Multiple-element Textures (Direct3D 9)
Traditional textures are considered to be single-element textures. Multiple-element textures enable applications to write simultaneously to multiple elements of a texture from the pixel shader. The result in the next rendering pass is that an application can use one or more of the elements as a single-element texture - that is, as inputs to the pixel shader. These additional elements can be thought of as a temporary store for intermediate results that will be used in a later pass by the application.
The first generation of hardware that exposes this feature has the following restrictions:
- All multiple-element texture surfaces will be allocated automatically. This limitation is addressed by treating this as a new type of surface format with multiple RGBA channels interleaved.
- All elements of the multiple element texture can have the same bit depth. This limitation is expressed by the name of the new surface formats.
- A multiple-element texture cannot be the primary/displayable. In other words, it must be off-screen only. This limitation is expressed by the surface-format enumeration.
- No dithering, alpha test, fogging, blending, raster-op, or masking is allowed. No post-pixel shader processing is done, except the z-test and stencil test.
- The texture cannot be a mipmap. Creation of the mip chain will fail.
- The same element cannot be set as a texture at the same time it is a render target. However, different elements of the same multiple-element texture surface can simultaneously be textures and render targets.
- No antialiasing is supported.
- Multiple-element texture surfaces, when used as a texture, cannot be filtered. This limitation can be verified using CheckDeviceFormat.
- Multiple-element texture surfaces cannot be locked.
- More than one multiple-element texture surface can be used simultaneously by assigning each to various stages, just as with normal textures.
- Multiple-element texture surfaces support conversion of gamma from 2.2 to 1.0 conversion on a read operation, just as with other texture formats.
- Some of the implementations do not apply the output write mask (D3DRS_COLORWRITEENABLE). Those that can have independent color write masks. This is expressed using a new capability bit. The number of independent color write masks available will be equal to the maximum number of elements of which the device is capable.
- Clear clears all elements of the multiple-element texture which is set as the render target.
The usage of multiple-element textures follows these steps:
- Applications discover support for this feature by checking for the availability of multiple-element texture formats.
- The application creates these surfaces by calling CreateTexture.
- The application sets the surface as a render target using the SetRenderTarget call. The pixel shader provides output to the surfaces using the mov - ps instruction.
- SetTexture is called to set a multiple-element texture surface to a particular stage. As with other textures, the same surface is allowed to be set to multiple stages at once.
- SetSamplerState is called to set D3DSAMP_ELEMENTINDEX to the appropriate element number in the multiple-element texture from which the sampler samples. Default value for this state is 0, which means non-multiple-element textures will work. Setting this state to an inappropriate number results in an undefined behavior - if the multiple-element texture is only two elements wide but the sampler is asked to sample from the fourth element, for example.
The following is a summary of the API elements that support multiple-element textures:
- The D3DFMT_MULTI2_ARGB8 surface format expresses the interleaved nature of the format.
- The D3DSAMP_ELEMENTINDEX sampler state indicates which element index to use.
- The following render states support multiple-element textures:
D3DRS_COLORWRITEENABLE applies to render target (or element) zero.
- The D3DPMISCCAPS_INDEPENDENTWRITEMASKS flag indicates that the device supports independent write masks for multiple element textures or multiple render targets.