When GL_POLYGON_OFFSET_FILL, GL_POLYGON_OFFSET_LINE, or GL_POLYGON_OFFSET_POINT is enabled, each fragment's value will be offset after it is interpolated from the 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.
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glPolygonOffset: man3/glPolygonOffset.xml
When GL_POLYGON_OFFSET_FILL, GL_POLYGON_OFFSET_LINE, or GL_POLYGON_OFFSET_POINT is enabled, each fragment's value will be offset after it is interpolated from the 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.