iOS: create an darker version of UIImage and leave transparent pixels unchanged?

Question

I found

Create new UIImage by adding shadow to existing UIImage

and

UIImage, is there an easy way to make it darker or all black

But the selected

Answer 1

How about trying a CoreImage Filter?

You could use the CIColorControls filter to adjust the input brightness and contrast to darken the image.

CIContext *context = [CIContext contextWithOptions:nil];
CIImage *inputImage = [[CIImage alloc] initWithImage:sourceImage]; //your input image

CIFilter *filter= [CIFilter filterWithName:@"CIColorControls"];
[filter setValue:inputImage forKey:@"inputImage"];
[filter setValue:[NSNumber numberWithFloat:0.5] forKey:@"inputBrightness"];

// Your output image
UIImage *outputImage = [UIImage imageWithCGImage:[context createCGImage:filter.outputImage fromRect:filter.outputImage.extent]];

Read more about the CIFilter parameters here:

http://developer.apple.com/library/mac/#documentation/graphicsimaging/reference/CoreImageFilterReference/Reference/reference.html%23//apple_ref/doc/filter/ci/CIColorControls