Graphics Reference
In-Depth Information
(a)
(b)
Figure 3.3
Ray sampling through voxels. The black triangles represent sample points; the corre-
sponding values are shown in the grayscale triangles at the right. (a) Normally the value is
assumed to be at the center of the voxel, so the sample point is the point on the ray closest
to the center. (b) Resampling works by sampling interpolated values at more points along
the ray.
known as
ray casting
in volume rendering, resamples the volume data along the
ray. In the simplest form, the sampled value comes from the value at the center of
a voxel (
Figure 3.3(a)
).
A smoother resampling can be obtained by interpolating
the volume data between voxels. In effect, this amounts to giving each voxel a
color (and opacity) gradation rather than a fixed value. More sample points using
the interpolated values produces a better overall result (
Figure 3.3(b)
)
.
The error introduced in ray casting is largest where the volume data changes
most abruptly. Ray casting can sometimes be optimized by using adaptive sam-
pling
(
Figure 3.4
), where more sample points are taken in sections where the
volume data is changing more quickly. The sampling interval is made narrower
when the difference in adjacent voxels values is large.
Volume
Figure 3.4
Adaptive ray sampling works by taking more samples where the density changes more
rapidly.
