Setting Up a 2D Coordinate System

Typical perspective frustums are set up such that the lower-left corner is coordinate (-1, -1) and the upper-right corner is (1, 1), which would make the center of the screen (0, 0). If you want to render primitives such as quads as if they were 2D, it will be convenient to define a coordinate system where the upper-left corner is (0, 0) and the lower-right, say, (640, 480).

The glOrtho and glFrustum methods can be used to set up the frustum and the coordinate system in which the objects are rendered. The difference between them is perspective and the sense of depth. When using a perspective projection, objects that are farther from the camera appear smaller and move more slowly. The glOrtho method can be used to produce a parallel projection in much the same way that the glFrustum method can be used to produce a perspective projection. The following line sets up a parallel projection matrix. The upper-left corner is (0, 0) and the lower-right corner is (640, 480).

//          left, right, bottom, top, zNear, zFar  gl.glOrtho(    0,   640,    480,   0,    -1,    1 );

Note that the zNear and zFar have been set to -1 and 1 correspondingly. When drawing, you can simply set the z-coordinate to 0 or use glVertex2i instead of glVertex3i. It is a good practice to avoid using perspective projection if you want to render polygons right in front of the camera. But if you want to use a perspective projection instead, you must render the geometry right passed the near clip and use the new and far clip appropriately. An example of a quad drawn using the orthographic projection matrix we set up earlier follows:

gl.glBegin(GL.GL_QUADS);        g gl.glVertex2i(100, 200); // South West g gl.glVertex2i(200, 200); // South East g gl.glVertex2i(200, 100); // North East g gl.glVertex2i(100, 100); // North West g gl.glEnd();

Unlike console games, PC games must be able to run in different screen resolutions, so menu systems may have to compensate for the possible changes. By using a virtual 640 x 480 coordinate system, you can choose to change the coordinate of the projection matrix, or leave it as is. If the width and height parameters that are passed to glOrtho change, the size of the components will shrink and allow more components to fit. On the other hand, by not changing them, the components take the same amount of space, regardless of the screen resolution.