glPolygonOffset — set the scale and units used to calculate depth values
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.
GL_POLYGON_OFFSET_FILL 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 , where is a measurement of the change in depth relative to the screen area of the polygon, and 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
glGet with argument
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