There are a number of interesting consequences to a unified shading model, some of which may not be immediately apparent. The most obvious addition is, of course, the ability to do texturing inside the vertex shader, and this is especially important for general-purpose displacement mapping, yet it need not be limited to that. A slightly less obvious addition is the ability to write directly to a vertex buffer from the vertex shader, allowing the caching of results for later passes. This is especially important when using higher-order surfaces and displacement mapping, allowing you to tessellate and displace the model once, store the results in a video memory vertex buffer, and simply do a lookup in all later passes.
But perhaps the most significant addition comes when you combine these two together with the virtual video memory mindset – with virtual video memory, writing to and reading from a texture becomes pretty much identical to writing to or reading from any other block of memory (ignoring filtering, that is). With this bit of insight, the General I/O Model of DirectX Next was born – you can now write any data you need to memory to be read back at any other stage of the pipeline, or even at a later pass.
