bug#41544: 26.3; Possible incorrect results from color-distance

[Simon Pugnet]

Hello,

I have noticed some potentially incorrect behaviour from the
`color-distance` elisp function.

As an example, take the following elisp: -
(list
(color-distance '(0 0 0) '(65535 65535 65535))
(color-distance '(65535 65535 65535) '(0 0 0))
(color-distance '(1 2 3) '(4 5 6))
(color-distance '(4 5 6) '(1 2 3)))

Result: (589568 584970 8 0)

Here, I would expect the first two elements to have the same 
result as
well as the third and fourth. This is because conceptually the 
distance
between colour (1 2 3) and (4 5 6) is the same as the distance 
between
(4 5 6) and (1 2 3), etc.

The problem comes from the `color_distance()` C function. In this
function, values are calculated via bit shifts to perform integer
divisions of 256 (>>8) and 512 (>>9). Take for example the 3rd and 
4th
items above (red channel): -
1 - 4 = -3
4 - 1 = 3
but: -
(1 - 4) >> 8 = -1
(4 - 1) >> 8 = 0

Therefore for negative values, there is a difference of 1 every 
time the
bit shift is performed, which is what leads to the discrepancy 
mentioned
above.

Modifying the function to remove these discrepancies causes the 
results
above to become (584970 584970 0 0) which appear to be more 
sensible.

My apologies in advance if this is in fact the correct behaviour 
of this
function.

Kind regards,

--
Simon Pugnet
https://www.polaris64.net/

---

In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 
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