Introduction

Techniques

• Trilinear Interpolation of intermediate slices:

In order to enhance the image quality of 2D-texture based volume rendering, an approach to remove the visual artifacts caused by the fixed number of slices is required. The idea to enable real trilinear interpolation is to compute intermediate slices on the fly. The missing third interpolation step is then performed within the rasterization hardware using multi-textures.
• Trilinear Interpolation of arbitrary slices:

At first the cross-section of the slicing plane with the bounding box of the volume is calculated. The resulting intersection polygon is then cut into a set of polygon strips at the intersection line with the object-aligned texture slices. Subsequently for each of these polygon strips the image information is obtained by interpolating the two adjacent texture images. This is achieved by specification of alpha values for the polygon vertices. In this case an alpha value of 0 indicates that the corresponding vertex should be textured with the image information from the first texture. Accordingly, if a value of 1 is specified the second texture image is applied. Within the polygon, Gouraud shading is used to interpolate between the alpha values specified at the polygon vertices. The interpolation between the two texture images is finally performed by the register combiners

• Performance Enhancements:

In addition to the optimization of image quality described above, multi-texturing can be used to speed up rendering. The idea of this approach is to reduce the necessary number of triangles by mapping the textures of multiple slice images onto a single polygon.
 
• Fast shaded Isosurfaces:

Westermann and Ertl [2] have introduced an efficient algorithm that exploits rasterization hardware to display shaded
isosurfaces. Using multi-stage rasterization, this method can be efficiently adapted to PC hardware.

• Shading for Semi-Tranparent Volumes:

The fast algorithm to display isosurfaces directly leads to a shading technique for semi-transparent volume rendering using multi-stage rasterization hardware. The approach is basically the same as the algorithm for shaded isosurfaces, with the exception that alpha-blending is used instead of the alpha-test.

• Clipping with arbitrary clipping geometry:

Multitexturing can also be used to perform clipping with arbitrary clipping geometry. For this purpose a second 'stencil' texture  is generated which determines which voxels of the volume are to be drawn or not. The clipping volume can be interactively moved by adapting texture coordinates.
Cubic clipping of a MRI volume
cubic clipping of a CT volume
cylindric clipping of a CT volume
spheric clipping of a CT volume

Results

• Performance Enhancements by mapping two textures on one slice polygon


• Frame Rates of shaded direct volume rendering


• Frame Rates of the GForce implementation compared to the 3D-texture solution on Onyx2 Base Reality

Download Demo

References

[2] R. Westermann, T. Ertl: Efficiently using Graphics Hardware in Volume Rendering Applications, In Proc. of SIGGRAPH 1998