DirectX 10 is the most significant update to Direct3D since DirectX 8, which introduced programmable shaders. DirectX 10 graphics offers several key enhancements to earlier versions:

• Unified Shader Model. Known as Shader Model 4.0, this unassuming name hides an entirely new approach to how DirectX handles programmable 3D. Shader Model 4.0 uses a consistent instruction set across all shader types now. Previously, pixel shader code looked different from vertex shader code. This played well on earlier hardware architectures, which had separate pixel and vertex shader units. This unified ISA (instruction set architecture) makes life much easier for graphics programmers.
• Geometry Shaders. In the past, vertex data would come into the 3D pipeline, be converted to triangle data. Vertex shader operations may modify the characteristics of the existing vertex data, but no new vertices are created. Then the data is rasterized, and those fragments are processed by pixel shaders. DirectX 10 adds a new stage, geometry shaders, which lie between vertex and pixel operations. It's possible to actually create new vertices or destroy existing ones with geometry shaders, and to perform operations on entire triangles or groups of triangles.
• Stream Out. Stream output means that data generated from geometry shaders can be shipped back to video memory, then reused and shipped back to the GPU to be processed again. This allows for more-complex geometries, advanced lighting and even more efficient GPU-based physics simulations.
• Longer Shader Programs. Shader programs can now be 64K (65,535) instructions long, instead of the previous 512 instructions.
• More Flexible Shader Programming. Flow control is now completely dynamic, and the number of constant registers has dramatically increased (from 256 to 16 x 4096, broken down into 16 input registers and 4096 temp registers). Load and integer ops now exist. While still very focused on graphics, the ISA is beginning to offer more general-purpose attributes.
• Improved Instancing. Instancing was a technique whereby a single DirectX draw call would send an object to the GPU, with some additional data that told the GPU where to place the object, what color it might be, animation data, and so on. But that created multiple objects that often looked and behaved nearly the same. That data was used to replicate the single object many times in a scene using just one draw call. In DX10, this data is in the form of array indices that point back to arrays. Each array entry contains different instance data, so the multiple objects can look and behave differently.
• Windows Vista Only. DirectX 10 will only run on Windows Vista, because it will require the new device driver model that's one of the core pieces of Vista. However, DirectX 10 capable hardware will run applications written for earlier versions of DirectX just fine, and will run DX9 and earlier apps on Windows XP.

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