Re: Drawing UI elements behind text

[Cecilio Pardo <cpardo@imayhem.com>]

On 27/11/2024 15:36, Eli Zaretskii wrote:
>> Date: Wed, 27 Nov 2024 00:19:43 +0100
>> Cc: emacs-devel <emacs-devel@gnu.org>
>> From: Cecilio Pardo <cpardo@imayhem.com>
>>
>> I haven't found a way to draw behind the text that is not too
>> complicated or affects redisplay too much. So now the idea is drawing
>> over the text. For the applications I have in mind this works equally well.
>>
>> The starting drawing elements are going to be vertical and horizontal
>> segments, with color (maybe with alpha), width and different patterns.
>> They can be placed on pixel positions or character positions when using
>> fixed size fonts, using floating point numbers so you can put, for
>> example, indent lines in the middle of characters.
>>
>> They are associated with a buffer, and have a 'source' marker so the
>> facility can be shared by several programs.
> 
> How/whether are these drawing elements connected to the glyph
> matrices?
> 
> If they are completely disconnected, how does this work when the
> window is scrolled several lines?

They are anchored at locations on the buffer, specified by line/column. 
This is transformed into pixels using the size of the default char cell 
for the buffer.  The buffer scroll position on each window is used to 
compute the final position on the frame/screen.

For buffers that use different font sizes or images this is probably not 
very useful. Direct pixel positions can be used too.

They are drawn after switching the back paint buffer, so that the 
redisplay result is left alone.

At first I tried intercepting the background clearing to draw behind the 
text. But to allow for redisplay to reuse the output I had to redraw the 
glyphs affected with a clear background, and this was too costly in the 
worst cases.

I will have a patch in a few days.