PDA

View Full Version : 2D innaccuracy



halo
08-29-2005, 06:34 PM
I am writing some 2D commands, and need to draw pixel-perfect lines, points, and rectangles.

Let's say the client area is 400x300, and I want to draw a line (0,0,400,0), across the very top of the viewport.

1) I set glViewport to 0,0,400,300.

2) I set glOrtho to -200,200,-150,150.

3) I draw a line ((Ax-200),(300-Ay-150),(Bx-200),(300-By-150)) or ((0-200),(300-0-150),(400-200),(300-0-150)).

The line is one pixel too high, and lies offscreen. If I make a line across the very bottom, it lies right on the bottom edge. What is the correct mathematical change I need to make so that I can draw an arbitrary size window and get pixel-perfect results?

Thanks.

halo
08-29-2005, 08:09 PM
hmmm...This seems to work:

-Add 0.5 to each value.
-Subtract 1.0 from y component.
-Subtract 1.0 from x component (polys only)
-Draw lines both ways, like this, to draw that last pixel:

Instead of this:
glBegin GL_LINES
glvertex2f Ax,Ay
glvertex2f Bx,By
glend

Use this:
glBegin GL_LINE_STRIP
glvertex2f Ax,Ay
glvertex2f Bx,By
glvertex2f Ax,Ay
glend