How do I get a combined effect of
diff -r left_dir right_dir and
diff <(cat left_file | myfilter) <(cat right_file | myfilter) without much scripting?
If your file names are simple (no whitespace etc.), you can use
If this is already "much scripting" and there are still problems (whitespace in file names, non-existing files on one side, etc.), you are probably out of luck.