Unit?

OpenGL OpenGL: Basic Coding
1

I’m using gluLookAt to pan a 2D image (textured). What are the units of the parameters (eyeX for example)? How can I construct a relation between these units and pixels?

thanks,

2

There are no units in OpenGL. If you place your viewpoint at (1, 0, 0), you have placed the viewpoint one unit away from the origin along the positive X-axis. Whether this unit, in your scene, correspond to one meter, one yard, one light year, one micrometer, one inch, one foot, one astronomical unit, or three apples and a banana, is up to you to decide.

And there is no relation betwee these units and a pixel. All coordinates in OpenGL are resolution independent.

You can, of course, set the matrices, so that a unit has some relation to pixel coordinates, but that is just in your eyes, and nothing OpenGL cares about. Here’s an example.

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.5, windowWidth + 0.5, 0.5, windowHeight + 0.5);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

This piece of code will set the matrices so that you can, using for example glVertex, set a vertex in window coordinates. You then have a 1:1 correspondence between units and pixels. But that’s just in your eyes. In OpenGL’s eyes, these are just regular projection and modelview matrices, and are not treated special in any way.

3

lol, Bob: an apple and a banana

4

A graphics engineer I once knew used to refer to these units as a “bananas per furlong”, I think he was probably closest to the true default graphics unit.