Sometimes I work in low light and I need to invert the color of specific windows to reduce my eye strain. This could be my pdf viewer to web pages for reference work.
I know how to invert the entire screen:
xcalib -invert -alter
But I can't find any man pages, docs or posts how to invert a single window only. I did this before yet cannot find how. Bizarre!
Searching the repos for other xorg tools I found xpdf, it has invert support but won't help for anything other than viewing pdf's. I don't use compositing so please don't suggest Compiz. I use Openbox WM. Thanks all.
Instead of a general all-purpose tool to invert any window, I focused my search on how to invert colors for a Google Chrome tab instead. I found Color Bookmarklets, you drag them to the browser toolbar and hit them to change the page content to invert the colors (invert lightness).