Re: Introducing emacs-webkit and more thoughts on Emacs rendering (was Rethinking the design of xwidgets)

[Eli Zaretskii <eliz@gnu.org>]

> From: Arthur Miller <arthur.miller@live.com>
> Cc: Akira Kyle <akira@akirakyle.com>,  larsi@gnus.org,  emacs-devel@gnu.org
> Date: Sat, 28 Nov 2020 09:57:39 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > As long as the widget can be told to use a given rectangular portion
> > of the screen, we should be fine: the current Emacs display engine is
> > perfectly capable of handling display elements that occupy an area of
> > certain dimensions.
> 
> How can I tell any drawing routines to use certain rectangular area on
> display so it is in harmony with Emacs display engine?

On the display engine level, you need to define a new kind of "display
element" (see 'enum glyph_type' in dispextern.h) and new kind of
method for producing such a display element (see 'enum it_method' in
dispextern.h).  You will also need to write a BUILD_<FOO>_GLYPH_STRING
macro that produces a "glyph string" from one or more display elements
of this new type; see BUILD_XWIDGET_GLYPH_STRING in xdisp.c as an
example.  Glyph strings are the interface between xdisp.c, which
parses buffers and strings for display and performs display layout,
and xterm.c (see below) which actually draws the resulting display
elements.

On the Lisp level, you will probably use a special 'display' property
whose value specifies the widget with the necessary parameters, like
we do with images.

> Are there callbacks I can connect too if I would like to update opengl
> viewport that I can hook easily into? Where do I look? I am not sure I
> understand it from image.c; it was there I was looking. I never looked
> into xwidgets though. Can be just me; but if you can point me to right
> place to look at I am thankful. 

The actual drawing is in xterm.c (for X; it's w32term.c for
MS-Windows, nsterm.m for NS, and term.c for TTY frames).  Each type of
display element has a function there, which is called to draw the
display element.  For example, x_draw_image_glyph_string for drawing
images.  You will need to write a new function for this new display
element, whose parameters are passed from the new it_method method
above.  Not sure if this is what you meant by "callback to connect to
in order to update opengl viewport".

(image.c just prepares the data required for drawing an image; its
functions are not called when drawing the image, we just use the
pixmap produced by image.c and various other parameters, like image
dimensions etc.)