Windows Vista Service Pack 1 brings to the table an evolution focused on softening the rough edges of the RTM version of the operating system, but at the same time introduces new aspects into the fabric of the Windows client. Case in point: DirectX 10.1. Moving from version 10 in Vista RTM to 10.1 in Vista SP1, DirectX's growth is closely correlated with Direct3D 10.1. And if at this point in time there is still a shadow of a doubt about the resources connected with the successor of DirectX 10, Microsoft has confirmed that end users will need both Vista SP1 as well as 10.1 compatible hardware in order to enjoy the benefits of DirectX 10.1.

This even if Direct3D is considered a "strict superset and a minor update" of Direct3D 10. Via Vista SP1 and 10.1 capable graphics cards, Direct3D 10.1 is designed to enable what Chuck Walbourn, Software Design Engineer XNA Developer Connection Microsoft, referred to as the 10.1 class of hardware features. Among them Walbourn enumerated MSAA per-sample shaders, MSAA depth read-back, independent blend modes per render target, cube map arrays and render to block-compressed formats.

Walbourn explained that DirectX 10.1 is set up to extend the 10.0 application programming interface with a total of three new items. The new interfaces are tailored specifically to 10.1 class hardware features, and in this context none of them can be built on 10 compatible hardware, even though barely a year separates versions 10 and 10.1 of DirectX. With DirectX 10.1 the Redmond company also introduced new HLSL 4.1 shader profiles.

"Using Direct3D 10.1 [and implicitly DirectX 10.1] requires a 10.1 capable video card and Windows Vista SP1," Walbourn said while explaining that at the same time "10.1 class hardware will work as 10 class hardware on Windows Vista without the service pack."

For developers Microsoft has been offering a new DirectX software development kit since November 2007 as a technology preview. The SDK was released to manufacturing in March 2008 and is available for download here.