ShaderX Shader Programming Tips & Tricks With DirectX 9

Using Vertex Shaders for
Geometry Compression
Dean Calver
This article is a follow-up to an article I wrote in Direct3D ShaderX: Vertex and
Pixel Shader Tips and Tricks. DirectX 9 has introduced new data types and added
new capabilities to the vertex stream model. This, combined with more complex
and faster vertex shaders, allows us to explore more advanced forms of vertex
and geometry compression.
What’s New in DirectX 9?
Vertex Shaders
In most cases I still use vertex shader version 1.1, as this is executed in hardware
on the greatest number of machines. The new cards do benefit in the extra constant
space available. This improves the amount of batching that can occur. Static
branching also makes it easier to use different compression methods on different
models. Vertex shader version 3.0 potentially offers a number of new capabilities,
the most prominent being vertex texturing. This will offer a new range of compression
methods but isn’t explored here due to current lack of hardware support.
New Vertex Stream Declaration Format
Limitations
The vertex stream declaration system from DirectX 8 was completely overhauled
to make it both easier to use and add new capabilities. From a compression point
of view, the most interesting items are the new vertex data types and the extra
control over where each element comes from in the stream (stream offset).
When under DirectX 8 drivers (you can check via the D3DDEVCAPS2_STREAM-
OFFSET cap bit), most new capabilities of the DirectX 9 vertex stream declarations
can’t be used. Under DirectX 7 drivers, you must stick to FVF-style
declarations. Also, if a declaration’s stream offsets produce overlapping vertex
elements, then even on DirectX 9 drivers, the D3DDEVCAPS2_VERTEXELE-
MENTSCANSHARESTREAMOFFSET cap bit must be set. Another limitation is
that stream offsets must align on DWORD boundaries (4 bytes).
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