This is something that is interesting and I am surprised there is no topic or at least not one that I know about and this happens in a few films where shadow details are lost and crushed in to nothing.
I am curious of ways of attempting to fix this problem using various techniques.
Upon looking at Empire strikes back for a long time. I wondered about this and without actually doing any tests. I am assuming that 2 sources could be mixed either 50/50 or 75/25 and color matched to retain shadow detail.
But then I started to think about it differently again. all you want is the bottom end in this case or perhaps if you have the other end of the spectrum blown highlights you would want to only replace highlights from another source and mix. without altering the image in the bottom or middle range.
Now obviously we can match and blend 2 images 50/50 and 25/75 as whole images for instance, but after thinking about this some more I realized the middle of the image is generally the part that is good most of the time when looking at value.
So I want to talk about developing a technique that allows for not mixing an image as a whole in any percentage but mixing an image as either top end complete replacement but geometrically matched, or bottom end replacement but geometrically matched.
Basically my thoughts are on how to develop this technique lie within a few different areas.
Assuming what you want to replace is actually black or white and without any detail. would a bucket fill technique work to chroma key out the black crush or blown highlights? Splitting a clip to individual frames and simply bucket fill the lot with either red / green / blue screen and then keying out replacing the affected areas with the alternate source without crushed black or blown whites.
obviously this could be matted in but without having to get into complex roto-scoping in some instances even if a selected area is colored and then keyed out to reveal underneath it shadow detail from an alternate source or highlights that are not blown from another source.
once what is keyed in to reveal a geometrically matched source underneath either restoring highlights or shadow detail then if detail is also lost in this process then another layer is added of the original source you want to use but opaque to a percentage where detail is restored again? or masked in, and in such a way that it is geometrically common sense around the affected areas of either Black crush or Blown Highlights but this would be a simple mask without having to do precise rotoscope work.
Without testing this does anyone have any thoughts or has anyone ever attempted to restore these types of things before and what technique if you are willing to share did you use to restore detail?
This is all theory at the moment. But aside from simply masking in shadow detail I want to develop a technique that if there is any movie with either Blown Highlights or Crushed Blacks a simple resource that can fix it or tutorial or technique providing another source even if it is in lower quality can yield better results and restore either shadow detail or blown out whites.