"Fake" blurred window transparency with almost zero GPU cycles

[Seven Systems]

Hello everyone. First post!

Thank you for the amazing work that everyone is doing on the KDE apps
and Plasma.

I've come across a potentially novel strategy for simulating a global
blurred transparency effect (similar to macOS Finder etc.) for the
entire desktop and all windows. The implementation is resource-
efficient, requiring minimal GPU usage and trivial coding effort.

It's so simple that someone MUST have thought of it before. Yet, I
cannot find any references online.

Screenshot:

https://seven.systems/global-transparency.png

It works like this:

- Draw the wallpaper as normal, with alpha = 1, as the bottom layer
- Draw the desktop and all windows as usual
- Draw a pre-rendered, blurred version of the wallpaper on top of
everything else, with alpha = desired_window_transparency (say, 0.15).
- Done!

This creates a convincing ILLUSION of individually blurred transparent
windows, without the need for dynamic blur or extensive code changes.

Drawbacks:

- Slight reduction of overall window contrast
- Individual areas of windows cannot be excluded
- Excluding individual windows from blurred transparency would require
changing z-order (drawing them on top of the blurred wallpaper copy)

Despite these trade-offs, it's incredibly effective and lightweight,
and probably "good enough" for the majority of real-world use cases
(I've been using this for years and have not come across any
"showstoppers" in practice).

Just wanted to drop this into the wild and capable hands that could
potentially make good use of it!

Hope this is the right place.

Thanks!

Best regards

-- 
Alexander Ewering

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