Subject: 27.1; PNG binary transparency is ignored

From: [email-masked]
Date: Wed, 10 Mar 2021 19:45:58 +0900

If a PNG file has a aplpha plain composeed of only 0 or 255,
its transparency is ignored.

The attached file transparent1.png is a red sqaure framed with a green
line, where red pixels have 0 for alpha value and green pixels have 255
for alpha value.
transparent2.png is a similar image, but the alpha values for green
pixels are 254.

Red pixels in the both images have 0 for alpha value, so they should be
transparent and invisible, but transparent1.png is displayed as a red
square with a green frame in a emacs buffer.

transparent2.png is displayed as a white square with a green frame as
expected.

When they are displayed in a chrome browser, inner squares of the both
images are displayed as transparent.

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:
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  global-eldoc-mode: t
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  file-name-shadow-mode: t
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  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:
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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
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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 81166 41710)
 (symbols 48 9599 0)
 (strings 32 27679 2980)
 (string-bytes 1 803013)
 (vectors 16 15294)
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