HP Visualize c160L Overview of the Visualize fx graphics - Page 5

Fast Virtual Texturing Texture mapping, which is wrapping a picture over a three dimensional object, has been used over the years as a key feature to enhance photorealism, reduce data set sizes, perform visual analysis, and aid in simulations (see Figure 1). Since texturing calculations are computationally expensive and memory access for large textures can be prohibitively slow, various workstation graphics vendors have provided hardware-accelerated texture mapping as a key differentiator for their product. A primary drawback of these attempts at hardware acceleration is that dedicated local hardware texture memory is limited Figure 1 A 3D textured skull. The VISUALIZE fx4 and fx6 subsystems support a texture map acceleration option. Pictured here is the use of 3D texture mapping OpenGL extensions with this option. This feature allows visualization of 3D data sets such as MRI images. in size and is expensive. To take advantage of the performance boost, graphics applications were constrained to textures that fit in the local hardware texture memory. In other words, the application was responsible for managing this hardware resource. Noticing this obvious artificial application limitation in texturing functionality, performance, and portability, Hewlett-Packard introduced, in the VISUALIZE-48, a new concept in hardware texture mapping called virtual texture mapping. Virtual texture mapping uses the dedicated local hardware texture memory as a true texture cache, swapping in and out of the cache the portions of textures that are needed for rendering a 3D image. Thus, for texturing applications, these limitations were eliminated. The application could define and use a texture map of any size (up to a theoretical limit of 32K texels × 32K texels*) that would be hardware accelerated, eliminating the need for the application to be responsible for managing local texture memory. Using the local hardware texture memory as a cache also means that this memory uses only the portions of the texture maps needed to render the image. This efficiency translates to more and larger texture maps being hardware accelerated at the same time. Applications that previously could not run because of texture size limits can now run because of the unlimited virtual texture size. Also, with only the used portions of the texture map being downloaded to the cache, far less graphics bus traffic occurs. The system design of virtual texture mapping involved changes in the HP-UX operating system to support graphics interrupts, onboard firmware support for these interrupts, the introduction of an asynchronous texture interrupt managing daemon process, and the associated texturing hardware described in this *A texel is one element of a texture. The output of the geometry chip's daisy chain is passed back through the interface chip. Generally, for triangle based primitives, the output takes the form of plane equations. As these floating-point plane equations are returned from the geometry chip to the interface chip and passed on to the texture chips, certain addressed locations in the interface chip will result in the floating-point values being converted to fixed-point values as they pass through. These fixed-point values are in a form the raster chips need to rasterize the primitive. The daisy-chain design allows up to eight of the geometry chips to be used although only three are applied in the case of the VISUALIZE fx6 product at this time. Article 4 • © 1998 Hewlett Packard Company 32 May 1998 • The Hewlett-Packard Journal