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Filtering in image processing is a process that cleans up appearances and allows for selective highlighting of specific information. A number of techniques are available and the best options can depend on the image and how it will be used. Both analog and digital image processing may require filtering to yield a usable and attractive end result. This can be a routine part of the editing process used to prepare images for distribution.
In the case of film photography, when a photographer develops prints, it may be necessary to use filtering to get the desired effects. Filters can be mounted in the enlarger to improve image quality, or for activities like developing black and white prints from color negatives. The photographer may perform tests with several filters to find the most appropriate.
Film photographers can use filtering in image processing for activities like sharpening up contrast. The filter can change the wavelength of the light as it passes through the enlarger, altering the resulting exposure and developed image. Kits of common filters for enlargers and cameras are widely available commercially.
Digital filtering offers a number of advanced photo manipulation options beyond the basic filters used in photo development. One common use of filtering in image processing is to remove blur. Images may be blurry because of file degradation, moving objects in the frame when the photo was taken, and other issues. The photographer can use a filtration algorithm to selectively target pixels and smooth the image out. More complex filters may be able to reconstruct partially damaged images through averaging, using available data to estimate missing contents in an image.
Another use for filtering in image processing is in the handling of images where technicians want to highlight specific objects of interest in the picture. For example, astronomers might pass an image through filters to selectively restrict data from certain wavelengths. This can allow other information in the image to pop into relief. Filters can also remove noise like haze from images to make them cleaner and clearer, even if they are not specifically blurred.
Software programs allow for very complex filtering in image processing. Many come with presets that people can use for basic tasks like adding soft filters to portraits or sharpening up contrast in dim images. Users can also develop their own filters, coding in specific parameters to make a custom version for a particular need or project. This may require advanced programming skills, as well as a thorough knowledge of how photography works to yield the best results.
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