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several frames as the camera does a complete rotation around each of the principal
axis for an equal amount of frames. As the animation progresses, we observe that
the Hausdorff distance decreases as the process converges to a full voxelization.
6.6
Limitations
One limitation of our method is that the cleanup phase will only remove invalid
voxels that are visible in any of the current image buffers (camera multiple ren-
der targets and light RSMs). The visible invalid voxels will be removed from the
voxelization the next time they appear in the image buffers. However, the cor-
rectness of the voxelization cannot be guaranteed for existing voxels that are not
visible in any buffer. For moving geometry, some progressively generated voxels
may become stale, as shown in the case of the bottom right of Figure 6.1. Never-
theless, in typical dynamic scenes, the stale voxels are often eliminated either in
subsequent frames due to their invalidation in the moving camera buffer or due
to their invalidation in other views in the same frame (see Figure 6.11).
Another limitation is that the extents of the voxelization region must remain
constant throughout volume updates; otherwise computations are performed with
stale buffer boundaries. When the bounding box of the scene is modified or the
scene changes abruptly or it is reloaded, the attribute volumes must be deleted
and progressively populated again. This is also the reason why the cascaded
light propagation volumes method of [Kaplanyan and Dachsbacher 10] could not
take advantage of progressive voxelization for the cascades near the user, as the
method assumes that they follow the user around, constantly modifying the cur-
rent volume extents.
Figure 6.11. Correct indirect shadowing effects and color bleeding: Stale voxels from
one view (behind the tank) are effectively invalidated in other views (reflective shadow
map).
 
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