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#### funzoom

Name

funzoom Image reduction by projection on a B-spline space

Command Synopsis

funzoom [-z z] [-x tx] [-y ty] [-o o] in out

-z z : unzoom factor (default 2.0)

-x tx : to first translate (x) the original image

-y ty : to first transalte (y) the original image

-o o : spline space order, 0..5, default 0

in : input Fimage

out : output Fimage

Function Summary

Fimage funzoom (in , out , z , o , tx , ty )

Fimage in , out ;

float *z , *tx , *ty ;

int *o ;

Description

This module unzooms an image by a factor z. The input image is first projected orthogonally onto the space of B-splines of order n (see the fcrop module). The order n can be specified by the -o option. Then, an appropriate subsampling is performed. Hence, the L2 norm of the error caused by image reduction is minimized. This method, described in [UAE95], allows to avoid undesirable artifacts like aliasing.

If n = 0, the method is a simple averaging under the assumption that pixel are adjacent squares. The input image u is supposed to take constant values in each 1×1 square (nearest neighbor interpolation), and the output value at a given pixel is given by the average of u on the corresponding z×z square. In particular, this yields the classical simple block averaging'' for integer reduction factors.

For n 1, the algorithm consists in two steps :

1. a zoom by a certain cubic (non-regular) spline, *, where is the B-spline of order 1 and (x) = (x/z).

2. an inverse spline transform of order 3, performed by the finvspline module.

Version 2.3

Last Modification date : Tue Feb 19 13:45:23 2002

Author

Lionel Moisan

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mw 2004-05-05