glPolygonOffset — set the scale and units used to calculate depth values
void glPolygonOffset (GLfloat factor,
GLfloat units);
Specifies a scale factor that is used to create a variable depth offset for each polygon. The initial value is 0.
Is multiplied by an implementation-specific value to create a constant depth offset. The initial value is 0.
When GL_POLYGON_OFFSET_FILL, GL_POLYGON_OFFSET_LINE, or GL_POLYGON_OFFSET_POINT is enabled, each
fragment's depth value will be offset after it is interpolated from the depth values of the appropriate vertices. The
value of the offset is factor × DZ + r × units, where DZ is a measurement of the change in depth relative to the screen area
of the polygon, and r is the smallest value that is guaranteed to produce a resolvable offset for a given implementation. The offset
is added before the depth test is performed and before the value is written into the depth buffer.
glPolygonOffset is useful for rendering hidden-line images, for applying decals to surfaces, and for rendering solids with highlighted edges.
glPolygonOffset is available only if the GL version is 1.1 or greater.
glPolygonOffset has no effect on depth coordinates placed in the feedback buffer.
glPolygonOffset has no effect on selection.
GL_INVALID_OPERATION is generated if glPolygonOffset is executed between the execution of glBegin and
the corresponding execution of glEnd.
glIsEnabled with argument GL_POLYGON_OFFSET_FILL, GL_POLYGON_OFFSET_LINE, or
GL_POLYGON_OFFSET_POINT.
glGet with argument GL_POLYGON_OFFSET_FACTOR or GL_POLYGON_OFFSET_UNITS.