int10h-oldschoolpc-fonts/ratio.txt

== Introduction ==

In classic IBM PC text mode, the graphics card stretches the font
such that it fills the screen.

A 9x16 font over 80x25 columns is displayed as a 720x400 image.
Assuming a 4:3 monitor as typically was present in the days, this
leads to a pixel aspect ratio of 20:27.

 Font size  Image size  aspect   decimal  inverse
 ------------------------------------------------------------
    8x8       640x200     5:12     0.42     2.40
    8x14      640x350    35:48     0.73     1.37
    8x16      640x400     5:6      0.83     1.20
    9x16      720x400    20:27     0.74     1.35

To faithfully recreate the same visual look as such a monitor would
show, this aspect ratio needs to be applied when making use of the
font.


== Application support for stretching ==

For cool-retro-term, there is a "Font Width" slider in the settings;
set it to 74%, 83%, or whatever is necessary. (Notice the pattern of
the "decimal" column of our table.)

For xterm, a fontconfig matrix can be specified like so:

	xterm -fa "Px437 ATI 9x16:matrix=0.74 0 0 1"
	xterm -fa "Px437 ATI 9x16:matrix=1 0 0 1.35"

One can either horizontally compress the glyphs, or vertically
stretch them to get to the result. The compress/stretch action
influences how many characters will be visible in a fullscreen
setting, so the basic font size may need to be adjusted.

	xterm -fa "Px437 ATI 9x16:size=32:matrix=0.74 0 0 1"
	xterm -fa "Px437 ATI 9x16:size=24:matrix=1 0 0 1.35"

produce an equivalent result.

Extra caveat: When using a matrix, you should also specify the -fd
parameter *and* specify the same matrix inside the -fd font
specification. Otherwise, CJK characters will be rendered in a
different size than the main characters. For a stretching matrix like
1/1.35, not specifying -fd and, as a result, having smaller CJK chars
is not as bad a problem as having bigger truncated CJK chars with a
compressing matrix like 0.74/1.


== Application-independent approach ==

Not all programs support specifying a matrix or a scaling factor.
Under systems using fontconfig (such as Linux distributions),
it is possible to override the stretch for an existing font
like so (~/.config/fontconfig/fonts.conf):

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
 <match target="font">
  <test qual="any" name="family">
   <string>Px437 IBM VGA 9x16</string>
  </test>
  <edit name="matrix" mode="assign">
   <times>
    <name>matrix</name>
    <matrix>
     <double>0.73</double>
     <double>0</double>
     <double>0</double>
     <double>1</double>
    </matrix>
   </times>
  </edit>
 </match>
 <dir>~/.fonts</dir>
</fontconfig>


== Framework-independent approach ==

The int10h project offers font files that have their aspect
correction applied at the font level. As a result, they won't need
support from fontconfig or applications at all. These font files are
provided in the

	int10h-oldschoolpc-fonts-stretched

subpackage in openSUSE.