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

From: Simon Pugnet <[email-masked]>
Date: Tue, 26 May 2020 17:29:16 +0100

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:
  tooltip-mode: t
  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 
epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies 
mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail 
rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair 
time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd 
tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace 
newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock 
font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham 
georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech 
european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray 
minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting 
font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

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))