Image Processing Reference
In-Depth Information
4.3 Extracting Hair from the
Background—a Tricky Task
Masking out and extracting a portrait of a woman with
cascading hair or a tree with a maze of branches are
difficult tasks even for professional digital editors. You'll
have more success if there is a good degree of contrast
between objects you wish to extract and the background.
You can tackle this task relatively simply by using the
Fuzzy Select tool (Magic Wand)
and the
Select by Color
tool
.
However, you may have to prepare your image first by
increasing the contrast. Finding the correct tool (and tool
options) requires practice; you'll soon get a feel for it. The
next sections will give you insight into the use of these
select tools.
4.3.1 The Threshold Function
The
Threshold
function (
Colors > Threshold
) converts a
color or grayscale image into a pure black-and-white
graphic. More specifically, it represents areas with a
brightness value of less than 50% black and a brightness
value of more or equal to 50% white.
If the
Preview
control is checked, you will see a pure
black-and-white representation in the image window.
The
Threshold Range
input boxes allow you to manually
select the upper and lower intensity ranges, i.e., the
black-white distribution in the image. You can either
enter numerical values in the intended fields or you can
move the sliders beneath the histogram curve with the
left mouse button.
The
Linear
and
Logarithmic
buttons in the upper-
right corner can be clicked to determine how the
histogram will be represented in the visual graph.
Figure 4.15
The Threshold window with a preview of the results in the
image window
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