The pixel cross-dissolving of 2D morphing suggests a linear 
.
Indeed, it works well for blending the color information of 
and 
. However, it fails to interpolate opacities in a manner
such that the rendered morph sequence appears smooth. This is due to
the exponential dependence of the color of a ray cast through the
volume on the opacities of the voxels it encounters. This phenomenon
is illustrated in the morph of figure 5. In
particular, the morph abruptly snaps into the source and target
volumes if a linear 
is used:
figure 5g shows that at time 
, very
early in the morph, the empty space towards the front of the human
head has already been filled in by the warped orangutan volume.