A two-step approach for interactive pre-integrated volume rendering of unstructured grids

In the area of volume visualization the cell projection technique is used widely to display unstructured tetrahedral grids. In this field, the pre-integration of the ray integral has proven to be a powerful method for the display of tetrahedral grids with high quality and at interactive frame rates. Besides the actual performance of the projected tetrahedra algorithm of Shirley and Tuchman, the degree of interactivity is also limited by the update rate of the pre-integrated ray integral and the performance of the graphics card at high screen resolutions. In order to widen those two bottlenecks, we propose a hardware-accelerated pre-integration approach and a rendering method which applies standard 2D texture mapping instead of the previously used 3D texturing. As a result, the update rate of the transfer function is increased by almost a factor of hundred and the rasterization of the tetrahedra is sped up by up to a factor of three without degrading accuracy. As a side effect, pre-integrated unstructured volume rendering can be utilized on a broader range of graphics platforms, since the support of 2D texture mapping is much more common. In summary, our two-step approach greatly increases the interactivity of unstructured pre-integrated volume rendering allowing for a much wider area of application.