[Octave-bug-tracker] [bug #40669] rgb2ind: uses system ("gm convert....") for dither option

[Carnë Draug]

Follow-up Comment #2, bug #40669 (project octave):

I would not recommend to make use of GM for the following reasons:

* because of other issues when reading and writing images, I'm guessing we'll
eventually have to replace GM with something else;

* the Floyd and Steinberg algorithm is quite easy to implement, no need to add
a dependency because of that;

* the algorithm can be done in different ways. The diffusion of the error is
usually done from left to right, and top to bottom but other variants do it
from bottom to top. My previous attempt at it found that Matlab does not
implement the standard one so depending on GM for this will not allow us to be
Matlab compatible;

* GM only does this for 8bit images;

Finally, dithering should not be implemented in rgb2ind, there's a missing
function "dither" that rgb2ind should be calling. My guess (from trying to do
this last year) is that for the complete rgb2ind to be implemented, the
functions imapprox and dither need to be implemented first (each of the
functions implementing different methods to perform the conversion).

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