Bitmap Alpha

Bitmap Alpha

A bitmap selection channel is one in which each pixel is either
fully opaque or fully transparent. This is the type of selection
generated by the Magic Wand tool in Photoshop. You
can feather or blur the resulting edge, but the initial selection
contains no semitransparent pixels.
This type of selection may have an occasional use, but it
belongs to the world of computers, not nature (or optics).
An edge made up of pixels that are either fully opaque or
invisible cannot describe a curve or angle smoothly, and
even a straight line looks unnatural in a natural image if it
completely lacks edge thresholding (Figure 3.6).

Feathered Alpha

Although it’s easy enough to see that a bitmap edge does
not occur in nature, it’s hard to imagine that hard objects
should have transparent, feathered edges. Examine the
edge of this book. It appears sharp.
But now study an image of the same thing, and you’ll fi nd
some degree of edge softness. Adding softness, threshold,
or “feather” to an edge approximates this softness in
the hard digital world of single pixels, which are square
and fundamentally either on or off. Properly feathered
edges can
. Approximate organic curves (Figure 3.7)
. Mimic optics behavior
The fi rst point is intuitive enough once you’ve gained
some experience working with raster images, which are digital
images made up of pixels.
Optics can be observed in any photo with no compositing
whatsoever (Figure 3.8). Viewed close-up, areas at the edge
of objects become a fi ne wash of color combining the foreground
and background. This is not due to inaccuracy in
the camera; it is what happens to light as it travels around
objects in the physical world and then through the lens of
the camera (or your eye).