Graphics Reference
In-Depth Information
In the simplest case,
a
might merely be a linear function in terms of the current frame number and the
range of frame numbers over which the morph is to take place. However, as Wolberg [
37
] points out, a
nonlinear blend is often more visually appealing. It is also useful to be able to locally control the cross-
dissolve rates based on aesthetic concerns. For example, in one of the first popular commercials to use
morphing, in which a car morphs into a tiger, the front of the car is morphed into the head of the tiger at
a faster rate than the tail to add to the dynamic quality of the animated morph.
Animated images are morphed by the user-defined coordinate grids for various key images in each
of two animation sequences. The coordinate grids for a sequence are then interpolated over time so that
at any one frame in the sequence a coordinate grid can be produced for that frame. The interpolation is
carried out on the
x
-
y
positions of the grid intersection points; cubic interpolation such as Catmull-Rom
is typically used. Once a coordinate grid has been produced for corresponding images in the animated
sequences, the morphing procedure reduces to the static image case and proceeds according to the pre-
vious description (see
Figure 4.47
)
.
4.5.2
Feature-based morphing
Instead of using a coordinate grid, the user can establish the correspondence between images by using
feature lines [
2
]. Lines are drawn on the two images to identify features that correspond to one another
and feature lines are interpolated to form an intermediate feature line set. The interpolation can be
based on interpolating endpoints or on interpolating center points and orientation. In either case, a map-
ping for each pixel in the intermediate image is established to each interpolated feature line, and a
key grids
interpolated
grid
Source image sequence
intermediate grid
Destination image sequence
interpolated
grid
key grids
FIGURE 4.47
Morphing of animated sequences.
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