iphone - UIImage with transparent rounded corners

Question

I'm using following code to add rounded corners to my UIImage, but the problem is that the rounded corners are showing "white" area instead of transparent or "clear". What am i doing wrong here:

- (UIImage *)makeRoundCornerImageWithCornerWidth:(int)cornerWidth cornerHeight:(int)cornerHeight {
    UIImage * newImage = nil;

    if (self != nil)    {
        NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
        int w = self.size.width;
        int h = self.size.height;

        CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
        CGContextRef context = CGBitmapContextCreate(NULL, w, h, 8, 4 * w, colorSpace, kCGImageAlphaPremultipliedFirst);

        CGContextBeginPath(context);
        CGRect rect = CGRectMake(0, 0, self.size.width, self.size.height);
        [self addRoundedRectToPath:context rect:rect width:cornerWidth height:cornerHeight];

        CGContextClosePath(context);
        CGContextClip(context);

        CGContextDrawImage(context, CGRectMake(0, 0, w, h), self.CGImage);
        CGImageRef imageMasked = CGBitmapContextCreateImage(context);
        CGContextRelease(context);
        CGColorSpaceRelease(colorSpace);

        newImage = [[UIImage imageWithCGImage:imageMasked] retain];
        CGImageRelease(imageMasked);

        [pool release];
    }

    return [newImage autorelease];
}


- (void)addRoundedRectToPath:(CGContextRef)context rect:(CGRect)rect width:(float)ovalWidth height:(float)ovalHeight {
    float fw, fh;

    // If the width or height of the corner oval is zero, then it reduces to a right angle,
    // so instead of a rounded rectangle we have an ordinary one.
    if (ovalWidth == 0 || ovalHeight == 0) {
        CGContextAddRect(context, rect);
        return;
    }

    //  Save the context's state so that the translate and scale can be undone with a call
    //  to CGContextRestoreGState.
    CGContextSaveGState(context);

    //  Translate the origin of the contex to the lower left corner of the rectangle.
    CGContextTranslateCTM (context, CGRectGetMinX(rect), CGRectGetMinY(rect));

    //Normalize the scale of the context so that the width and height of the arcs are 1.0
    CGContextScaleCTM (context, ovalWidth, ovalHeight);

    // Calculate the width and height of the rectangle in the new coordinate system.
    fw = CGRectGetWidth (rect) / ovalWidth;
    fh = CGRectGetHeight (rect) / ovalHeight;

    // CGContextAddArcToPoint adds an arc of a circle to the context's path (creating the rounded
    // corners).  It also adds a line from the path's last point to the begining of the arc, making
    // the sides of the rectangle.
    CGContextMoveToPoint(context, fw, fh/2);                // Start at lower right corner
    CGContextAddArcToPoint(context, fw, fh, fw/2, fh, 1);   // Top right corner
    CGContextAddArcToPoint(context, 0, fh, 0, fh/2, 1);     // Top left corner
    CGContextAddArcToPoint(context, 0, 0, fw/2, 0, 1);      // Lower left corner
    CGContextAddArcToPoint(context, fw, 0, fw, fh/2, 1);    // Back to lower right

    // Close the path
    CGContextClosePath(context);

    // Restore the context's state. This removes the translation and scaling
    // but leaves the path, since the path is not part of the graphics state.
    CGContextRestoreGState(context);
}

Answer 1

Here's a simpler formulation using UIKit calls:

- (UIImage*) roundCorneredImage: (UIImage*) orig radius:(CGFloat) r {
    UIGraphicsBeginImageContextWithOptions(orig.size, NO, 0);
    [[UIBezierPath bezierPathWithRoundedRect:(CGRect){CGPointZero, orig.size} 
                                cornerRadius:r] addClip];
    [orig drawInRect:(CGRect){CGPointZero, orig.size}];
    UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return result;
}

Notice the NO parameter - this makes the image context transparent, so the clipped-out region is transparent.