[bug #68044] Typo in "__magick_read__.cc"

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URL:
  <https://savannah.gnu.org/bugs/?68044>

                 Summary: Typo in "__magick_read__.cc"
                   Group: GNU Octave
               Submitter: dasergatskov
               Submitted: Fri 13 Feb 2026 11:27:47 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name:
        Originator Email:
             Open/Closed: Open
         Discussion Lock: Unlocked
                 Release: stable
        Operating System: Any
           Fixed Release: None
         Planned Release: None


    _______________________________________________________

Follow-up Comments:


-------------------------------------------------------
Date: Fri 13 Feb 2026 11:27:47 PM UTC By: Dmitri A. Sergatskov <dasergatskov>
See:
https://octave.discourse.group/t/typo-in-magick-read-cc/7338

 "__magick_read__.cc" has an obvious typo around line 367


365               const octave_idx_type nPixels = alpha.numel ();
 366               for (octave_idx_type pix = 0; pix < nPixels; pix++)
 367                 alpha_fvec[pix] = 1 - amap_fvec[static_cast<int>
(img_fvec[3])];
 368 
 369               retval(2) = alpha;


The diff:

diff -r 4707e181cf09 libinterp/corefcn/__magick_read__.cc
--- a/libinterp/corefcn/__magick_read__.cc      Wed Feb 11 08:41:16 2026
+0100
+++ b/libinterp/corefcn/__magick_read__.cc      Fri Feb 13 18:10:04 2026
-0500
@@ -364,7 +364,7 @@
               // 1 for transparent and 0 for opaque so we fix that here.
               const octave_idx_type nPixels = alpha.numel ();
               for (octave_idx_type pix = 0; pix < nPixels; pix++)
-                alpha_fvec[pix] = 1 - amap_fvec[static_cast<int>
(img_fvec[3])];
+                alpha_fvec[pix] = 1 - amap_fvec[static_cast<int>
(img_fvec[pix])];
 
               retval(2) = alpha;
             }

fixes that for me. That can be verified by using attached GIF
(file test16.gif) GIF seems to be the only format affected by that.
With original Octave:

octave:1> [im, map, alpha] = imread ("test16.gif")
im =

   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
...
alpha =

   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1
   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1
   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1
   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1   1
...

incorrectly shows uniform opacity.
With patched

octave:2> [im, map, alpha] = imread ("test16.gif")
im =

   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
...
alpha =

   1   1   1   1   1   0   1   1   1   1   1   1   1   1   1   1
   1   1   1   1   1   0   1   1   1   1   1   1   1   1   1   1
   1   1   1   1   1   0   1   1   1   1   1   1   1   1   1   1
   1   1   1   1   1   0   1   1   1   1   1   1   1   1   1   1
...

is correct. That can also be verified by extracting alpha channel by
GraphicsMagick

gm convert test16.gif -channel Opacity -depth 16 gray:alpha_gm.raw

(file alpha_gm.raw)
and inspect the result with e.g. "od -o alpha_gm.raw"

$ od -o alpha_gm.raw 
0000000 000000 000000 000000 000000 000000 177777 000000 000000
0000020 000000 000000 000000 000000 000000 000000 000000 000000
0000040 000000 000000 000000 000000 000000 177777 000000 000000
0000060 000000 000000 000000 000000 000000 000000 000000 000000
0000100 000000 000000 000000 000000 000000 177777 000000 000000
...


Dmitri.
-- 







    _______________________________________________________
File Attachments:

Name: test16.gif                     Size: 179B
    <https://file.savannah.gnu.org/file/test16.gif?file_id=58238>

Name: alpha_gm.raw                   Size: 512B
    <https://file.savannah.gnu.org/file/alpha_gm.raw?file_id=58239>



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