[Octave-bug-tracker] [bug #57504] Scaling error for imresize in Image 2.0.0

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?57504>

                 Summary: Scaling error for imresize in Image 2.0.0
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Mon 30 Dec 2019 01:49:04 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: Pawel M Stasik
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 5.1.0
        Operating System: Any

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Details:

I've noticed this bug some time ago.

When the function scales the image, it uses the indexes of the pixels as the
coordinates. This is wrong, as the actual approach should be assuming the
points of interest should be in the middle of pixels - so shifted by 0.5. The
way the interpolation is implemented in Image 2.0.0 results in a small zoom.
I've checked the newest release of the image package (2.6.0) and it seems the
error is still in there.

I use the following procedure to resolve the issue for myself (at the basis).

*****

XI = 1:targetWidth;
XI = size(im,2)/targetWidth*(XI-0.5)+0.5;
                
YI = 1:targetHeight;
YI = size(im,1)/targetHeight*(YI-0.5)+0.5;

im_scaled = imremap(im, XI, YI);

*****
I can't recall it now perfectly, but I might have included some code to ensure
the indexes don't exceed the image. So it's worth taking into account that
border-cases also might need being taken care of. As I've said, I don't recall
perfectly now id imremap requires that.

I have not updated my Octave in some time, but I reckon unless the Image
package was updated, the error is still in it.




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