orthographic projection question

[flatland]

Hi everyone,

I'm fairly new to Vizard and am working with orthographic projection. I have a basic question. In the program below, the application window has an aspect ratio of 1, as does the width and height of the 100x100 region between the left/right/top/bottom clipping planes in the orthographic projection setup. But when I run the program, the 90x90 vertical quad centered at (0,0,0) doesn't appear centered vertically in the application window. It is slightly displaced in the downward direction, and I don't understand why. (This is causing me problems when I try to connect mouse coordinates in the application window with objects in the scene.) Anyone have an idea why this is happening?

Robin

import viz

# set application window size
viz.window.setSize( 700, 700 )

# set mainWindow
viz.MainWindow.ortho(-50,50,-50,50,-1,1)
viz.MainWindow.clearcolor( viz.BLACK )

viz.startLayer(viz.QUADS)
viz.vertexColor(1,0,0)
viz.vertex(-45, 45, 0)
viz.vertex(-45,-45, 0)
viz.vertex( 45,-45, 0)
viz.vertex( 45, 45, 0)
viz.endLayer()

viz.go()