pyopengl-users

[PyOpenGL-Users] gluPickMatrix and glPointSize

[<pa...@cs...>]

I have a question about the effect of glPointSize and glLineWidth on 
picking. I have code that is able to return a hitlist for polygons on 
the screen for a one pixel picking box. When I try to pick points and 
lines, it also works fine. When I change the point size or the line 
width, however, the picking no longer works for points and lines 
unless I pick the exact center of the points and lines.

I could have sworn that picking is an operation that is applied after 
rasterization in the OpenGL pipline. Can anyone explain this behavior 
to me? Is there a work around?

Thanks,
Pete

Re: [PyOpenGL-Users] gluPickMatrix and glPointSize

[<il...@ya...>]

 --- pa...@cs... wrote: > I have a question
about the effect of glPointSize
> and glLineWidth on 
> picking. I have code that is able to return a
> hitlist for polygons on 
> the screen for a one pixel picking box. When I try
> to pick points and 
> lines, it also works fine. When I change the point
> size or the line 
> width, however, the picking no longer works for
> points and lines 
> unless I pick the exact center of the points and
> lines.
> 
> I could have sworn that picking is an operation that
> is applied after 
> rasterization in the OpenGL pipline. Can anyone
> explain this behavior 
> to me? Is there a work around?
> 
> Thanks,
> Pete
> 

One work around might be to draw multiple parallel
lines instead?

Do you have the code?  Maybe it is an issue with your
opengl implementation?

Which opengl implementation do you use?


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Re: [PyOpenGL-Users] gluPickMatrix and glPointSize

[Peter P. <pa...@cs...>]

Hi Rene,

I actually found the answer in the OpenGL advanced notes after some
searching. Note the starred section.

"The OpenGL selection method determines that an object has been hit if  it
intersects the view volume. Bitmap and pixel image primitives will  generate
a hit record only if a raster positioning command is sent to  the pipeline
and the transformed position lies within the viewing volume. To generate hit
records for any point within a pixel image or bitmap, a bounding rectangle
should be sent rather than the image, and selection testing performed on the
interior of the rectangle. ****Similarly, wide lines and points will be
selected only if the equivalent infinitely thin line or small point would be
selected. To facilitate selection testing of wide lines and points, proxy
geometry representing the true footprint of the primitive should be used
instead****"

Pete