[CVS] :mask property on images imprecise on GNU/Linux

[Oliver Scholz]

>From the Emacs Lisp Reference Manual:

,----[ Display => Images => Image Descriptors ]
| `:mask MASK'
|      If MASK is `heuristic' or `(heuristic BG)', build a clipping mask
|      for the image, so that the background of a frame is visible behind
|      the image.  If BG is not specified, or if BG is `t', determine the
|      background color of the image by looking at the four corners of
|      the image, assuming the most frequently occurring color from the
|      corners is the background color of the image.  Otherwise, BG must
|      be a list `(RED GREEN BLUE)' specifying the color to assume for
|      the background of the image.
`----

Unfortunately this is rather imprecise on a GNU/Linux system (tested
with Debian Woody):

If I take an example PPM image like this:

(setq my-test-ppm
      (concat "P6 8 8 255 "
              [0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 
               0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
               0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
               0 0 1 0 0 0 0 0 0 255 255 255 255 255 255 0 0 0 0 0 0 0 0 1
               0 0 1 0 0 0 0 0 0 255 255 255 255 255 255 0 0 0 0 0 0 0 0 1
               0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
               0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
               0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1]))

Then the following should -- after evaluation -- insert a hollow black
(#000001) square with a tiny white (#FFFFFF) dot in the middle. The
area between the black lines of the square and the white dot should be
transparent.

(insert-image (create-image my-test-ppm 'pbm t :mask '(heuristic (0 0 0))))

On GNU/Linux, however, this appears as a tiny white dot only, nothing
else. As far as the :mask property is concerned, Emacs treats the
colours #000000 and #000001 as the same. This is bad. Emacs should
recognize *especially* the colour used to specify transparent areas
very precisely.

Ironically this works correctly on MS Windows XP.

FYI, it works correctly on GNU/Linux only when the difference between
the colours is big enough.  For example, if I define the PPM above in
the following way, it is o.k.:

(setq my-test-ppm
      (concat "P6 8 8 255 "
              [0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 
               0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5
               0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5
               0 0 5 0 0 0 0 0 0 255 255 255 255 255 255 0 0 0 0 0 0 0 0 5
               0 0 5 0 0 0 0 0 0 255 255 255 255 255 255 0 0 0 0 0 0 0 0 5
               0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5
               0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5
               0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5]))

    Oliver
-- 
Oliver Scholz               7 Ventôse an 211 de la Révolution
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