How to gray scale an image with transparent pixels making all of them opaque?

Question

I'm using this function I found in this post to convert a colored image to a gray scaled one:

func grayScaleImage() -> UIImage {
    let imageRect = CGRectMake(0, 0, self.size.width, self.size.height);
    let colorSpace = CGColorSpaceCreateDeviceGray();

    let width = UInt(self.size.width)
    let height = UInt(self.size.height)
    let context = CGBitmapContextCreate(nil, width, height, 8, 0, colorSpace, .allZeros);
    CGContextDrawImage(context, imageRect, self.CGImage!);

    let imageRef = CGBitmapContextCreateImage(context);
    let newImage = UIImage(CGImage: imageRef)
    return newImage
}

I did some tests with some images that have transparent background, and the result I get is an image with black background. I'd like the transparent/translucent pixels to behave as if the background of the image were white, how could I do that?

Answer 1

To get the transparent part of your image to be white (or any other color), just draw a filled rectangle in the drawing context.

Swift 3:

extension UIImage {
    func grayScaleImage() -> UIImage {
        //let imageRect = CGRectMake(0, 0, self.size.width, self.size.height);
        let imageRect = CGRect(origin: CGPoint.zero, size: self.size)
        let colorSpace = CGColorSpaceCreateDeviceGray();

        let width = Int(self.size.width)
        let height = Int(self.size.height)

        let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: 8, bytesPerRow: 0, space: colorSpace, bitmapInfo: .allZeros)

        // set Fill Color to White (or some other color)
        context?.setFillColor(UIColor.white.cgColor)
        // Draw a white-filled rectangle before drawing your image
        context?.fill(imageRect)

        context?.draw(self.cgImage!, in: imageRect)

        let imageRef = context?.makeImage()
        let newImage = UIImage(cgImage: imageRef!)
        return newImage
    }
}

Swift 2.2:

extension UIImage {
    func grayScaleImage() -> UIImage {
        let imageRect = CGRectMake(0, 0, self.size.width, self.size.height);
        let colorSpace = CGColorSpaceCreateDeviceGray();

        let width = Int(self.size.width)
        let height = Int(self.size.height)
        let context = CGBitmapContextCreate(nil, width, height, 8, 0, colorSpace, .allZeros);

        // set Fill Color to White (or some other color)
        CGContextSetFillColorWithColor(context, UIColor.whiteColor().CGColor)
        // Draw a white-filled rectangle before drawing your image
        CGContextFillRect(context, imageRect)

        CGContextDrawImage(context, imageRect, self.CGImage!);

        let imageRef = CGBitmapContextCreateImage(context);
        let newImage = UIImage(CGImage: imageRef!)
        return newImage
    }
}