How to return UIImage with certain alpha

Question

I am writing a method that takes a image and a alpha value then return a uiimage with that alpha value.

I have found some code like this

+ (UIImage *)imageByApplyingAlpha:(UIImage*) originalImage andAlpha:(CGFloat) alpha {
    UIGraphicsBeginImageContextWithOptions(originalImage.size, NO, 0.0f);

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

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

    CGContextSetBlendMode(ctx, kCGBlendModeMultiply);

    CGContextSetAlpha(ctx, alpha);

    CGContextDrawImage(ctx, area, originalImage.CGImage);

    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return newImage;
}

but this only work on single image. It will generate malformed GIF when I use the returned image to form GIF.

Please help me to find a easy way to get UIImage with alpha.

Thanks all!

Answer 1

Unfortunately, the GIF format doesn't support alpha channels, so you won't be able to set alpha values on one. Transparency in GIF uses index transparency, which is all or nothing.