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How it Works
Trace Contour looks at relative brightness levels
rather than the usual contrast edges, and
surrounds similar areas with a one-pixel-wide line.
For a good example of the filter in action, try running
it on a simple black-to-white gradient. When you
move the Threshold slider, you can clearly see the
difference it makes, moving from the dark areas at a
low Threshold value to light areas at higher Threshold
values. As with Find Edges and Glowing Edges, it’s
best to immediately follow this filter by modifying the
blending mode and opacity.
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The gray gradation
on the target image runs
from absolute black (0 in
brightness terms) to white
(255). Running the filter at
level 128 (half way) with
Lower Edge selected
produces a contour halfway
across the gradient. The
four white quadrants at
the target’s center are also
clearly outlined. Here, the
contour lines have been
overlaid on a faint version
of the target. You can also
see contour lines in the color
gradients where the
brightness level is 128.
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Selecting Lower Edge
shifts the boundary of the
detected area to sit within
the darker area instead
of the lighter. In practical
terms, this is of little
interest. Colored lines show
where contour lines have
been created in each of the
three color channels (four in
CMYK mode). In this
composite example in RGB,
the cyan lines (on the left
side of the left hand)
indicate outlines at the 128
level in the Red channel,
magenta lines indicate the
Green channel, and yellow
lines indicate
the Blue channel.
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Using the Trace Contour Filter
This filter often produces a
wealth of confusing lines on
a white ground when it is
used on an RGB image.
More interesting results
can be found by switching
to Lab color mode.
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Duplicate the
background image layer
twice (to make three layers
in total), then go to Image >
Mode > Lab Color to change
to Lab color mode. Run
the Trace Contour filter
on the top layer, choosing
Lower Edge for a simpler
line structure, and a Level
value of 200 to outlines
the image. Hit OK.
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Hide the top layer,
activate the middle layer,
and open the Trace Contour
filter again. Select a lower
Level value—say, 100.
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Move to the bottom layer
and apply a still lower
figure—50, in this case.
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 Change the blending
mode of the top two layers
to Linear Burn.
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For a final flourish, flatten
the image by clicking on the
fly-out button at the top
right of the Layers palette,
and apply the first two
filters in the Stylize suite.
Use Diffuse with Anisotropic
selected to soften the lines
a little, followed by Emboss
at maximum Height and
Amount to produce this
effect that’s reminiscent
of screen printing.
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Roger Pring is an author, graphic designer, and educator who runs the Cooper Dale design consultancy in London, England.
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