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<h2 class="title" style="clear: both"><a id="filters-edge"></a>5. Edge-detect filters</h2>
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Edge detect filters search for borders between different colors
and so can detect contours of objects.
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They are used to make selections and for many artistic purposes.
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Most of them are based on gradient calculation methods and give
thick border lines. Look at fig.1 which represents color
intensity variations. On the left is a slow color gradient which
is not a border. On the right is a quick variation which is an
edge. Now, let us calculate the gradient, the variation speed, of
this edge, i.e the first derivative (fig.2). We have to decide
that a border is detected when gradient is more than a threshold
value (the exact border is at top of the curve, but this top
varies according to borders). In most cases, threshold is under
top and border is thick.
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The Laplacian edge detection uses the second derivative (fig.3).
The top of the curve is now at zero and clearly identified. That's
why Laplace filter renders a thin border, only a pixel wide. But
this derivative gives several zeros corresponding to small
ripples, resulting in false edges.
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Some blurring before applying edge filters is often necessary: it
flattens small ripples in signal and so prevents false edges.
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