iOS UIImage how to convert black to transparent programmatically?

Question

I have an icon that's hovering over a button. The icon is black an white and I would like to "Cut" black regions out programmatically.

How can I convert black regions of an icon to transparent programmatically?

Answer 1

You can use Core Image Filters - in particular, the filter CIMaskToAlpha

For detailed instructions on using CIFilters here is Apple's Core Image Programming Guide and a RayWenderlich tutorial

Edit: this filter takes black pixels and makes them completely transparent, and replaces pixels white with opaque, gray with partially transparent, etc.

-(void)setImage:(UIImage *)image_
{

    UIImage *entryImage  = image_;
    CIContext *context = [CIContext contextWithOptions:nil];

    CIImage *image = [CIImage imageWithCGImage:[entryImage CGImage]];
    CIFilter *filter = [CIFilter filterWithName:@"CIMaskToAlpha"];
    [filter setDefaults];
    [filter setValue:image forKey:kCIInputImageKey];
    //    CIImage *result = [filter valueForKey:kCIOutputImageKey];
    CIImage *result = [filter outputImage];

    CGImageRef cgImage = [context createCGImage:result fromRect:[result extent]];

    UIImage *newImage = [UIImage imageWithCGImage:cgImage scale:[entryImage scale] orientation:UIImageOrientationUp];
    CGImageRelease(cgImage);
    [super setImage:newImage];
}