bug#47043: 27.1; transparent PNG image scaled larger may be blurred

[ynyaaa@gmail.com]

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When a transparent PNG image is scaled larger, it may be displayed as a
blurred image.

The attached image transparent3.png is a green square with red dots.
Its alpha values change one by one from left to right with step 1.
transparent4.png is a similar image whose alpha values are floored into
multiples of four.

When transparent3.png is scaled much larger, the displayed image is
blurred.
  (insert-image (create-image "transparent3.png" 'png nil :scale 5))
  =>t ;; blurred dots are displayed
transparent4.png is displayed with square dots.
  (insert-image (create-image "transparent4.png" 'png nil :scale 5))
  =>t ;; square dots are displayed


[-- Attachment #2: transparent3.png --]
[-- Type: image/png, Size: 408 bytes --]

[-- Attachment #3: transparent4.png --]
[-- Type: image/png, Size: 1188 bytes --]

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In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-08-22 built on CIRROCUMULUS
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.18363
System Description: Microsoft Windows 10 Pro (v10.0.1909.18363.1379)

Recent messages:

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: JPN
  locale-coding-system: cp932

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:
(jka-compr mailcap mule-util eieio-opt cl-extra speedbar sb-image
ezimage dframe apropos image-mode exif dired-aux gnutls network-stream
nsm mailalias smtpmail auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs json map misearch multi-isearch pp shadow sort mail-extr
emacsbug message rmc puny dired dired-loaddefs format-spec rfc822 mml
mml-sec password-cache epa derived epg epg-config gnus-util rmail
rmail-loaddefs text-property-search time-date subr-x seq byte-opt gv
bytecomp byte-compile cconv mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils help-fns radix-tree cl-print
debug backtrace help-mode easymenu find-func cl-loaddefs cl-lib
term/bobcat japan-util tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type mwheel dos-w32 ls-lisp disp-table term/w32-win
w32-win w32-vars term/common-win tool-bar dnd fontset image regexp-opt
fringe tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer 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 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 w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 88647 33894)
 (symbols 48 9671 7)
 (strings 32 28513 2952)
 (string-bytes 1 836070)
 (vectors 16 15468)
 (vector-slots 8 287514 21412)
 (floats 8 50 279)
 (intervals 56 2245 282)
 (buffers 1000 31))