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Interpolation
In the previous example of merging images to construct an extended focus composite, it is important for the images to be aligned. This is also the case for arithmetic image combinations such as tracking motion by subtracting one image from another, removing background, or Boolean combinations that will be described later on. Sometimes, for instance with the confocal light microscope, a sequence of aligned images can be captured directly. In other cases, such as typical serial section work, computer processing is needed to perform the alignment. In general this may require rotating an image and perhaps stretching to compensate for distortions in cutting.
Typical printing requirements for images are from 150 to 300 pixels per inch. Preparing the image to fit on a page or in a report may require either enlargement or reduction. It is important to bear in mind that enlarging an image does not create any more information, and may in some cases make seeing details more difficult rather than easier. But it is also useful to be able to resize images when needed.
When an image is resized or rotated, the new pixel locations will not in general align exactly with the original ones, so that interpolation is needed to determine the values for the new pixels, as shown in the Interpolation Techniques interactive tutorial. The simplest method is select the nearest pixel to the new location. This preserves pixel values but produces visual artifacts such as stair-stepping along edges. Interpolation using the four nearest neighbors (bilinear) or a larger neighborhood of 16 pixels (bicubic) produce visually more pleasing results, but can blur fine lines and detail.
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Contributing Authors
John C. Russ - Materials Science and Engineering Dept., North Carolina State University, Raleigh, North Carolina, 27695.
Matthew Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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