ALE
Image Processing Software
Deblurring, Anti-aliasing, and Superresolution.
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# Drizzling Renderer

ALE versions 0.4.1 and later implement a drizzling algorithm based on that outlined in research by Richard Hook and Andrew Fruchter.1

## Algorithm

In the drizzle rendering method, pixels from source images are associated with a square region of given radius in the source image coordinate system. Similarly, each pixel in the accumulated image, or target image, is associated with a square region, such that each square region coincides with its neighbors exactly at its edges. The source regions are then transformed according to the known alignment between source and target, and the regions from all transformed source images are combined to form the target image, where each source pixel region contributes to each target pixel region linearly with the area of overlap between the regions (as calculated in the target coordinate system).

ALE uses an approximation to the above approach, wherein the area of overlap is calculated in the coordinates of the source image and then multiplied by a factor approximating any difference in scale between the source and target images; for the purpose of these calculations, the region of the target pixel is approximated by a rectangle in the source coordinate system having all sides parallel to the source image coordinate axes.

## Properties

Assuming small drizzling radius, drizzling is approximately the same as convolution of discrete pixel data with a box filter having the same radius as the chosen drizzling radius. Given the box filter approximation, jittering assumption, point sampling, and sufficiently small radius, drizzling should provide an acceptable approximation of T. Until this is proven, however, this result should be considered unreliable.

1 More information on drizzling can be found in the paper by Hook and Fruchter, "Variable-Pixel Linear Combination", published in vol. 125 of the ASP Conference Series (eds. Gareth Hunt and H. E. Payne). This paper was also published on-line at:

http://www.cv.nrao.edu/adass/adassVI/hookr.html

Copyright 2002, 2003, 2004 David Hilvert

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