bug#22253: Background of transparent PNG images uses default background color, instead of current one

[Clément Pit--Claudel]

Hi all,

When no :background is specified for a transparent PNG, the background color of 
the 'default face is used. This causes aliasing effect to appear then e.g. the 
image is highlighted (transient-mark-mode).

To reproduce (on latest emacs-25):

* Save https://i.stack.imgur.com/BqrLK.png as ~/rooster.png
* emacs -Q
  M-: (insert #(" 🐤" 1 2
          (display
           (image :type imagemagick :file "~/rooster.png" :ascent center :mask 
heuristic :height 150))))
* Notice that the picture looks perfectly antialiased against the default white 
background
* C-x h
* Notice the aliasing

Can I provide more information on this bug?

Cheers,
Clément.

In GNU Emacs 25.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2015-10-20
Repository revision: 13274cd5d9eb87273c14656084286d5321193858
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:     Linux Mint 17.2 Rafaela

Configured using:
 'configure CFLAGS=-O3'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LC_TIME: en_DK.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-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 gnus-util mail-extr emacsbug message dired format-spec
rfc822 mml mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util help-fns help-mode easymenu cl-loaddefs pcase cl-lib mail-prsvr
mail-utils time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment
elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
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 charscript
case-table epa-hook jka-cmpr-hook help simple abbrev 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 dbusbind inotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 81322 5072)
 (symbols 48 19200 0)
 (miscs 40 53 159)
 (strings 32 12798 5368)
 (string-bytes 1 370572)
 (vectors 16 10812)
 (vector-slots 8 418744 6298)
 (floats 8 134 160)
 (intervals 56 313 16)
 (buffers 976 11)
 (heap 1024 38000 1041))

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