Re: [newsletters] Re: [PyOpenGL-Users] OpenGl + Mouse Selection

[Mike C. F. <mcf...@ro...>]

Setup the projection matrix to be slightly larger than a single point 
(OpenGLContext gives you a 2x2 area).  Here's the code from 
OpenGLContext/renderpass's SelectRenderPass class (matrixSize is just a 
tuple of (2,2) in this case):

    def PickProjection( self ):
        """Setup the constrained picking matrix and result buffer

        We set up the view frustum to be a
        box centered around the picking point of size
        self.matrixSize, projecting back into the screen
        from our current viewpoint.

        We then set up the selection buffer into which
        our results will be saved, and then switch to
        selection-mode rendering.
        """
        glMatrixMode(GL_PROJECTION)
        glLoadIdentity()
        glInitNames()
        x,y = self.pickPoint
        gluPickMatrix(
            x,y,
            self.matrixSize [0], self.matrixSize [1],
            self.getViewport()
        )
        glSelectBuffer(self.bufferSize)
        glRenderMode(GL_SELECT)

You can find documentation on gluPickMatrix here:
http://pyopengl.sourceforge.net/documentation/manual/gluPickMatrix.3G.html

HTH,
Mike

sebastien HEITZMANN wrote:

> I use this method with good success.
>
> I just have a little problem. My program paint some line on the OpenGL 
> panel and when i try to select one, I need to be very close to select 
> it. Is it a way to make the select area more large ?
>
> Thanks in advance.
>
> SEB
>
> Jasper Phillips a écrit:
>
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>>
>> That sounds like what I want.  Thanks for the search reference!
>> - -Jasper
>>
>> On Mon, 21 Apr 2003, Mike C. Fletcher wrote:
>>  
>>
>>> There are a number of ways to do this, but the most common way I've 
>>> encountered is to use the glSelectBuffer-based selection mechanisms 
>>> which come with OpenGL.  You can find sample code for the 
>>> glSelectBuffer stuff in OpenGLContext or just about any OpenGL 
>>> reference or programming Web site (such as NeHe).
>>>
>>> The basic approach is to re-render the scene in "select" mode, 
>>> pushing "names" (integers) onto the selection stack as you render to 
>>> a frustum which is the projection of your picking point to 
>>> infinity.  When you exit the select rendering mode you receive a 
>>> list of records indicating which names were active when items were 
>>> rendered to the frustum.
>>>
>>> Hope this helps,
>>> Mike
>>>   
>>
>>
>> [snip my original question]
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>>
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-- 
_______________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://members.rogers.com/mcfletch/