Opacity-weighted color interpolation, for volume sampling

Abstract

volume rendering, compositing, ray tracing Volume rendering creates images from sampled volumetric data. The compute intensive nature of volume rendering has driven research in algorithm optimization. An important speed optimization is the use of preclassification and preshading. We demonstrate an artifact that results when interpolating from preclassified or preshaded colors and opacity values separately. This method is flawed, leading to visible artifacts. We present an improved technique, opacity-weighted color interpolation, evaluate the RMS error improvement, hardware and algorithm efficiency, and demonstrated improvements. We show analytically that opacity-weighted color interpolation exactly reproduces material based interpolation results for certain volume classifiers, with the efficiencies of preclassification. Our proposed technique may also have broad impact on opacity-texture-mapped polygon rendering. Abstract Volume rendering creates images from sampled volumetric data. The compute intensive nature of volume rendering has driven research in algorithm optimization. An important speed optimization is the use of preclassification and preshading. We demonstrate an artifact that results when interpolating from preclassified or preshaded colors and opacity values separately. This method is flawed, leading to visible artifacts. We present an improved technique, opacity-weighted color interpolation, evaluate the RMS error improvement, hardware and algorithm efficiency, and demonstrated improvements. We show analytically that opacity-weighted color interpolation exactly reproduces material based interpolation results for certain volume classifiers, with the efficiencies of preclassification. Our proposed technique may also have broad impact on opacity-texture-mapped polygon rendering.

A Practical Evaluation of Popular Volume Rendering Algorithms

“…These renderers use the Pre-DVRI optical model with pre-multiplied opac- [48], which benefits their rendering speed but limits the image quality under magnification.…”

“…In Figure 3, we observe that the image quality achieved with TEX shows severe color-bleeding artifacts due to the non-opacity weighted colors [48], as well as staircasing. The latter is due to the limited precision of the framebuffer and can be reduced by increasing the number of slices.…”

“…al. [48] that it is incorrect to interpolate voxel colors and opacities separately and then compute the product C(s i )·α(s i ). Rather, one must multiply color and opacity beforehand at each voxel and then interpolate the product.…”

“…For the pre-shaded renderers, the color is opacityweighted [48] (i.e., the c b α b product in (10) is interpolated).…”

A Spectral Analysis of Function Composition and its Implications for Sampling in Direct Volume Visualization

“…Wittenbrink et al [24] made the observation that it is important to interpolate f in order to properly super-sample (g • f ), while recently Younesy et al [26] pointed out that it is important to low-pass filter g in order to sub-sample (g • f ).…”