSum[i] > 0 ) {
            crop.top = i; break;
        }
    }

    for ( int i = rows; i >= 0; i-- ) {     //bottom
        if ( rowSum[i] > 0 ) {
            crop.bottom = MAX(0, rows-i-1); break;
        }
    }

    for ( int i = 0; i<cols; i++ ) {        //left
        if ( colSum[i] > 0 ) {
            crop.left = i; break;
        }
    }

    for ( int i = cols; i >= 0; i-- ) {     //right
        if ( colSum[i] > 0 ) {
            crop.right = MAX(0, cols-i-1); break;
        }
    }

    free(bitmapData);
    free(colSum);
    free(rowSum);

    if ( crop.top == 0 && crop.bottom == 0 && crop.left == 0 && crop.right == 0 ) {
        //no cropping needed
        return self;
    }
    else {
        //calculate new crop bounds
        rect.origin.x += crop.left;
        rect.origin.y += crop.top;
        rect.size.width -= crop.left + crop.right;
        rect.size.height -= crop.top + crop.bottom;

        //crop it
        CGImageRef newImage = CGImageCreateWithImageInRect(cgImage, rect);

        //convert back to UIImage
        return [UIImage imageWithCGImage:newImage];
    }
}

here it is in 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;
    }
}