Basics Of Polygon Offset
It's difficult to render coplanar primitives in OpenGL for two reasons:
- Given two overlapping coplanar primitives with different vertices, floating point round-off errors from the two polygons can generate different depth values for overlapping pixels. With depth test enabled, some of the second polygon's pixels will pass the depth test, while some will fail.
- For coplanar lines and polygons, vastly different depth values for common pixels can result. This is because depth values from polygon rasterization derive from the polygon's plane equation, while depth values from line rasterization derive from linear interpolation.
Setting the depth function to GL_LEQUAL or GL_EQUAL won't resolve the problem. The visual result is referred to as stitching, bleeding, or Z fighting.
Polygon offset was an extension to OpenGL 1.0, and is now incorporated into OpenGL 1.1. It allows an application to define a depth offset, which can apply to filled primitives, and under OpenGL 1.1, it can be separately enabled or disabled depending on whether the primitives are rendered in fill, line, or point mode. Thus, an application can render coplanar primitives by first rendering one primitive, then by applying an offset and rendering the second primitive.
While polygon offset can alter the depth value of filled primitives in point and line mode, under no circumstances will polygon offset affect the depth values of GL_POINTS, GL_LINES, GL_LINE_STRIP, or GL_LINE_LOOP primitives. If you are trying to render point or line primitives over filled primitives, use polygon offset to push the filled primitives back. (It can't be used to pull the point and line primitives forward.)
Because polygon offset alters the correct Z value calculated during rasterization, the resulting Z value, which is stored in the depth buffer will contain this offset and can adversely affect the resulting image. In many circumstances, undesirable "bleed-through" effects can result. Indeed, polygon offset may cause some primitives to pass the depth test entirely when they normally would not, or vice versa. When models intersect, polygon offset can cause an inaccurate rendering of the intersection point.