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What does the # symbol mean when it follows a variable name. e.g.

arrow_len# vs arrow_len

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up vote 7 down vote accepted

Metafont works in a grid determined by the current resolution and only with numbers, not dimensions. In a parameter file you specify dimensions such as pt#, for example

ht#:=6pt#;

and then you can work in the pictures with ht, after having said

define_pixels(ht);

For example,

ht#:=6pt#; define_pixels(ht);show ht;

will print 216 when the value of hppp is 36 (that corresponds to a resolution of 2602 dots per inch, the default when a mode has not been specified). In this way, ht will always represent the number of pixels necessary to cover a distance of 6pt. In the drawing of a character it would be safe to say

ht:=6pt;

since the resolution is already known. In a parameter file such as cmr10.mf it's necessary to specify parameters as "sharp dimensions", so they will be resolution independent, since it's not known a priori when the resolution will be set (before or after setting the font parameters). For example, logo10.mf says

% 10-point METAFONT logo
font_size 10pt#;  % the "design size" of this font
ht#:=6pt#;    % height of characters
xgap#:=0.6pt#;    % horizontal adjustment
u#:=4/9pt#;    % unit width
s#:=0;      % extra space at the left and the right
o#:=1/9pt#;    % overshoot
px#:=2/3pt#;    % horizontal thickness of pen
input logo    % now generate the font
bye      % and stop.

and logo.mf starts with mode_setup which sets, among other things, the resolution.

Similarly, dimensions that must be "output" should be specified in "sharp form"; from digits.mf we get

beginchar("1",9u#,fig_height#,0);

because this will write the correct value in points in the gf file; but in the definition of the character's shape u is used. It's a bit complicated; one should keep in mind that

a#=1pt#;define_pixels(a);

will assign the value 1 to a# and the value of hppp to a and you always know when you need the measure in points or the actual number of pixels.

Notice also that changing the value of a will not change the value of a#, it's the other way around with the help of the define_pixels function.

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