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20.3.2. Polyphase Algorithmic Description
The filtering part of the polyphase scaler works by passing a windowed sinc function over the input data.
- For up scaling, this function performs interpolation.
- For down scaling, it acts as a low-pass filter to remove high-frequency data that would cause aliasing in the smaller output image.
During the filtering process, the mid-point of the sinc function must be at the mid-point of the pixel to output. This is achieved by applying a phase shift to the filtering function.
If a polyphase filter has Nv vertical taps and Nh horizontal taps, the filter is a Nv ×Nh square filter.
Counting the coordinate space of the filter from the top-left corner, (0, 0), the mid-point of the filter lies at ((Nv –1)/2, (Nh –1)/2). As in the bilinear case, to produce an output pixel at (i, j), the mid-point of the kernel is placed at
The filter function is positioned over the real solution by adjusting the function’s phase:
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