title: AVFoundation how to add watermark with specified time and duration
body: <p>I want to add a watermark to a video using AVFoundation with specified frame and duration. I can't find the answer from Google and tutorial. A Post on StackOverflow <a href="https://stackoverflow.com/questions/16166433/how-to-watermark-your-video-with-different-images-and-different-cmtimes-using-av">How to watermark your video with different images and different CMTimes using AVFoundation</a> doesn't solve the problem. It just change opacity from start time to end time. Do you have any ideas. Thanks!</p>