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FW: imrotate: Differences between Matlab and Octabe


From: Journeaux, Ian
Subject: FW: imrotate: Differences between Matlab and Octabe
Date: Mon, 15 Mar 2010 12:35:55 -0500

In answer to one of my previous question, someone pointed out that the default settings for imrotate were different for Matlab (“nearest”) and Octave (“bilinear”). That was an oversight on our part but as we have dug in a little more, using the “nearest” procedure, there are still small differences between Matlab and Octave.

 

We cut a 200 x 200 pixel image around a feature in one of our samples. We then did a rotation on the image by writing a small Matlab / Octave program, that we ran on both platforms with the 3 (bilinear, bicubic, nearest) techniques, giving 6 results.  

 

For our purposes, we were mainly interested in the nearest procedure as we want to minimize smoothing of the pixel values in the images.

We observed that the nearest versions of Matlab and Octave are not quite the same producing small differences in the results. About half (roughly) of Matlab pixels are 1 grey level above that of Octave, the rest being equal. This suggests a different way of rounding. The central pixel (100,100) where the major shift occurs there is a much larger difference - 8 grey levels. In all cases Matlab grey levels are exactly where they should be wrt the original image.

 

The results for “nearest” are probably good enough for us but since we had dug into it, and observed some differences, I  though we should share the results.

 

Ian Journeaux

Manager – Process Technology

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