Re: scroll-down with pixel transition

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 12 Apr 2017 21:13:35 +0900 (JST)
> Cc: emacs-devel@gnu.org, tkk@misasa.okayama-u.ac.jp
> From: Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>
> 
> GOAL: Pixel-level vertical scrolling up and down
> 
> > I'm still not sure what you mean by "overshoot", but if that's the
> > extra full-line scroll for each of the 10 iterations
> 
> Yes, that's what I meant!
> 
> > then you see the same in the snippet you shown that scrolls in the
> > other direction, if you look carefully.
> 
> This I do not see (even with careful look).  At least, following lines
> give me pixel-level and smooth scrolling.  Not for you?

That's because the last iteration of the inner loop comes within 1
pixel of the next screen line, so you don't see the jump, which is
visible when you scroll in the other direction.  I've provided a
snippet to demonstrate the basic issue which causes that; if something
is unclear in my explanation, I can try clarifying.

> Let's say the lines above give smooth scrolling.  If so, to make Emacs
> scroll both up and down with pixel-to-pixel resolution, requirement is
> that `set-window-vscroll' accepts negative number.

Like I explained, it won't help, because the moment you do something
that potentially requires a redisplay, the vscroll will be zeroed, and
the text will scroll back to the state where the first screen line
visible in the window is fully visible.  So this cannot be the
solution to your problem, I think.