I tried writing a vertical version of the display_line function but could
not get it to work. Emacs does not display any character after these
changes.
(maybe I should have also rewrote move_it_in_display_line_to?)

Therefore forgive me for asking again, if for now I just want to make the
characters appear column wise instead of row wise, disregarding line
overflow, or termination of a line etc,
do I still have to look into the display_line function, because as I recall
I had once removed most of the lines of the display_line function and Emacs
was still displaying lines fines.

Again, thank you for your patience.

On Fri, Sep 30, 2022 at 12:00 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: समीर सिंह Sameer Singh <lumarzeli30@gmail.com>
> > Date: Fri, 30 Sep 2022 01:27:01 +0530
> > Cc: emacs-devel@gnu.org
> >
> >  Before you modify display_line, you need to decide how will the
> >  vertical-layout display produce glyphs.
> >
> > Can you please expand more on this?
> > How to produce glyphs in a column wise order?
>
> I did that in the original message.  Quoting myself:
>
>   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 looked at gui_produce_glyphs and that led me to append_glyph, but I
> could not figure out how the glyphs are
> > produced.
> > There is also draw_glyphs but I think working with that will require
> editing the low level *term files (if it even is
> > the right function)
>
> I didn't mean gui_produce_glyphs and its subroutines, I mean the logic
> of how glyphs are produced, as described above, in display_line
> itself.  The current code in display_line produces glyph of a single
> screen line, in visual order, from left to right.  Above I suggested
> to write a similar display_column function, which will produce glyphs
> of a single column, top to bottom, and will make the decisions
> regarding when the column is full and should be terminated, like
> display_line does with respect to screen lines.  Does this sounds like
> a good approach to you?
>
> If the approach sounds good, but something in my description is
> unclear, please ask more specific questions about those unclear parts
> of what I wrote.
>