Depth Bias (Direct3D 9)

Polygons that are coplanar inyour 3D space can be made to appear as if they are not coplanar byadding a z-bias to each one. This is a technique commonly used toensure that shadows in a scene are displayed properly. For instance, ashadow on a wall will likely have the same depth value as the walldoes. If you render the wall first and then the shadow, the shadowmight not be visible, or depth artifacts might be visible. You canreverse the order in which you render the coplanar objects in hopes ofreversing the effect, but depth artifacts are still likely.

Anapplication can help ensure that coplanar polygons are renderedproperly by adding a bias to the z-values that the system uses whenrendering the sets of coplanar polygons. To add a z-bias to a set ofpolygons, call the IDirect3DDevice9::SetRenderState method just before rendering them, setting the Stateparameter to D3DRS_DEPTHBIAS, and the Value parameter to a valuebetween 0-16 inclusive. A higher z-bias value increases the likelihoodthat the polygons you render will be visible when displayed with othercoplanar polygons.

Offset = m * D3DRS_SLOPESCALEDEPTHBIAS + D3DRS_DEPTHBIAS

where m is the maximum depth slope of the triangle being rendered.

m = max(abs(delta z / delta x), abs(delta z / delta y)) 

The units for the D3DRS_DEPTHBIAS andD3DRS_SLOPESCALEDEPTHBIAS render states depend on whether z-bufferingor w-buffering is enabled. The application must provide suitable values.

Thebias is not applied to any line and point primitive. However, this biasneeds to be applied to triangles drawn in wireframe mode.

// RenderStates
D3DRS_SLOPESCALEDEPTHBIAS, // Defaults to zero
D3DRS_DEPTHBIAS,           // Defaults to zero

// Caps
D3DPRASTERCAPS_DEPTHBIAS           
D3DPRASTERCAPS_SLOPESCALEDEPTHBIAS