Re: Zero height line causing arithmetic errors

[Kenichi Handa <handa@m17n.org>]

In article <jwvtzfwc4kq.fsf-monnier+emacs@gnu.org>, Stefan Monnier <monnier@iro.umontreal.ca> writes:

> My MPC.el package uses an overlay placed at the end of a line with an
> after-string of #("\n" 0 1 (face (:height 0.05 :inverse-video t)))
> in order to place a horizontal line in the display.

> In Emacs-22, this worked fine (except the line was a bit thicker than
> 0.05 times the base line height).  In Emacs-CVS until recently this
> worked except that the line was full-height (i.e. 13 pixels in my case).
> Recently, it has started to cause more problems because now
> FRAME_SMALLEST_FONT_HEIGHT returns 0, and that value is used at 2 places
> in the divisor position.

> I guess the 0 is because 13 pixels (default font height) * 0.05 -> 0.65
> pixels which are truncated to 0.

> I currently use the patch below, which makes it all work again (tho
> still with the problem that the 0.05 isn't taken into account and line
> line is 13 pixel think), but I suspect it's not the right place to fix
> it,

For the problem of FRAME_SMALLEST_FONT_HEIGHT being set to
0, I installed the similar change.

And, for the problem of too tall line-height, I found the
culprit is XftTextExtents8.  When the pixelsize of a font is
less than 5 or so, for some font, it returns strange values
in `extents' argument.   For instance, with the attached
test problem, I get this result:

% ./xfttest 'bitstream vera sans mono'
pixelsize=9, ascent=8, descent=2
pixelsize=8, ascent=7, descent=2
pixelsize=7, ascent=6, descent=2
pixelsize=6, ascent=5, descent=2
pixelsize=5, ascent=4, descent=2
pixelsize=4, ascent=4, descent=1
pixelsize=3, ascent=3, descent=1
pixelsize=2, ascent=2, descent=1
pixelsize=1, ascent=1, descent=1
pixelsize=0, ascent=1, descent=1
% ./xfttest 'dejavu sans mono'
pixelsize=9, ascent=8, descent=2
pixelsize=8, ascent=7, descent=2
pixelsize=7, ascent=6, descent=2
pixelsize=6, ascent=5, descent=1
pixelsize=5, ascent=5, descent=1
pixelsize=4, ascent=5, descent=2  <-- increasing!
pixelsize=3, ascent=4, descent=4
pixelsize=2, ascent=5, descent=5
pixelsize=1, ascent=5, descent=5
pixelsize=0, ascent=5, descent=5

I don't know which component of these has a bug:
  o the font "dejavu sans mono",
  o freetype
  o xft

Anyway, I installed a quick dirty workaround; not believe
the result of XftTextExtents8 if the pixelsize is less than
5.

With that change, if your default font is scalable, the
following code works as expected.

(let ((buf (get-buffer-create "test"))
      overlay)
  (set-buffer buf)
  (erase-buffer)
  (insert "first line\n")
  (setq overlay (make-overlay (1- (point)) (point)))
  (overlay-put overlay 'after-string
	       (propertize "\n" 'face '(:height 0.01 :inverse-video t)))
  (switch-to-buffer buf))

But, note that the line height is still 2, not 1.  That's
because the display engine calculate the line height by
ascent+descent, and both ascent and descent doesn't become 0
in the structure XftFont even though `height' member is 1.
It may be another Xft (or freetype) problem.

---
Kenichi Handa
handa@ni.aist.go.jp

PS. You can compile xfttest.c by this command:
% gcc -g -o xfttest `xft-config --cflags` `xft-config --libs` xfttest.c

----- xfttest.c ----
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <X11/Xlib.h>
#include <X11/Xft/Xft.h>

char *str = "!\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~";

void
show_height (Display *display, XftFont *font)
{
  XGlyphInfo extents;

  XftTextExtents8 (display, font, str, 94, &extents);
  printf (" ascent=%d, descent=%d\n", extents.y, extents.height - extents.y);
}

int
main (int argc, char **argv)
{
  int size;
  char name[256], *p;
  Display *display;
      
  if (argc < 2)
    {
      fprintf (stderr, "Usage: xfttest FAMILYNAME\n");
      exit (1);
    }
  display = XOpenDisplay (NULL);
  if (! display)
    {
      fprintf (stderr, "Can't open the display\n");
      exit (1);
    }
  strcpy (name, argv[1]);
  p = name + strlen (name);
  for (size = 9; size >= 0; size--)
    {
      XftFont *font;

      sprintf (p, ":pixelsize=%d", size);
      font = XftFontOpenName (display, 0, name);
      printf ("pixelsize=%d,", size);
      show_height (display, font);
      XftFontClose (display, font);
    }
  
  exit (0);
}