Cropping CIImage with CICrop isn't working properly

Are you sure the output image is the size you are expecting? How are you drawing the output image?

The CICrop filter does not reduce the size of the original image, it just blanks out the content you don't want.

To get the result you want you probably need to just do this:

[image drawAtPoint:NSZeroPoint fromRect:NSMakeRect(150, 150, 300, 300) operation:NSCompositeSourceOver fraction:1.0];

If you want an actual CIImage as output rather than just drawing it, just do this:

CIImage* croppedImage = [image imageByCroppingToRect:CGRectMake(150, 150, 300, 300)];

//you also need to translate the origin   
CIFilter* transform = [CIFilter filterWithName:@"CIAffineTransform"];
NSAffineTransform* affineTransform = [NSAffineTransform transform];
[affineTransform translateXBy:-150.0 yBy:-150.0];
[transform setValue:affineTransform forKey:@"inputTransform"];
[transform setValue:croppedImage forKey:@"inputImage"];
CIImage* transformedImage = [transform valueForKey:@"outputImage"];

It's important to note that the coordinate system of a view is top-left-corner, whereas CIImage is bottom left. This will make you crazy if you don't catch it when you're doing these transforms! This other post describes a one-directional conversion: Changing CGrect value to user coordinate system.