glPolygonOffset - set the scale and units used to calculate depth values
void glPolygonOffset(GLfloat factor, GLfloat units);
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.
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.
glDepthFunc(), glEnable(), glGet(), glIsEnabled()
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