Looking for references afterward, I found this bit about Cairo
integration in GTK+ in the er… GNOME Wiki Attic?

https://wiki.gnome.org/Attic/GtkCairoIntegration

> gdk-pixbuf only handles the pixel formats packed RGB and RGBA. Both of
> these formats are inefficient memory and performance wise.
>
> […]
>
> 2. Most Xservers uses the color format ARGB so to blit a GdkPixbuf you
>    need to convert each pixel from RGBA to ARGB which means that you are
>    taking an unnecessary performance hit. It is even worse if you are
>    blitting pixbufs using Cairo because then you have to convert from
>    premultiplied to non-premultiplied alpha.

See also the note on "Pixel formats" from a GNOME developer:

https://people.gnome.org/~federico/blog/my-gdk-pixbuf-braindump.html

> Every time we paint a GdkPixbuf to a cairo_t, there is a conversion
> from unpremultiplied RGBA to premultiplied, platform-endian ARGB.

(I find the GNOME Wiki Attic page a bit perplexing; they talk about
converting *from* premultiplied *to* non-premultiplied when working with
Cairo, which contradicts the Cairo manual, the GNOME developer's blog,
and my observations.)


Some questions I have about my patch:

- Is it OK to have array sizes in argument lists?  I know they are
  mostly decorative (e.g. sizeof still returns the size of a pointer),
  but I find the practice useful as a way to document intent (no idea
  if static analyzers are smart enough to use them nowadays).
  
  I can't say off the top of my head whether they come from C99 or
  later, but I found a few other functions using them
  (prepare_standard_handles in src/w32.h,
  get_lface_attributes_no_remap in src/xfaces.c), so I figured we
  already support the feature in practice.

- I originally divided by 255, since I figured that would be the
  maximal value for any given component and we want x*a / max to yield
  x when a is 255; however I later noticed that emacs_color_to_argb32
  divides by 256, so I went for that instead.  I am still not sure
  whether it's correct, or whether it matters much.

- I added my static function to the "Image type independent image
  structures" section so that it ends up close to emacs_color_to_argb32,
  but maybe I should move it down to the "SVG" section?  In which case I
  will probably need to add a new #ifdef USE_CAIRO block.

  (Alternately, should I just inline the function?)

- I haven't thought really hard about that "if (iconptr[3] == 0)"
  branch.  Is it still relevant?


Thank you for your time.



In GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.8, cairo version 1.16.0)
 of 2019-05-15 built on my-little-tumbleweed
Repository revision: 4fa6029f7ee30aa3316d6d73db61462730042908
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: openSUSE Tumbleweed

Configured using:
 'configure --with-xwidgets --with-cairo'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS XWIDGETS JSON PDUMPER
LCMS2 GMP

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

Major mode: Magit Process

Minor modes in effect:
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  icomplete-mode: t
  global-page-break-lines-mode: t
  page-break-lines-mode: t
  electric-pair-mode: t
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  global-diff-hl-mode: t
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  electric-indent-mode: t
  mouse-wheel-mode: t
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  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
  buffer-read-only: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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mule-util info package easymenu epg-config url-handlers url-parse
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
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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
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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 xwidget-internal cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 444631 125454)
 (symbols 48 47237 83)
 (strings 32 176476 9183)
 (string-bytes 1 5274843)
 (vectors 16 64009)
 (vector-slots 8 2012096 107118)
 (floats 8 522 596)
 (intervals 56 17166 2584)
 (buffers 992 85))