Graphics Reference
In-Depth Information
transparency or translucency inparts of the foreground. Thus, every image, nomatter
how high resolution, has fractional alpha values.
Matting is closely related to a computer vision problem called
image segmenta-
tion
, but they are not the same. In segmentation, the goal is to separate an image
into hard-edged pieces that snap together to reconstitute the whole, which is the
same as creating a binary alpha matte for each object. On the other hand, for matting
we expect that the foreground “piece” should have soft edges, so that pixels at the
boundary contain a combination of foreground and background colors. As illustrated
in Figure
2.2
, clipping a hard-segmented object out of one image and placing it in
another generally results in visual artifacts that would be unacceptable in a produc-
tion environment, while a continuous-valued alpha matte produces a much better
result. Nonetheless, we discuss a few hard segmentation algorithms in Section
2.8
and discuss how they can be upgraded to a continuous matte.
Unfortunately, the matting problem for a given image can't be uniquely solved,
since there aremany possible foreground/background explanations for the observed
colors.We can see this fromEquation (
2.2
) directly, since it represents three equations
•
•
=
+
=
+
I
α
F
1
α
B
Figure 2.1.
An illustration of the matting equation
I
B
. When
α
is 0, the image
pixel color comes from the background, and when
α
is 1, the image pixel color comes from the
foreground.
=
α
F
+
(
1
−
α)
(a)
(b)
(c)
(d)
Figure 2.2.
Image segmentation is not the same as image matting. (a) An original image, in
which the foreground object has fuzzy boundaries. (b) (top) binary and (bottom) continuous alpha
mattes for the foreground object. (c) Composites of the foreground onto a different background
using the mattes. The hard-segmented result looks bad due to incorrect pixel mixing at the soft
edges of the object, while using the continuous alpha matte results in an image with fewer visual
artifacts. (d) Details of the composites in (c).
