New answers tagged overlay
BgMask is a script that does what you want. It has automatic and manual background extraction mode. Load your images as layers and do Extract Background, and if that leaves artifacts, use manual background extraction tool. See also a video tutorial.
I don't know of any general way to do that, and indeed, in some cases an automatic solution might not even be possible without some way to distinguish the text from the background. (You could, for example, decide that the color a given pixel had in a majority of the images was the background, but that could fail for pixels that were actually covered by text ...
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