Re: Looking for colourful advice

On Tue, 2010-03-02 at 10:35 -0500, Joseph Scheuhammer wrote:
> Hi,
> Gnome-mag has a "cross-hair" feature for helping users find the mouse 
> cursor.  An example graphic can be viewed here:
> The issue is that the cross hairs cannot obscure what lies beneath 
> them.  As you can see in the png, the colours of the cross hair are 
> blended with the background.
> Is colour blending possible within GnomeShell?  My approach would be to 
> use clutter texture objects to form the cross hairs -- are there colour 
> blending functions for texture objects?  I've looked at the online 
> documentation (, and 
> found transparency manipulations, but that's probably not enough.  
> Should I be diving into COGL?  Looking somewhere else?
> Any advice would be appreciated.

To get this 'XOR' affect you'll need to go straight to OpenGL and use
glLogicOp(). You can find examples of this with a web search. This will
need to be done from C.

Two approaches to code organization:

 - You could write a custom clutter actor that draws this way
   (Using cogl_begin_gl() and cogl_end_gl() around the OpenGL code)

 - You could paint the cross-hairs on top of the stage after the
   stage is completely painted, similar to how shell-recorder.c
   draws the recording icon and progress bar. (You still need
   the cogl_begin_gl() and cogl_end_gl() calls.)

- Owen

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