Re: Implementing Vertical Text support in Emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: समीर सिंह Sameer Singh <lumarzeli30@gmail.com>
> Date: Fri, 9 Sep 2022 00:14:45 +0530
> Cc: emacs-devel@gnu.org
> 
> Well after getting some free time I started to work on this for a week or so and I have to say I am quite a bit
> lost :( 
> 
> Right now I don't care about harfbuzz or correct character composition, I only want to display rows vertically,
> but
> I don't know where to start, does modifying only display_line and move_it_in_display_line_to work?
> Even if this is the case, I don't know how to parse the huge display_line function and find the area of interest
> among the lines of wrap and hscroll code? where is the code where the matrix is filled?

display_line is the relatively low level which produces a single
screen line of a window, i.e. a single row of characters, images, and
other "display elements" that occupy a single visual line.

Before you modify display_line, you need to decide how will the
vertical-layout display produce glyphs.  The easiest would be to
produce them in column-wise order, because that basically processes
the buffer in the order of character positions, like Emacs does now.
This will allow you to leave the lower levels, which walk the buffer
and deliver characters, mostly intact.

So perhaps you should have a display_column function, which will look
like display_line, but instead of advancing the X coordinate, it will
advance the Y coordinate, and produce a single column, top to bottom.
Then the basic calculations should be the same as in display_line,
except the column ends when it reaches last_visible_y instead of
last_visible_x.  IOW, the glyph matrices will be made of glyph columns
instead of glyph rows.  Basically, the roles of the X and the Y
coordinates will be swapped.

I suggest to invest some time in reading the code of display_line and
studying its main ideas and tasks.  Because the hypothetical
display_column will have to do something very similar, just vertically
instead of horizontally.  Ask questions where you cannot figure stuff
out.  Then design your display_column in a similar way.

> Regrettably my inexperience  is causing hurdles therefore any guidance would be a huge help.

Don't give up.  And thanks for working on this.