TIP #222 Version 1.2: Add [wm attributes -alpha] Attribute on Windows

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes a new controlled attribute -alpha to control toplevel alpha transparency for the wm attributes command on Windows.

Rationale

Windows 2000/XP+ has the ability to create windows with alpha transparency. This attribute would expose control for that functionality for Tk users. This is mostly an eye candy feature, but could be useful for fade-in splash screens or fancy demos.

Specification

A new attribute -alpha will be recognized by wm attributes. It will take a value from 0 (opaque) to 100 (full transparent) that signifies the percentage transparency of the toplevel. Thus, -alpha 10 would make the toplevel 10% transparent (mostly visible).

There are some special considerations for this feature. It only works on Windows 2000/XP+. The code handles the dynamic lookup of the necessary functionality. If you do not have the functionality, then any attempt to set the -alpha value will be ignores, and queries to -alpha will always return 0 (opaque).

Layered windows cannot use the CS_CLASSDC class style, so an alternative class must be made available. This is handled in the implementation transparently, with UpdateWrapper choosing the appropriate class as necessary. Transparent windows will use TkTransparentToplevel as their class name (normal is TkToplevel).

Efforts have been made to reduce calls to UpdateWrapper.

Reference Implementation

See Tk Patch 892194 [1]

The reference patch implements the functionality along with one bug fix to UpdateWrapper - correct setting of the Z order of the window based on the previous wrapper window.

Comments

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Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.