The warping step has produced two warped volumes 
and
from the source and target volumes 
and 
.
Any practical warp is likely to misalign some features of 
and 
, possibly because these were not specifically delineated
by feature elements. Even if perfectly aligned, matching features may
have different opacities. These areas of the morph, collectively
called mismatches, will have to be smoothly faded in/out in the
rendered sequence, in order to maintain the illusion of a smooth
transformation. This is the goal of blending.
We have two alternatives for performing this blending step. It may
either be done by cross-dissolving images rendered from 
and
, which we call 2.5D morphing, or by cross-dissolving the
volumes themselves, and rendering the result, i.e. a full 3D morph.
The 2.5D approach produces smooth image sequences and provides the
view and lighting independence of 3D morphing discussed in
section 1.1; however, some disadvantages of 2D morphing
are reintroduced, such as incorrect lighting and
occlusions. Consequently, 2.5D morphs do not look as realistic as 3D
morphs. For example, the ``missing link'' of
figure 5f lacks distinct teeth, and the base
of the skull appears unrealistically transparent.
For this reason, we decided to investigate full 3D morphing, whereby
we blend the warped volumes by interpolating their voxel values. The
interpolation weight 
is a function that varies over time, where
``time'' is the normalized frame number
. We have the option of using either a linear or non-linear
.