Re: [PyOpenGL-Users] orthographic projection error

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

hi greg

Greg Ewing wrote:
 > altern wrote:
 >> The example draws a one pixel wide box in the borders of the window.
 >> In none of the sytems I tried so far the box was totally visible.
 >> There is always one or two sides missing just by one pixel.
 >
 > What coordinates are you using to draw the box, exactly?
 > You might need to offset them by half a pixel so that
 > the lines go through the centres of pixels. If you're
 > actually trying to draw a line between two pixels,
 > different systems may round it in different ways.

This is the way i set the projection and draw the lines, being
size=(800,600) in my example

	# the projection
	glOrtho(0, size[0],  size[1], 0, 1, 0)

  	# red marquee with diagonal lines cross
  	glBegin(GL_LINE_LOOP)
         glColor3f(1,0,0) #
         glVertex2i(0,0)
         glVertex2i(size[0], size[1])
         glVertex2i(0, size[1])
         glVertex2i(size[0], 0)
         glVertex2i(0,0) # marquee
         glVertex2i(size[0], 0)
         glVertex2i(size[0], size[1])
         glVertex2i(0, size[1])
         glEnd() # end line loop

I tried to replace all the 0 value in all vertexes by 0.5 but it doesnt 
work either. I also tried to change the glOrtho to
glOrtho(0.5, size[0], size[1], 0.5, 1, 0)
and also combining both with no luck.

However if I set glOrtho like this
glOrtho(-1, size[0], size[1]+1, 0, 1, 0)
then it works fine on my Debian desktop and on my debian laptop but not 
on my laptop with XP. So I decided to do tests in all systems on my 
machines. This is the result:

glOrtho(-1, size[0], size[1]+1, 0, 1, 0)
works on Debian desktop with a matrox card
works on Debian laptop with ATI

glOrtho(-1, size[0], size[1]+1, -1, 1, 0)
works on XP laptop with ATI

glOrtho(-1, size[0]+1, size[1], 1, 0)
works on XP Desktop with matrox card

I also tested on and old mac ibook with ATI card running 10.4.8 and on
that it works if i do
glOrtho(0, size[0], size[1]+1, -1, 1, 0)

Not very consistent. It seems to me that it might be more to do with the 
OS rather than with the cards themselves. The XP versions I am running 
are different but the Debian are pretty much the same.

I attach my example in case anyone wants to test it out his/her machine. 
   The glOrtho is on line 19 and i pasted different combinations of 
setup the glOrtho

thanks!

enrike