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Eye-Relative vs. Z-Based Depth

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To alleviate fog-related graphic artifacts caused by uneven distribution of z-values in a depth buffer, most hardware devices use eye-relative depth instead of z-based depth values for pixel fog. Eye-relative depth is essentially the w element from a homogeneous coordinate set. Microsoft® Direct3D® takes the reciprocal of the RHW element from a device space coordinate set to reproduce true w. If a device supports eye-relative fog, it sets the D3DPRASTERCAPS_WFOG flag in the RasterCaps member of the D3DCAPS8 structure when you call the IDirect3DDevice8::GetDeviceCaps method.

Note   With the exception of the reference rasterizer, software devices always use z to calculate pixel fog effects.

When eye-relative fog is supported, the system automatically uses eye-relative depth rather than z-based depth if the provided projection matrix produces z-values in world space that are equivalent to w-values in device space. You set the projection matrix by calling the IDirect3DDevice8::SetTransform method, using the D3DTS_PROJECTION value and passing a D3DMATRIX structure that represents the desired matrix. If the projection matrix is not compliant with this requirement, fog effects are not applied properly. For more information about producing a compliant matrix, see W-Friendly Projection Matrix. The perspective projection matrix provided in What Is the Projection Transformation? produces a compliant projection matrix.

Direct3D uses the currently set projection matrix in its w-based depth calculations. As a result, an application must set a compliant projection matrix to receive the desired w-based features, even if it does not use the Direct3D transformation pipeline.

Direct3D checks the fourth column of the projection matrix. If the coefficients are [0,0,0,1] (for an affine projection) the system will use z-based depth values for fog. In this case, you must also specify the start and end distances for linear fog effects in device space, which ranges from 0.0 at the nearest point to the user, and 1.0 at the farthest point.

See Also

Pixel Fog

 Last updated on Thursday, April 08, 2004

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