A filter is an image processing routine applied to an image to enhance
details. This is usually done in one of two ways: by smoothing
the image to remove noise and show subtle features hidden by the noise, or by sharpening
the image to reveal more fine details. For now we will look at the effects
of each filter. To see how each filter actually works visit the
Advanced
Image Processing page.
Low-Pass Filters
A low-pass filter is used to smooth an image. This reduces the effects
of noise by canceling out rapid variations from pixel to pixel. Noise
appears as random spots in the image which usually are very different in value
from their neighboring pixels. Light from the object being imaged very
rarely changes in such a rapid manner; rather the brightness changes more
gradually across many pixels. By analyzing the values of each pixel in the
image and reducing rapid variations in brightness.
Note: Many programs refer to
low-pass filters simply as a Smooth or Blur function.
Above: On top, an unfiltered M17 image. Below is the
same image after being smoothed with a low-pass filter. Note that the
lower image appears less noisy, but at the expense of fine detail.
High-Pass Filters
High-pass filters operate in the same way as low-pass filters: by analyzing
the values of each pixel and changing it based on the values of its
neighbors. However, the effect is the opposite. Instead of smoothing
and image, high-pass filters sharpen an image and bring out finer details.
Unfortunately, they also increase noise and can create interesting artifacts if
not used carefully. Over-sharpening an image should be avoided and
starting with an
image with very low noise is always preferable.
Above: Image of M20 before and after sharpening with a
high-pass filter. Sharpening brings out finer details and increases the
brightness of faint stars. But it also increases noise. This can be
seen in the fainter parts of the blue nebula, especially around the top and
bottom of the red nebula.
Note: Another common form of
filtering is Unsharp Masking. Not all programs offer this feature and so
it is covered under Advanced Image Processing.
Next, Color Image Processing....
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