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Re: Question about display engine

From: martin rudalics
Subject: Re: Question about display engine
Date: Fri, 30 Aug 2019 09:03:06 +0200

>> But where do we get that from?
> It will come out automatically from the face merging process.

You told me before and I'm still afraid that no such automatism

>> But when the display engine gets there, the respective face was not
>> even realized.
> It will be realized when the display engine comes to that place.
> That's exactly what we have been talking about here, right?  Or maybe
> I don't understand what difficulty you are seeing there.

But then "realizing" the face for the spaces at the end of line
becomes much harder: The display engine has to handle _every newline_
in every buffer as _if it were a stop position_ in order to merge all
faces at that position and find out which background to use.

>> The merging process will kick in iff we treat the newline character at
>> the end of line 2 as a stop position.  Since no text property changes
>> there, I don't see how this could happen.
> It will happen because we call append_space_for_newline and
> extend_face_to_end_of_line, and those functions _know_ they are at end
> of a line.

But they would still have to fully realize the face to use by merging
all faces at each newline.  This means we can do away with all those
precalculated extend_background, extend_... bits because they cease to
contribute anything to the solution.

The only practical solution I see is to, instead of extend_background
bits, use extend_background colors.  In my example, this would mean
that when, at the stop position prescribed by the comment start text
property change, the face merger sets the background value of the face
to realize to that of the region and the extend_background value of
the face to realize to that of the comment.  The display engine would
then realize the face for writing the remainder of the line right from
extend_background instead of from background + extend_background.

> Maybe I don't understand the question, because I see no problems
> here.  Just coding.

Maybe you're right.  But I still don't see the light at the end of
this tunnel.


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