| Microsoft DirectX 8.1 (Visual Basic) |
Vertex data specified with a flexible vertex format (FVF) code has the property of being either transformed or nontransformed. The terminology used for FVF-specified transformed vertices is TL (transformed and lit) vertices. Microsoft® Direct3D® continues to support TL vertex data, as in previous releases. However, TL vertices are subject to some conditions that are not applicable to nontransformed vertices.
TL vertex data is possible only for vertices specified by an FVF code, and it is not valid as input for a programmable vertex shader. Programmable vertex shaders have the flexibility to take various forms of transformed vertex data, but this is different from FVF-TL vertex data because the interpretation is part of the shader, not a property of the vertex data. Because it is defined by an FVF code, TL vertex data is also inherently single stream.
TL vertex data is not guaranteed to be clipped by Direct3D, thus it is required that TL vertex primitives be clipped prior to sending them to Direct3D for rendering. The x and y values are required to be clipped to the viewport, or, if available, the device guardband. The z values for TL vertices are required to be between 0. and 1, inclusive. It is also required that the RHW values be within the range [0 < 1/D3DCAPS8.MaxVertexW]. The RHW requirement is to guarantee that the w values are within the supported range of the hardware rasterization device. Note that the w, and RHW, range of vertices resulting from a perspective projection transformation can be modified by applying a scale to all values in the projection matrix.
Note If D3DPMISCCAPS_CLIPTLVERTS is set, then the device clips post-transformed vertex data to the z and (x, y) viewport limits. Clipping to user-defined clip planes is not supported for post-transformed vertex data. If the D3DPMISCCAPS_CLIPTLVERTS capability is not set, then the application is required to clip these primitives to the z limit and at least to the guardband extent in x and y. On the other hand, clipping for pre-transformed vertices is fully supported in both drawing and vertex processing calls to a destination vertex buffer followed by another drawing call. This includes user-defined clip planes as well as the z and (x, y) viewport limits.
TL vertex data is always passed directly to the driver for rendering. When using vertex buffers with TL vertex data, there can be significant performance advantages to having the driver allocate these vertex buffers in AGP or video memory. Note that the allocation of a TL vertex buffer may be driver-allocated even when non-TL vertex data —either FVF or non-FVF—is not allowed to be driver-allocated, as would be the case when running on a hardware device that does not support transformation and lighting.
For more information, see Transformed and Lit Vertices.