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bug#41544: 26.3; Possible incorrect results from color-distance
From: |
Simon Pugnet |
Subject: |
bug#41544: 26.3; Possible incorrect results from color-distance |
Date: |
Tue, 26 May 2020 17:29:16 +0100 |
User-agent: |
mu4e 1.2.0; emacs 26.3 |
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
3.24.14)
of 2020-03-26, modified by Debian built on lcy01-amd64-020
Windowing system distributor 'The X.Org Foundation', version
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- bug#41544: 26.3; Possible incorrect results from color-distance,
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