ShaderX Shader Programming Tips & Tricks With DirectX 9

}
// get the 10,10,10 portion of the position
float3 cpos = input.position;
// decode our 2 bits (0-3)
float two bits = floor(input.enc2Bit.w / 64.0);
// factor in the extra bits and convert back into the 0-1 range
cpos.z = (cpos.z + two bits) * 0.25;
// transform by the inverse compression matrix
float4 pos = mul( float4(cpos,1), InvCompressionTransform );
Displacement Compression
My previous article covered the use of vertex shaders to render displacement
maps. This capability can be extended to a very powerful technique that Tom
Forsyth has termed “displacement compression.” It’s a complete family of techniques
that includes patch rendering, displacement mapping, and subdivision surfaces
that any vertex shader-capable hardware can do and is a powerful form of
geometry compression.
Usually tessellation levels are decided by the CPU, as we currently have no
programmable tessellation hardware, but there are a few fixed-function hardware
tessellation systems that you may be able to use. This is the technique’s major
limitation — to a limited degree, we can remove triangles (by sending the vertices
to be clipped), but we cannot add triangles.
By using the vertex shaders as a function evaluator with the vertex stream
bringing in the function parameters, we can render many geometrical surfaces.
For the surfaces we use here, this consists of a barycentric surface function with
an additional displacement scalar, but other surfaces’ parameterizations are
possible.
There are two components that are needed for displacement compression.
� Displacement mapping: A method of retrieving a scalar displacement along
the surface normal. Without it, your displacement compression becomes
standard surface patch evaluation.
� Surface basis: Every displacement compression shader requires a basis system
that defines the base surface before displacement. The simplest is just
planar, although it could be as complex as a subdivision surface.
Displacement Mapping
Section I — Geometry Manipulation Tricks
Using Vertex Shaders for Geometry Compression
There are at least four ways to get the displacement value into the vertex shader.
The more advanced methods require explicit hardware support and are not covered
here. Refer to presentations from Mike Doggett and Tom Forsyth for details
[2]. Also, Tom Forsyth’s article covers actual generation of displacement data in
detail [1].
The technique presented here works on any vertex shader hardware by
treating the displacement map as a 1D vertex stream. It’s a generalization of the
technique that I presented in Direct3D ShaderX, which had an implied planar basis
that with a few minor modification works for any surface basis.
7