More fun with shaders - Copacity

...Conditional Opacity

This new plugin by Michael Callahan makes parts of the window that are similar to the background colour transparent, leaving things in the foreground as bright as they should be! At the moment it uses black as a background colour, but as the post says, that can change if people like it.

Michael's post explains all very well

Legacy applications don't support transparency. Composition managers work around this by allowing each window to have a global transparency value. However the correct behavior for most windows is to have the background color be translucent and the foreground be opaque so that text is more visible.

The conditional opacity filter works around this by using a fragment shader to only make colors that are close to the background color transparent. The rest of the colors remain opaque. The included screenshot shows a comparison of and emacs window using the copacity filter on the left and normal transparency on the right. The text fade is clearly visible between the two windows.

The plugin is hard coded to use BLACK as the background color pending a review of the usefulness of the plugin and what the user interface should look like. It also doesn't set the transparency of the window but rather preserves the existing value. The current usage is to set a window up with black background and light foreground, then use transset to set the transparency value to the desired amount, then -c to turn on the copacity effect.

And a screenshot (from the original post)