I have a marble rolling around an iPhone screen currently and behind it there is a background. I want the background to be obscured at first so it cannot be seen and as you move the ball around it is slowly revealed in the trail of the ball. A bit like "De Blob" in reverse (Screenshot attached). Does anyone have any idea how I might go about doing this? Obviously it's on the iPhone so in this sense I am somewhat limited.
Thanks in advance!
De Blob:
