bug#19117: 25.0.50; emacs on x11 chooses different fonts for the same face sometimes

[Dima Kogan <dima@secretsauce.net>]

I just looked a bit more.

As I mentioned in the last message, the font description that results in
ugly rendering is

 #<font-entity x adobe courier ## iso8859-1 medium i normal 0 0 0 0 nil>

Apparently the "0 0 0 0" represents a "scalable" font. Not 100% sure
what this means in the context of X11. In Emacs, we load this font with
XLoadQueryFont() in xfont_open(). I wrote a small program to see what
this means on my box:

 #include <stdio.h>
 #include <stdlib.h>
 #include <X11/Xlib.h>
 #include <X11/Xatom.h>
 int main(void)
 {
     Display* dpy = XOpenDisplay(":0.0");

     const char* font_input = "-adobe-courier-medium-i-normal-*-0-0-0-0-p-0-iso8859-1";
     const char* font_output;
     unsigned long value;

     XFontStruct *font = XLoadQueryFont(dpy, font_input);
     XGetFontProperty (font, XA_FONT, &value);
     font_output = XGetAtomName (dpy, (Atom) value);

     fprintf(stderr, "font '%s' loaded as '%s'\n", font_input, font_output);
     return 0;
 }

Here I load the scalable font that Emacs is choosing (and that xlsfonts
says exists on my machine). Then after it is loaded, I ask what the font
is called. The output is this:

font '-adobe-courier-medium-i-normal-*-0-0-0-0-p-0-iso8859-1' loaded as '-urw-nimbus mono l-regular-o-normal--17-120-100-100-p-100-iso8859-1'

So this completely explains why I'm getting ugly rendering. Emacs is
picking a scalable adobe font, which XLoadQueryFont() interprets as a
17-pixel urw bitmap font. Since I was asking for an 11-pixel font to
begin with, emacs is down-scaling this font by a factor of 11/17, which
does not look so good.