compizconfig-settings-manager can control all windows transparency. Googling I've find this simple command make it easier for selecting specific window, photo, video, anything on your desktop, and turn it in crystal effect. Making a shot think very useful for graphic design, video edition or simply fun windows compositions. Only one of multiple possibilities:
Comparing in CAD, impress, base, calc, used as layout template.
Overlapping correction templates for teachers.
Easy accurately check multiple progress bars or indicators together on different windows.
Tansparent background watermarks on screenshots.
Eye-meet differences faster overlapping pics or photos.
Copy cartoons by hand.
Indicate form fields to fill on different documents or grids.
Easily copiying long word chains with no paste or clipboard.
Securely copiying difficult passwords, without password managers.
Easily copy text from poor OCR scan image (old books).
#!/bin/bash sh -c 'xprop -f _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY $(printf 0x%x $((0xffffffff * 50 / 100)))'
In this case, "50 / 100" determine half opacity level. You can return 100% with "100 / 100". Or close window through panel if you are lost.
Also you can convert any window to a real GHOST-window, changing value to "0 / 100". Full transparent window (not minimized). Where is it? Fun! But be careful using this value (0 / 100), specially on panel or icons, they will become fully transparent.
On a mess, reboot and then you'll kill the process.