XOR support for mouse cursors

Graeme Gill graeme2 at argyllcms.com
Sun Nov 11 21:42:39 PST 2007

Ben Swartzlander wrote:

> Didn't Microsoft Windows 3.0 have XOR cursors? How is it possible to 
> patent something that already existed in a shipping product at the time? 

The U.S. Patent Office works in mysterious ways ?

Presumably the patent examiner was convinced that something
was slightly different in the 1994 patent from previous
related patents. As I understand it, a patent covers
the claims minus prior art.

> Just so I understand, is it the case that the X.org team has considered 
> implementing XOR cursors, and opted not to purely out of legal fears? Or 
> has it never seriously been considered?

At one stage all the X11 consortium members were threatened by the
holder of the XOR cursor patent, even though the X11 server didn't
actually support an XOR cursor itself. A client application was
the only way of implementing an XOR cursor, simply by using the XOR
rendering mode (one of the 16 possible modes).

Typically X11 doesn't need to use an XOR cursor, since it supports
overlay cursors. XOR cursors are usually regarded as a "trick"
to avoid having to save what's under the cursor, and restore
it every time the cursor moves.

Graeme Gill.

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