Question

I may point out that Drawing and Rendering in Objective-C is my weakness. Now, here’s my problem.

I want to add a ‘Day/Night’ feature to my game. It has got lots of objects on a map. Every object is a UIView containing some data in variables and some UIImageViews: the sprite, and some of the objects have a hidden ring (used to show selection).

I want to be able to darken the content of the UIView, but I can’t figure out how. The sprite is a PNG with transparency. I’ve just managed to add a black rectangle behind the sprite using this:

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextSaveGState(ctx);

CGContextSetRGBFillColor(ctx, 0, 0, 0, 0.5);
CGContextFillRect(ctx, rect);
CGContextRestoreGState(ctx);

As I’ve read, this should be done in the drawRect method. Help please!

If you want to understand better my scenario, the App where I’m trying to do this is called ‘Kipos’, at the App Store.

Answer 1

Floris497’s approach is a good strategy for a blanket darkening for more than one image at a time (probably more what you’re after in this case). But here’s a general purpose method to generate darker UIImages (while respecting alpha pixels):

+ (UIImage *)darkenImage:(UIImage *)image toLevel:(CGFloat)level
{
    // Create a temporary view to act as a darkening layer
    CGRect frame = CGRectMake(0.0, 0.0, image.size.width, image.size.height);
    UIView *tempView = [[UIView alloc] initWithFrame:frame];
    tempView.backgroundColor = [UIColor blackColor];
    tempView.alpha = level;

    // Draw the image into a new graphics context
    UIGraphicsBeginImageContext(frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [image drawInRect:frame];

    // Flip the context vertically so we can draw the dark layer via a mask that
    // aligns with the image's alpha pixels (Quartz uses flipped coordinates)
    CGContextTranslateCTM(context, 0, frame.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);
    CGContextClipToMask(context, frame, image.CGImage);
    [tempView.layer renderInContext:context];

    // Produce a new image from this context
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    UIImage *toReturn = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    UIGraphicsEndImageContext();
    [tempView release];
    return toReturn;
}