One of the
places in the program where Tolerance comes up is in tools like
Magic Wand, Flood Fill and Color Replacer, as well as the Background
Eraser. The first three of these all use a single pixel, and
therefore a single unique color, as the reference relative to
which the Tolerance is computed. This is significant in relation
to what happens elsewhere. The Background Eraser has a complex
definition of Tolerance that I can't go into, except to say it
is entirely different from a Magic Wand tolerance.
Superficially,
you might think that a filter like Select Similar would work
exactly like the Magic Wand. However, it can't. As a selection
modifier, Select Similar has to work on any selection created
in any fashion with any combination of selection tools used both
in "add to" or "subtract from" selection
mode (or, for that matter, with a selection loaded from disk,
etc.). That selection can contain many different colors. Select
Similar therefore has to do two things. First it has to define
some description of "the color of the selection". Second,
it has to apply a Tolerance relative to this description. In
PSP 7 this Tolerance was fixed inside the code to a value that
worked well. In PSP 8 you can adjust this Tolerance to better
meet your needs. Note, however, that this is not the same as
applying a Tolerance to a unique color as in the case of the
Magic Wand. We can probably fiddle with the displayed value of
the Tolerance in Select Similar to make it seems more like the
Tolerance in the Magic Wand but there is no way to make the two
identical in all respects.
Select Color
Range is another selection modifier with a Tolerance setting.
Like the Magic Wand it uses a unique color relative to which
the Tolerance is computed. Unfortunately, however, the metric
for color difference is not the same as in the Magic Wand. In
other words, the concept of a color difference in this filter
is not the same as in the Magic Wand. There are several reasons
for this, the main ones being to get a perceptually useful result
along with a working Sharpness control. The end result is that
the Tolerance in this filter is not directly comparable to the
Tolerance in either Select Similar or in the Magic Wand. Again,
it is possible to tweak the displayed values of the Tolerance
to make them seem more similar to the Tolerance values elsewhere
in the program but they will never be exactly identical in all
respects. (In fact, the Tolerance range in this filter also needs
to be increased a little so that, with white as the reference,
black can be selected (or the converse) but that is another matter.)
