How can I set the Alpha of a UIImage?

Question

When I combine two UIImage's (using drawinrect:) they are both the same alpha even though one is supposed to be less than 1.0. How can I change the alpha of specifically a UIImage?

Answer 1

You can't change the alpha of a UIImage. You could draw it with alpha to a new context and get a new image from that. Or you could extract the CGImage, then extract the data, then adjust the alpha bytes, then create a new CGImage from the data and a new UIImage from the CGImage.

But in this case, just use drawInRect:blendMode:alpha: instead of drawInRect:.

Answer 2

Here is UIImage Category.

usage >>

UIImage * imgNew = [imgOld cloneWithAlpha:.3];

code >>

- (UIImage *)cloneWithAlpha:(CGFloat) alpha {
        UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0f);

        CGContextRef ctx = UIGraphicsGetCurrentContext();
        CGRect area = CGRectMake(0, 0, self.size.width, self.size.height);

        CGContextScaleCTM(ctx, 1, -1);
        CGContextTranslateCTM(ctx, 0, -area.size.height);

        CGContextSetBlendMode(ctx, kCGBlendModeMultiply);

        CGContextSetAlpha(ctx, alpha);

        CGContextDrawImage(ctx, area, self.CGImage);

        UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

        UIGraphicsEndImageContext();

        return newImage;
    }

ref >> How to set the opacity/alpha of a UIImage?

Answer 3

If the UIImage is being displayed inside a UIImageView you can set the alpha property on the UIImageView.