Copyright 2007 TeX Users Group.
You may freely use, modify and/or distribute this file.

Erode:

Erode is called exactly once when a character is rendered (it is, of
course, not called when the character is already in the cache).

On the stack at the time of the call are three numbers, uo, vo, and s
say (typically uo = 1.0, vo = 1.0 and s = 10.0).
Erode returns two numbers on the stack, u and v say,
(normally either u = 1.0 and v = 1.0 or u = 0.0 and v = 1.5).
If there is not definition for Erode, the numbers on the stack before
the call are used, that is, in effect u = uo and v = vo.

The two paramaters u and v control erosion in character rendering.
Setting both of these parameters to zero suppresses erosion and leads
to rendering like that found in Type 3 fonts (that is there is an
average ``dot-growth'' of 1/2 pixel all the way around).
``Normal'' erosion is attained by setting both parameters to one.

The first parameter, u, controls the erosion of horizontal, vertical and
inclined parts of the outline.
The second parameter, v, controls additional erosion of slanted parts of
the outline only.
When u is zero, vertical and horizontal stems are not eroded,
independent of the value of v. 
Negative values for u and v cause the outline to grow rather than to
shrink.

Erode decides what values to return based on a calculation
involving the device pixel size.
If the number of device pixels corresponding to a given character space
interval is above some threshold, then u = 1.0 and v = 1.0 is returned
(This is the normal mode, where all part of the character are eroded
more or less equally).
If the number of device pixels is below the threshold, 
then u = 0.0 and v = 1.5 are returned
(This means that horizontal and vertical stems are not eroded).

% reduction of overshoot phenomena at small point sizes ?

Erode itself contains two (integer) parameters, n and m say.
The decision is based on whether

	( (n + 1/2) - [(s/10) * n] )^2  <  d(m)^2

where [x] denotes the integer part of x, while d(m) is the distance
in device pixels corresponding to the character space distance m.
Note that the left hand side equals a quarter, independent of n,
when s = 10 (as it typically is).

Thus a character will be rendered normally when m maps into more than
1/4 device pixel.  When m maps to a smaller number of device pixels,
erosion is curtailed somewhat.

Typical values for m range from 60 to 200, with smaller numbers used
for light faces and larger ones for heavy faces.

Typical values for n range from 5 to 24.  Again with some tendency
for heavier faces to have larger values.  
As noted above, however, when s = 10, the decision is independent of n.

On the Apple LaserWrite, a font has to be printed at some
ridiculously small size (like 1pt or 2pt) for anything but the normal
erosion to kick in.