чт, 24 дек. 2020 г. в 17:46, Eli Zaretskii <eliz@gnu.org>:

> > From: Evgeny Zajcev <lg.zevlg@gmail.com>
> > Date: Thu, 24 Dec 2020 13:52:21 +0300
> > Cc: emacs-devel <emacs-devel@gnu.org>
> >
> >  Doesn't the doc string of create-image (which does the scaling) answer
> your question?
> >
> > Sorry, I don't understand it, why 10 pixels? In my case `auto' creates
> very large images (i.e.
> > (frame-char-width) ==> 30), so scaling factor is 3.  This creates very
> large images by default.
> >
> > I know I can set `image-scaling-factor` to whatever I need.  However I
> want to understand the default
> > behaviour decision, why 10 pixels is considered as "typical character"
> width?
> >
> > I think, by default `create-image' should create images that look good
> when inserted into buffer, that is *not*
> > what I get by default.
>
> See bug#22172.  And if that doesn't explain it, I hope Lars (who wrote
> that code) could elaborate.
>

Lars, could you please elaborate the logic in the
`image-compute-scaling-factor'.  As I understand, it means "the larger font
you have, the larger image you get"?  But why?  I use large fonts (say 30
pixels in width), in 1920x1080 screen, and I'm totally ok with scale factor
1 for images, but `auto' image-scaling-factor gives me 3 as default scale.
This results in very large images.

Also, the problem with `image-compute-scaling-factor' is that it returns
different scale factors for `auto` in different window configurations,
because `(/ (window-width nil t) (window-width))` is not always the same

For better `auto' scaling we should consider the physical size of the
display and its resolution (that is called DPI I think?) not the size of
the font.
But probably this info is not always available.  If we have that DPI info,
we can then calculate `auto' scale factor using "typical DPI" (where images
looks ok).
Something like:

  (defun image-compute-scaling-factor (scaling)
    (cond
     ((numberp scaling) scaling)
     ((eq scaling 'auto)
      (let* ((mm-height (when (display-graphic-p)
                          (display-mm-height)))
             (dpi (if mm-height
                      (round (/ (display-pixel-height)
                                (/ mm-height 25.4)))
                    ;; DPI is unavailable, use typical DPI
                    72)))
        ;; If we assume that a typical DPI is less then 100, then we should
        ;; scale all images according to how current display DPI is larger,
        ;; then typical DPI.  But don't scale images down.
        (if (< dpi 100)
            1
          (/ (float dpi) 100))))
     (t
      (error "Invalid scaling factor %s" scaling))))

What do you think?

Thanks

-- 
lg