C++ w/ OpenGL Pixel Perfect Plot

Question

I need to plot a pixel on the window, according to its position in pixels in integers, not floats. like, for example, if I was to plot 0, 0 it would be the very top left corner of the window, 1,1 would be one pixel to the right of 0, 0, and 1 pixel below 0. I guess what I need is it to be kind of like I grid. I don't care if 0, 0 is the origin like how it normally is, but I need it to count by pixels, not having 1 as the max and -1 as the min, or whatever.

Things I have already tried: I thought I could just do something like this Window Width ^-1 * X for plotting, and that actually does work, but not all the time. Anyways, how would I go by doing this? I have also done X position / Screen Width, and that works the best, but it stops working for every 10 or so pixels.

Answer 1

First of all, OpenGL doesn't guarantee pixel perfect placement in all cases. However, there is a very common gotcha: the center of each pixel will not have integer coordinates if you pixel coordinates.

Here's a diagram of the bottom-left pixel in an OpenGL viewport.

+Y
^
|     
+-----+
|     |
|  o  |
|     |
+-----+---> +X

If your data is transformed so that each pixel has a width and height of 1 unit, then the center of the pixel is at (0.5, 0.5). If you draw a GL_POINT at that pixel, the coordinates should be (0.5, 0.5). If you draw a rectangle, it will go from (0,0) to (1,1). If you draw a vertical line through it, the line should have an x-coordinate of 0.5.

Et cetera.