Drawing Coplanar Primitives Widthout Polygon Offset

You can simulate the effects of polygon offset by tinkering with glDepthRange(). For example, you might code the following:

  glDepthRange (0.1, 1.0);
  /* Draw underlying geometry */
  glDepthRange (0.0, 0.9);
  /* Draw overlying geometry */

This code provides a fixed offset in Z, but doesn't account for the polygon slope. It's roughly equivalent to using glPolygonOffset with a factor parameter of 0.0.

You can render coplanar primitives with the Stencil buffer in many creative ways. The OpenGL Programming Guide outlines one well-know method. The algorithm for drawing a polygon and its outline is as follows:

1. Draw the outline into the color, depth, and stencil buffers.
2. Draw the filled primitive into the color buffer and depth buffer, but only where the stencil buffer is clear.
3. Mask off the color and depth buffers, and render the outline to clear the stencil buffer.

On some SGI OpenGL platforms, an application can use the SGIX_reference_plane extension. With this extension, the user specifies a plane equation in object coordinates corresponding to a set of coplanar primitives. You can enable or disable the plane. When the plane is enabled, all fragment Z values will derive from the specified plane equation. Thus, for any given fragment XY location, the depth value is guaranteed to be identical regardless of which primitive rendered it.