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Draw a mask image with Core Graphics API in Swift

MaskImage

Draw a mask image like Left-3 to blend a rounded corner:

func UIGraphicsDrawAntiRoundedCornerImageWithRadius(radius: CGFloat, outerSize: CGSize, innerSize: CGSize, fillColor: UIColor) -> UIImage{  
    UIGraphicsBeginImageContextWithOptions(outerSize, false, UIScreen.mainScreen().scale)
    let currentContext = UIGraphicsGetCurrentContext()
    
    let bezierPath = UIBezierPath()
    
    let xOffset = (outerSize.width - innerSize.width) / 2
    let yOffset = (outerSize.height - innerSize.height) / 2
    
    let hLeftUpPoint = CGPointMake(xOffset + radius, yOffset)
    let hRightUpPoint = CGPointMake(outerSize.width - xOffset - radius, yOffset)
    let hLeftDownPoint = CGPointMake(xOffset + radius, outerSize.height - yOffset)
    
    let vLeftUpPoint = CGPointMake(xOffset, yOffset + radius)
    let vRightDownPoint = CGPointMake(outerSize.width - xOffset, outerSize.height - yOffset - radius)
    
    let centerLeftUp = CGPointMake(xOffset + radius, yOffset + radius)
    let centerRightUp = CGPointMake(outerSize.width - xOffset - radius, yOffset + radius)
    let centerLeftDown = CGPointMake(xOffset + radius, outerSize.height - yOffset - radius)
    let centerRightDown = CGPointMake(outerSize.width - xOffset - radius, outerSize.height - yOffset - radius)
    
    bezierPath.moveToPoint(hLeftUpPoint)
    bezierPath.addLineToPoint(hRightUpPoint)
    bezierPath.addArcWithCenter(centerRightUp, radius: radius, startAngle: CGFloat(M_PI * 3 / 2), endAngle: CGFloat(M_PI * 2), clockwise: true)
    bezierPath.addLineToPoint(vRightDownPoint)
    bezierPath.addArcWithCenter(centerRightDown, radius: radius, startAngle: 0, endAngle: CGFloat(M_PI / 2), clockwise: true)
    bezierPath.addLineToPoint(hLeftDownPoint)
    bezierPath.addArcWithCenter(centerLeftDown, radius: radius, startAngle: CGFloat(M_PI / 2), endAngle: CGFloat(M_PI), clockwise: true)
    bezierPath.addLineToPoint(vLeftUpPoint)
    bezierPath.addArcWithCenter(centerLeftUp, radius: radius, startAngle: CGFloat(M_PI), endAngle: CGFloat(M_PI * 3 / 2), clockwise: true)
    bezierPath.addLineToPoint(hLeftUpPoint)
    bezierPath.closePath()
    
    //If draw drection of outer path is same with inner path, final result is just outer path.
    bezierPath.moveToPoint(CGPointZero)
    bezierPath.addLineToPoint(CGPointMake(0, outerSize.height))
    bezierPath.addLineToPoint(CGPointMake(outerSize.width, outerSize.height))
    bezierPath.addLineToPoint(CGPointMake(outerSize.width, 0))
    bezierPath.addLineToPoint(CGPointZero)
    bezierPath.closePath()
    
    fillColor.setFill()
    bezierPath.fill()
    
    CGContextDrawPath(currentContext, .FillStroke)
    let antiRoundedCornerImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    
    return antiRoundedCornerImage
}

Draw a mask image like Left-2 to blend a rouned corner:

func UIGraphicsDrawAntiRoundedCornerImageWithRadius(radius: CGFloat, rectSize: CGSize, fillColor: UIColor) -> UIImage {
    UIGraphicsBeginImageContextWithOptions(rectSize, false, UIScreen.mainScreen().scale)
    let currentContext = UIGraphicsGetCurrentContext()
    
    let bezierPath = UIBezierPath()
    
    let hLeftUpPoint = CGPointMake(radius, 0)
    let hRightUpPoint = CGPointMake(rectSize.width - radius, 0)
    let hLeftDownPoint = CGPointMake(radius, rectSize.height)
    
    let vLeftUpPoint = CGPointMake(0, radius)
    let vRightDownPoint = CGPointMake(rectSize.width, rectSize.height - radius)
    
    let centerLeftUp = CGPointMake(radius, radius)
    let centerRightUp = CGPointMake(rectSize.width - radius, radius)
    let centerLeftDown = CGPointMake(radius, rectSize.height - radius)
    let centerRightDown = CGPointMake(rectSize.width - radius, rectSize.height - radius)
    
    bezierPath.moveToPoint(hLeftUpPoint)
    bezierPath.addLineToPoint(hRightUpPoint)
    bezierPath.addArcWithCenter(centerRightUp, radius: radius, startAngle: CGFloat(M_PI * 3 / 2), endAngle: CGFloat(M_PI * 2), clockwise: true)
    bezierPath.addLineToPoint(vRightDownPoint)
    bezierPath.addArcWithCenter(centerRightDown, radius: radius, startAngle: 0, endAngle: CGFloat(M_PI / 2), clockwise: true)
    bezierPath.addLineToPoint(hLeftDownPoint)
    bezierPath.addArcWithCenter(centerLeftDown, radius: radius, startAngle: CGFloat(M_PI / 2), endAngle: CGFloat(M_PI), clockwise: true)
    bezierPath.addLineToPoint(vLeftUpPoint)
    bezierPath.addArcWithCenter(centerLeftUp, radius: radius, startAngle: CGFloat(M_PI), endAngle: CGFloat(M_PI * 3 / 2), clockwise: true)
    bezierPath.addLineToPoint(hLeftUpPoint)
    bezierPath.closePath()
    
    //If draw drection of outer path is same with inner path, final result is just outer path.
    bezierPath.moveToPoint(CGPointZero)
    bezierPath.addLineToPoint(CGPointMake(0, rectSize.height))
    bezierPath.addLineToPoint(CGPointMake(rectSize.width, rectSize.height))
    bezierPath.addLineToPoint(CGPointMake(rectSize.width, 0))
    bezierPath.addLineToPoint(CGPointZero)
    bezierPath.closePath()
    
    fillColor.setFill()
    bezierPath.fill()
    
    CGContextDrawPath(currentContext, .FillStroke)
    let antiRoundedCornerImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    
    return antiRoundedCornerImage
}
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