Image-Order Volume Rendering

The image-order approach to volume rendering determines, for each pixel on the image plane, the data samples which contribute to it. Raycasting [24] is an image-order algorithm that casts viewing rays through the volume. At discrete intervals along the ray, the three-dimensional function is reconstructed from the samples and the optical model is evaluated. This process is illustrated in Figure 2.1. As the accumulation is performed in front-to-back order, viewing rays that have accumulated full opacity can be terminated. This very effectively avoids processing of occluded regions and is one of the main advantages of raycasting. One challenge in raycasting is the efficient skipping of non-contributing regions (i.e., regions that have been classified as transparent). As typical medical datasets commonly contain a large number of such voxels, this has a major performance impact.

Figure 2.1: Illustration of raycasting. A ray starting at an image pixel is cast through the volume, evaluating the optical model at each resample location.

Numerous approaches for improving the performance of raycasting have been presented. Most of them rely on one or more of the following principles:

Subsections

• Image-Space Coherency
• Object-Space Coherency
• Inter-Ray Coherency
• Inter-Frame Coherency
• Empty Space Skipping
• Efficient Memory Access