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    <p>Unfortunately, I cannot find a link to the original survey, but as Hollywood cinematographers moved from film to digital images, this question came up a lot, so someone (maybe SMPTE, maybe the ASC) gathered a bunch of professional cinematographers and showed them footage that had been rescaled using a bunch of different algorithms. The results were that for these pros looking at huge motion pictures, the consensus was that <strong>Mitchell</strong> (also known as a high-quality Catmull-Rom) is the best for scaling up and <strong>sinc</strong> is the best for scaling down. But sinc is a theoretical filter that goes off to infinity and thus cannot be completely implemented, so I don't know what they actually meant by 'sinc'. It probably refers to a truncated version of sinc. <strong>Lanczos</strong> is one of several practical variants of sinc that tries to improve on just truncating it and is probably the best default choice for scaling down still images. But as usual, it depends on the image and what you want: shrinking a line drawing to preserve lines is, for example, a case where you might prefer an emphasis on preserving edges that would be unwelcome when shrinking a photo of flowers.</p> <p>There is a good example of the results of various algorithms at <a href="http://www.cambridgeincolour.com/tutorials/image-resize-for-web.htm">Cambridge in Color</a>. </p> <p>The folks at <em>fxguide</em> put together <a href="http://www.fxguide.com/featured/Keeping_Your_Renders_Clean/">a lot of information</a> on scaling algorithms (along with a lot of other stuff about compositing and other image processing) which is worth taking a look at. They also include test images that may be useful in doing your own tests.</p> <p>Now ImageMagick has an <a href="http://www.imagemagick.org/Usage/filter/">extensive guide on resampling filters</a> if you really want to get into it. </p> <p>It is kind of ironic that there is more controversy about scaling down an image, which is theoretically something that can be done perfectly since you are only throwing away information, than there is about scaling up, where you are trying to add information that doesn't exist. But start with Lanczos. </p>
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