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Re: Continuous image scrolling

From: Clément Pit-Claudel
Subject: Re: Continuous image scrolling
Date: Tue, 3 Apr 2018 17:57:35 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 2018-04-03 15:59, Juri Linkov wrote:
>> i.e. it is not possible to finely scroll the sequence of images up and
>> down, you go one image at the time.

> It's because next-line goes to the next file in Dired by default,
> this is why I disabled this inconvenience in my ~/.emacs by using
> normal cursor motion keybindings:

Maybe I'm misunderstanding the original request, but if not then I don't think 
this is the issue.

AFAIU, one thing that is not too easy to implement today in ELisp is continuous 
("smooth") scrolling over multiple images, as shown in e.g. evince: if I drag 
the scroll bars or press the up and down keys, the current page and the 
following one move up and down by a bit.

We usually call this pixel-scroll in Emacs, and we have an implementation of it 
based on set-window-vscroll in lisp/pixel-scroll.el.  This is also what Docview 
and PDFTools use to scroll images that don't fit in the selected window.  But I 
don't think that trick works for sequences of images, at least not straight out 
of the box.

The main issue, IIUC, is that images occupy one (tall) line, and Emacs tries to 
always keep the line containing the point in full view, which means that 
set-window-vscroll does not always work if the point is on a line containing an 

You can see what I mean concretely with the following example. Open a new 
buffer, and run (insert (propertize " " 'display '(image :type jpeg :file 
"/path/to/some/large/picture.jpg" :scale 1))).  Then insert 5 lines of "A".  
Your buffer will look like this:

|  Picture here  |
|                |

Put the point on the third A and run (set-window-vscroll (selected-window) 8 
t); note how the images moves a bit up.  That's the kind of scrolling we want, 
which is encouraging :)

But now put the point on the picture's line, and try the same experiment; 
notice how the picture doesn't move, because we try to keep the line in full 
view. (if the image is larger than the window, it does work, and that's how 
docview scrolls single pages. I don't know if there's a way to get that 
behavior when the image fits in the window).  That behavior becomes an issue 
when you have sequences of images, because you can't leave the point on the 
line of the first image, but moving the point to the line of the second image 
scrolls it entirely into view.

The two techniques to work around this that I'm aware of are (1) cutting the 
images into thin horizontal strips (and showing one strip per line), and (2) 
inserting a thin text line between each image on which to place the point while 
scrolling.  I don't know if either of these would work for pdftools.  Hopefully 
there's a third, simpler technique to achieve this?


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