Splatting without the blur

Abstract

Splatting is a volume rendering algorithm that combines efficient volume projection with a sparse data representation: Only voxels that have values inside the iso-range need to be considered, and these voxels can be projected via efficient rasterization schemes. In splatting, each projected voxel is represented as a radially symmetric interpolation kernel, equivalent to a fuzzy ball. Projecting such a basis function leaves a fuzzy impression, called a footprint or splat, on the screen. Splatting traditionally classifies and shades the voxels prior to projection, and thus each voxel footprint is weighted by the assigned voxel color and opacity. Projecting these fuzzy color balls provides a uniform screen image for homogeneous object regions, but leads to a blurry appearance of object edges. The latter is clearly undesirable, especially when the view is zoomed on the object. In this work, we manipulate the rendering pipeline of splatting by performing the classification and shading process after the voxels have been projected onto the screen. In this way, volume contributions outside the iso-range never affect the image. Since shading requires gradients, we not only splat the density volume, using regular splats, but we also project the gradient volume, using gradient splats. However, alternative to gradient splats, we can also compute the gradients on the projection plane, using central differencing. This latter scheme cuts the number of footprint rasterization by a factor of four, since only the voxel densities have to be projected. Our new method renders objects with crisp edges and well-preserved surface detail. Added overhead is the calculation of the screen gradients and the per-pixel shading. Both of these operations, however, may be performed using fast techniques employing lookup tables.