Re: [PyOpenGL-Users] bug on glcanvas?

[George P. <ge...@ri...>]

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