How do I draw on an image in Swift?

Question

I need to be able to programmatically draw on an image, and save that image for later use. Say, draw a line on specific x and y coordinates on the image, save the image, and

Answer 1

All you need to do is create and get an Image Context object and access all its powerful drawing methods. You can learn more about the CGContext object features here.

This function draws a line and a circle on an UIImage and returns the modified image:

Swift 4

func drawOnImage(_ image: UIImage) -> UIImage {
     
     // Create a context of the starting image size and set it as the current one
     UIGraphicsBeginImageContext(image.size)
     
     // Draw the starting image in the current context as background       
     image.draw(at: CGPoint.zero)

     // Get the current context
     let context = UIGraphicsGetCurrentContext()!

     // Draw a red line
     context.setLineWidth(2.0)
     context.setStrokeColor(UIColor.red.cgColor)
     context.move(to: CGPoint(x: 100, y: 100))
     context.addLine(to: CGPoint(x: 200, y: 200))
     context.strokePath()
     
     // Draw a transparent green Circle
     context.setStrokeColor(UIColor.green.cgColor)
     context.setAlpha(0.5)
     context.setLineWidth(10.0)
     context.addEllipse(in: CGRect(x: 100, y: 100, width: 100, height: 100))
     context.drawPath(using: .stroke) // or .fillStroke if need filling
     
     // Save the context as a new UIImage
     let myImage = UIGraphicsGetImageFromCurrentImageContext()
     UIGraphicsEndImageContext()
     
     // Return modified image
     return myImage
}