3.1 Introduction
When dealing with large surfaces and selections, I found channels to
be very useful. They can be modified and painted like a grayscale layer,
and can be loaded instantly as a selection. By default, the selected
areas are drawn black, and shades of grey show some partial selection.
Some useful operations can be done between channels, and a selection
can be saved as a channel, even to another document (it
must be the same size). Thus, to make thinks clearer, we will save/load
all the selections to/from channels of a work document, and make changes
to the shapes and create shadows inside this document.
The first time you save a selection, go to SELECT > SAVE SELECTION
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select document > new, name the channel. This will automatically
create a new grayscale document. For further saves, choose the document
name (usually Untitled-1 by default) and a new channel (give it a name).
You can stack the channels and delete them as soon as you dont
need them.
Calculations with channels
Well make extensive use of two similar Photoshops functions
: IMAGE > CALCULATIONS and IMAGE > APPLY IMAGE.
CALCULATIONS is when you want to save the result in a new
channel; APPLY IMAGE is to modify the active channel.
The operation is defined by the blending mode. Well use three
of them. The results may seem paradoxical, but remember that its
much like mathematic operations on each pixel, blacks value is
0 and whites ~1 (255/256). Thus to add doesnt
mean that you put together all black surfaces. The addition is between
the same pixel of each channel, and for instance, white added to black
results in white. When you are not sure, refer to the examples given
here :
Channels:
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Alpha 1
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Alpha 2
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Calculations :
Effect on selection :
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Multiply Alpha 1 with Alpha 2
Gathering of areas
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Add Alpha 1 and Alpha 2
Intersection of areas
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Add reverse of Alpha 2 to Alpha 1
Clip Alpha 2 from Alpha 1 area
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