Re: Add a function that returns pixel distance between points?

[Yuan Fu]

> On Jan 31, 2021, at 3:13 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Sun, 31 Jan 2021 14:41:52 -0500
>> Cc: emacs-devel@gnu.org
>> 
>>> Finally, I'm not sure why the behavior of window-text-pixel-size is a
>>> problem: if you use :align-to with pixel units (as opposed to column
>>> units), then you should _want_ it to account for stuff like
>>> line-prefix, no?
>> 
>> Yes, value given to :align-to needs to account for line-prefix. But when I 
>> want to calculate the pixel width of a column, I don’t want to include the 
>> prefix: that makes the returned value larger than the actually width of the 
>> column (because the line-prefix width is added to it).
> 
> But the pixel coordinate to which you need to align also needs to be
> increased due to line-prefix, doesn't it?
> 
> IOW, can you show a test case where using window-text-pixel-size
> returns incorrect results for :align-to due to line-prefix?

Here is an example:

(let ((width))
  (insert "woome")
  (setq width (car (window-text-pixel-size
                    nil (line-beginning-position) (point))))
  (put-text-property (line-beginning-position) (point)
                     'line-prefix "   ")
  (message "true width: %d returned width: %d"
           width (car (window-text-pixel-size
                       nil (line-beginning-position) (point)))))

Run it in an empty buffer and it will print the expected width and returned 
width. In the snippet I’m trying to measure the width of the text.

Yuan