Exactly what does this function do?
func CMVideoFormatDescriptionGetCleanAperture()
It is in Apple's own SquareCam and quite a lot of other sample codes out there has this function too (copy&paste?). Since Apple's documentation isn't helping me much:
Discussion
The clean aperture is a rectangle that defines the portion of the encoded pixel dimensions that represents image data valid for display.
Can anyone explain it in simpler words?
It's a sub-region of the video frame where the image is "clean". It's better explained with some pictures. Here's the guide from Final Cut Pro.