I remember some old video editing programs used a grayscale image to be used as a transition pattern.
Basically what they did was using the different levels of a grayscale image as a step for the transition.
Here is an example. Given the first image, when the program was analyzing the gray level marked with the red line as a threshold, the transition would look like the image below.
Do you know if that can be done in a free program like Davinci Resolve, Hit Film, probably Blender?
And a tip of how can be implemented?
images by pixabay