Re: Set-window-vscroll sometimes doesn't work

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Sun, 18 Oct 2020 13:51:14 -0400
> Cc: help-gnu-emacs@gnu.org
> 
> > NOFORCE non-nil means Emacs doesn't have to display the window
> > starting at the position if there are valid reasons, so yes.  But my
> > suggestion is not to use set-window-start when you want to vscroll,
> > it's unsafe.  You in effect create a conflict for the display engine,
> > and have no control on how it will resolve that conflict.
> 
> Thanks. I see what you mean. What I want to do is to scroll over images as if 
> it is made of several lines (like sliced images). Which requires scrolling 
> over logical lines by set-window-start and scrolling over images by 
> set-window-vscroll. Do you think I would be better off implementing this in, 
> say, window_scroll_pixel_based rather than in Lisp?

It sounds like you want to do what we already have implemented in
next-line and previous-line and their subroutines.  Typing C-n on a
tall image scrolls the image the number of pixels that is equal to the
line height.  Maybe you can just use that code?