Re: [PyOpenGL-Users] bug on glcanvas?

[altern <al...@gm...>]

hi again

i also forgot to say that exactly the same code using pygame instead of 
wxpython works fine in both windows, linux and OSX

enrike

George Paci wrote:
> altern wrote:
> 
>  > This is the detailed description of error:
>  > I use  glOrtho(0, 800,  600, 0, 5000, 0)
>  > to set up the projection,
> 
>  From the docs at
> http://pyopengl.sourceforge.net/documentation/manual/glOrtho.3G.xml
> :
> 
> Python Specification
> 
> glOrtho	(left, right, bottom, top, zNear, zFar) -> None
> 
> Parameters
> 
> left, right -- Specify the coordinates for the left and right
>                 vertical clipping planes.			
> bottom, top -- Specify the coordinates for the bottom and top
>                 horizontal clipping planes.
> zNear, zFar -- Specify the distances to the nearer and farther
>                 depth clipping planes. These values are negative
>                 if the plane is to be behind the viewer.
> 
> 
> So you're calling it with bottom=600 and top=0, which is the reverse
> of the way OpenGL handles y coordinates: greater Y in OpenGL is UP.
> 
> The depth values also seem to be backwards: the docs say they're the
> *distances* to the nearer and farther clipping planes.  So you probably
> want zNear=0 and zFar=5000 .
> 
> Try this and see if it works:
> 
> glOrtho(0, 800, 0, 600, 0, 5000)
> 
> 
> Except this looks like you're trying to do things in pixels,
> not OpenGL's units.  A much more natural way would be:
> 
> glOrtho(-400, 400, -300, 300, 0, 5000)
> 
> ... and then (0,0,0) would be at the center of the viewport.
> 
>  > This is true in windows but under Linux I get the (0,0) on the
>  > center of  the screen and if I increase the y value of a vertex
>  > it moves up instead  of down. On top of this the values of the
>  > opengl are not mapped to  pixels and moving a vertex by 1 unit
>  > (should move 1 pixel) moves it out of the window.
> 
> It sounds like your glOrtho() call flat-out failed on Linux
> (more precisely, using whatever driver is associated with
> your rendering context).  Are you sure you're not eating an
> exception thrown by the call?  My guess is it doesn't like
> your depth coordinates.
> 
> To sum up:
> 
> Try:
>    glOrtho(0, 800, 0, 600, 0, 5000)
> 
> Then, if you really want y=0 at the top, try:
>    glOrtho(0, 800, 600, 0, 0, 5000)
> 
> In case you're losing polygons in the front part of
> the scene, try a negative near depth:
>    glOrtho(0, 800, 0, 600, -100, 5000)
> 
> Finally, consider putting everything near the origin
> (i.e. (0,0,0)) and projecting that in the center:
>    glOrtho(-400, 400, -300, 300, -100, 5000)
> 
> 
> --George Paci
> 
>    The desire to remain the same is the single most powerful motivator
>    for change.  In order to preserve something, people will change
>    anything that's less important.  -- Dale Emery
> 
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