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Concept
Image scaling
Summary
In computer graphics and digital imaging, image scaling refers to the resizing of a digital image. In video technology, the magnification of digital material is known as upscaling or resolution enhancement.
When scaling a vector graphic image, the graphic primitives that make up the image can be scaled using geometric transformations, with no loss of . When scaling a raster graphics image, a new image with a higher or lower number of pixels must be generated. In the case of decreasing the pixel number (scaling down) this usually results in a visible quality loss. From the standpoint of digital signal processing, the scaling of raster graphics is a two-dimensional example of sample-rate conversion, the conversion of a discrete signal from a sampling rate (in this case the local sampling rate) to another.
Image scaling can be interpreted as a form of image resampling or image reconstruction from the view of the Nyquist sampling theorem. According to the theorem, downsampling to a smaller image from a higher-resolution original can only be carried out after applying a suitable 2D anti-aliasing filter to prevent aliasing artifacts. The image is reduced to the information that can be carried by the smaller image.
In the case of up sampling, a reconstruction filter takes the place of the anti-aliasing filter.
File:160 by 160 thumbnail of 'Green Sea Shell'.png | Original 160x160px image
File:160 by 160 thumbnail of 'Green Sea Shell' - 0. in fourier domain.png | Original image in spatial-frequency domain
File:160 by 160 thumbnail of 'Green Sea Shell' - 1. fourier filtered for downsampling to 40 x 40.png | 2D [[low-pass filter]]ed, but still at 160x160px
File:160 by 160 thumbnail of 'Green Sea Shell' - 1.1. fourier filtered image for downsampling to 40 x 40 in fourier domain.png | [[Filter (signal processing)|Filtered]] image in spatial-frequency domain
File:160 by 160 thumbnail of 'Green Sea Shell' - 2. downsampling to 40 x 40 (nearest neighour).png | low-pass filtered 160x160px image 4× downsampled to 40x40px{{nbsp}}
File:160 by 160 thumbnail of 'Green Sea Shell' - 3.
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