GNU bug report logs - #41544
26.3; Possible incorrect results from color-distance

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Simon Pugnet <simon <at> polaris64.net>
To: bug-gnu-emacs <at> gnu.org
Subject: 26.3; Possible incorrect results from color-distance
Date: Tue, 26 May 2020 17:29:16 +0100

[Message part 1 (text/plain, inline)]

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 
11.0.12008000
System Description:	Ubuntu 20.04 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
(589568 584970 8 0)
You can run the command ‘eval-print-last-sexp’ with C-j
Mark activated
kill-line: End of buffer
Making completion list...
You can run the command ‘kill-region’ with C-w

Configured using:
'configure --build x86_64-linux-gnu --prefix=/usr
--sharedstatedir=/var/lib --libexecdir=/usr/lib
--localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --enable-libsystemd --with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.3/site-lisp:/usr/share/emacs/site-lisp
--with-sound=alsa --without-gconf --with-mailutils --build
x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
--libexecdir=/usr/lib --localstatedir=/var/lib
--infodir=/usr/share/info --mandir=/usr/share/man 
--enable-libsystemd
--with-pop=yes
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.3/site-lisp:/usr/share/emacs/site-lisp
--with-sound=alsa --without-gconf --with-mailutils --with-x=yes
--with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
-fdebug-prefix-map=/build/emacs-mEZBk7/emacs-26.3+1=. 
-fstack-protector-strong
-Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
-D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions 
-Wl,-z,relro''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS 
GLIB
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT 
ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS LIBSYSTEMD LCMS2

Important settings:
 value of $LANG: en_GB.UTF-8
 value of $XMODIFIERS: @im=none
 locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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 global-eldoc-mode: t
 eldoc-mode: t
 electric-indent-mode: t
 mouse-wheel-mode: t
 tool-bar-mode: t
 menu-bar-mode: t
 file-name-shadow-mode: t
 global-font-lock-mode: t
 font-lock-mode: t
 blink-cursor-mode: t
 auto-composition-mode: t
 auto-encryption-mode: t
 auto-compression-mode: t
 line-number-mode: t
 transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired 
dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived 
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custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting 
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Memory information:
((conses 16 95338 8459)
(symbols 48 20386 1)
(miscs 40 50 168)
(strings 32 28445 1160)
(string-bytes 1 747812)
(vectors 16 13900)
(vector-slots 8 500980 11290)
(floats 8 51 264)
(intervals 56 301 25)
(buffers 992 12))

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