Core Graphics CGImage mask not affecting

Question

What I'm essentially trying to do is have a text label 'cut' a text-shaped hole through the view. I've tried using self.mask = uiLabel but those refused to position the text correctly so I'm approaching this through Core Graphics.

Here's the code that isn't working (in the draw(_ rect: CGRect)):

    let context = (UIGraphicsGetCurrentContext())!

    // Set mask background color
    context.setFillColor(UIColor.black.cgColor)
    context.fill(rect)

    context.saveGState()


    let paragraphStyle = NSMutableParagraphStyle()
    paragraphStyle.alignment = .center

    let attributes = [
        NSParagraphStyleAttributeName: paragraphStyle,
        NSFontAttributeName: UIFont.systemFont(ofSize: 16, weight: UIFontWeightMedium),
        NSForegroundColorAttributeName: UIColor.white
    ]

    let string = NSString(string: "LOGIN")

    // This wouldn't vertically align so we calculate the string size and create a new rect in which it is vertically aligned
    let size = string.size(attributes: attributes)
    let position = CGRect(
        x: rect.origin.x,
        y: rect.origin.y + (rect.size.height - size.height) / 2,
        width: rect.size.width,
        height: size.height
    )

    context.translateBy(x: 0, y: rect.size.height)
    context.scaleBy(x: 1, y: -1)
    string.draw(
        in: position,
        withAttributes: attributes
    )

    let mask = (context.makeImage())!

    context.restoreGState()

    // Redraw with created mask
    context.clear(rect)

    context.saveGState()
    // !!!! Below line is the problem
    context.clip(to: rect, mask: mask)
    context.restoreGState()

Essentially I've successfully created the code to create a CGImage (the mask variable) which is the mask I want to apply to the whole image.

The marked line when replaced with context.draw(mask, in: rect) (to view the mask) correctly displays. The mask shows (correctly) as:

:

However once I try to apply this mask (using the context.clip(to: rect, mask: mask)) nothing happens!. Actual result:

Desired result is:

but for some reason the mask is not being correctly applied.

---

This code seems like it should work as I've read the docs over and over again. I've additionally tried to create the mask in a separate CGContext which didn't work. Also when I tried to convert the CGImage (mask) to CGColorSpaceCreateDeviceGray() using .copy(colorSpace:), it returned nil. I've been at this for two days so any help is appreciated

Answer 1

I'm not actually sure that you will enjoy the code below. It was made by using the PaintCode. Text will be always at the center of rounded rectangle. Hope it will help you.

Code, put it inside draw(_ rect: CGRect):

    //// General Declarations
    let context = UIGraphicsGetCurrentContext()

    //// Color Declarations - just to make a gradient same as your button have
    let gradientColor = UIColor(red: 0.220, green: 0.220, blue: 0.223, alpha: 1.000)
    let gradientColor2 = UIColor(red: 0.718, green: 0.666, blue: 0.681, alpha: 1.000)
    let gradientColor3 = UIColor(red: 0.401, green: 0.365, blue: 0.365, alpha: 1.000)

    //// Gradient Declarations
    let gradient = CGGradient(colorsSpace: nil, colors: [gradientColor.cgColor, gradientColor3.cgColor, gradientColor2.cgColor] as CFArray, locations: [0, 0.55, 1])!


    //// Subframes
    let group: CGRect = CGRect(x: rect.minX, y: rect.minY, width: rect.width, height: rect.height)


    //// Group
    context.saveGState()
    context.beginTransparencyLayer(auxiliaryInfo: nil)


    //// Rectangle Drawing
    let rectanglePath = UIBezierPath(roundedRect: group, cornerRadius: 5)
    context.saveGState()
    rectanglePath.addClip()
    let rectangleRotatedPath = UIBezierPath()
    rectangleRotatedPath.append(rectanglePath)
    var rectangleTransform = CGAffineTransform(rotationAngle: -99 * -CGFloat.pi/180)
    rectangleRotatedPath.apply(rectangleTransform)
    let rectangleBounds = rectangleRotatedPath.cgPath.boundingBoxOfPath
    rectangleTransform = rectangleTransform.inverted()
    context.drawLinearGradient(gradient,
                               start: CGPoint(x: rectangleBounds.minX, y: rectangleBounds.midY).applying(rectangleTransform),
                               end: CGPoint(x: rectangleBounds.maxX, y: rectangleBounds.midY).applying(rectangleTransform),
                               options: [])
    context.restoreGState()


    //// Text Drawing
    context.saveGState()
    context.setBlendMode(.destinationOut)

    let textRect = group
    let textTextContent = "LOGIN"
    let textStyle = NSMutableParagraphStyle()
    textStyle.alignment = .center
    let textFontAttributes = [
        NSFontAttributeName: UIFont.systemFont(ofSize: 16, weight: UIFontWeightBold),
        NSForegroundColorAttributeName: UIColor.black,
        NSParagraphStyleAttributeName: textStyle,
        ]

    let textTextHeight: CGFloat = textTextContent.boundingRect(with: CGSize(width: textRect.width, height: CGFloat.infinity), options: .usesLineFragmentOrigin, attributes: textFontAttributes, context: nil).height
    context.saveGState()
    context.clip(to: textRect)
    textTextContent.draw(in: CGRect(x: textRect.minX, y: textRect.minY + (textRect.height - textTextHeight) / 2, width: textRect.width, height: textTextHeight), withAttributes: textFontAttributes)
    context.restoreGState()

    context.restoreGState()


    context.endTransparencyLayer()
    context.restoreGState()

Results:

Answer 2

If you want the label to have fully translucent text, you can use blend modes instead of masks.

public class KnockoutLabel: UILabel {
    public override func draw(_ rect: CGRect) {
        let context = UIGraphicsGetCurrentContext()!
        context.setBlendMode(.clear)

        self.drawText(in: rect)
    }
}

Make sure to set isOpaque to false though; by default the view assumes it is opaque, since you use an opaque background color.

Answer 3

Cutting image into a rectangle

Inspired from the answer of @Woof

A little different but I needed a similar functionality. I wanted to cut the shape of an image into the background. Basically this can be used to invert an image as well.

I tried this with .PNG images as they have a clear background.

I draw a coloured rectangle and then using blend modes and clipping I cut the shape of an image into the rectangle. For this I have to convert the UIImage into CGImage to set the colorSpace to linearGray.

func cutImageIntoRectangle(_ image: UIImage) -> UIImage {
        let rect = CGRect(x: 0, y: 0, width: 100, height: 100)
        let renderer = UIGraphicsImageRenderer(size: rect.size)
        let img = renderer.image { ctx in
            let context = ctx.cgContext
            context.setFillColor(UIColor.red.cgColor)
            context.addRect(rect)
            context.fillPath()
            context.setBlendMode(.destinationOut)
            context.clip(to: rect)
            let cgImage = image.cgImage?.copy(colorSpace: CGColorSpace(name: CGColorSpace.linearGray)!)
            context.drawFlipped(cgImage!, in: rect)
        }
        return img
    }

For some reason when copying to CGImage the image is flipped. For this I use the extension method drawFlipped, which I copied from here: https://stackoverflow.com/a/58470440/15006958

extension CGContext {
    /// Draw `image` flipped vertically, positioned and scaled inside `rect`.
    public func drawFlipped(_ image: CGImage, in rect: CGRect) {
        self.saveGState()
        self.translateBy(x: 0, y: rect.origin.y + rect.height)
        self.scaleBy(x: 1.0, y: -1.0)
        self.draw(image, in: CGRect(origin: CGPoint(x: rect.origin.x, y: 0), size: rect.size))
        self.restoreGState()
    }
}