  We report empirical study results on the color encoding of ensemble scalar
and orientation to visualize diffusion magnetic resonance imaging (DMRI) tubes.
The experiment tested six scalar colormaps for average fractional anisotropy
(FA) tasks (grayscale, blackbody, diverging, isoluminant-rainbow,
extended-blackbody, and coolwarm) and four three-dimensional (3D) directional
encodings for tract tracing tasks (uniform gray, absolute, eigenmap, and Boy's
surface embedding). We found that extended-blackbody, coolwarm, and blackbody
remain the best three approaches for identifying ensemble average in 3D.
Isoluminant-rainbow coloring led to the same ensemble mean accuracy as other
colormaps. However, more than 50% of the answers consistently had higher
estimates of the ensemble average, independent of the mean values. Hue, not
luminance, influences ensemble estimates of mean values. For ensemble
orientation-tracing tasks, we found that the Boy's surface embedding (greatest
spatial resolution and contrast) and absolute color (lowest spatial resolution
and contrast) schemes led to more accurate answers than the eigenmaps scheme
(medium resolution and contrast), acting as the uncanny-valley phenomenon of
visualization design in terms of accuracy.
