Opengl pixel perfect 2D drawing

In OpenGL, lines are rasterized using the "Diamond Exit" rule. This is almost the same as saying that the end coordinate is exclusive, but not quite...

This is what the OpenGL spec has to say: http://www.opengl.org/documentation/specs/version1.1/glspec1.1/node47.html

Also have a look at the OpenGL FAQ, http://www.opengl.org/archives/resources/faq/technical/rasterization.htm, item "14.090 How do I obtain exact pixelization of lines?". It says "The OpenGL specification allows for a wide range of line rendering hardware, so exact pixelization may not be possible at all."

Many will argue that you should not use lines in OpenGL at all. Their behaviour is based on how ancient SGI hardware worked, not on what makes sense. (And lines with widths >1 are nearly impossible to use in a way that looks good!)

Note that OpenGL coordinate space has no notion of integers, everything is a float and the "centre" of an OpenGL pixel is really at the 0.5,0.5 instead of its top-left corner. Therefore, if you want a 1px wide line from 0,0 to 10,10 inclusive, you really had to draw a line from 0.5,0.5 to 10.5,10.5.

This will be especially apparent if you turn on anti-aliasing, if you have anti-aliasing and you try to draw from 50,0 to 50,100 you may see a blurry 2px wide line because the line fell in-between two pixels.