[pdftex] auto letter spacing

[Thanh Han The]

Hi all,

I am working on a primitive \letterspacefont that generates
a letter-spaced font from another font like this:

,--------
| \font\f=cmr10
| \letterspacefont\fx=\f 100 % letterspace \f by a factor 100/1000
| \fx This is a text in letter-spaced cmr10
`--------

the above example creates \fx as a virtual font, in which
each charater is taken from \f, and have a kern of
50*quad(f)/1000 at each side. All this happens in pdftex
memory.

However I am not sure the "more right" way to do this is that
each character c should have a kern of 5*width_of_c/1000 at
each side, ie the kern inserted around each character is
proportional to its width, instead of having a fixed global
value for all characters. This is more closed to the way font
expansion is done, but is different from what fontinst does
for letter-spacing.

How do you see it?

Thanh