[Octave-bug-tracker] [bug #55780] [octave forge] (image) imresize with 'cubic' option is very wrong

[anonymous]

Follow-up Comment #4, bug #55780 (project octave):

Carnë makes a good point, there should always be a comment when
special-casing something. Is the original author still around to explain this
special case?

If cleaning up `imremap` anyway, I suggest you also remove the redundant
segment

  valid = !isna(warped);
  warped(!valid) = extrapval;

This bit is repeated twice: once inside `grayinterp`, applied to each channel,
and then again after the loop over the channels in `imremap` itself. Except
that `grayinterp` is called with `extrapval=0` (is this a bug? Why not pass
`extrapval`?).

Actually, it looks like `interp2` also should be called with `extrapval`
instead of `0`, then the non-valid parts will be filled with the right value
there. Does `interp2` really return NA values if passing `extrap=0`?

Ouch! There's no pre-allocation for `warped` either!

Looking at this code without the special case, I'd remove `grayinterp`
altogether, it does nothing. I'd leave the relevant bit in `imremap` as
follows:

  ## Interpolate
  sz = size (im);
  n_planes = prod (sz(3:end));
  warped = zeros(sz);     % PREALLOCATE!!!!
  for i = 1:n_planes
    warped(:,:,i) = interp2 (double(im(:,:,i)), XI, YI, interp, extrapval); %
JUST CALL INTERP2 DIRECTLY HERE, AND USE EXTRAPVAL
  endfor

  ## we return image on same class as input
  warped = cast (warped, class (im));


When removing `graybicubic`, `bc` also becomes irrelevant (`sym_sub2ind` is
removed in the patch). The functions `isgriddata` and `isequallyspaced` are
never used in the current code, I would remove them in this patch too.

Cheers,
Cris Luengo.


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