Primitive Assembly and Rasterization - Praise for OpenGL ES 30 Programming Guide - page 182

Figure 7-11 Polygon Offset

Figure 7-11 shows two coplanar polygons being drawn. The code to draw
these two coplanar polygons without polygon offset is as follows:

glClear ( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

// load vertex shader
// set the appropriate transformation matrices
// set the vertex attribute state

// draw the SMALLER quad
glDrawArrays ( GL_TRIANGLE_FAN, 0, 4 );

// set the depth func to <= as polygons are coplanar
glDepthFunc ( GL_LEQUAL );

// set the vertex attribute state

// draw the LARGER quad
glDrawArrays ( GL_TRIANGLE_FAN, 0, 4 );

To avoid the artifacts shown in Figure 7-11, we need to add a delta to the
computed depth value before the depth test is performed and before the
depth value is written to the depth buffer. If the depth test passes, the
original depth value—and not the original depth value + delta—will be
stored in the depth buffer.

The polygon offset is set using the following API call:

void glPolygonOffset (GLfloat factor , GLfloat units )

The depth offset is computed as follows:

depth offset = m * factor + r * units