David
Thanks for your post on digital imaging, which I am struggling to
assimilate in full but I think confirms what I already thought.
I think the equation you give is the usual one - which I repeat in slightly
different form in case others had the same difficulty as me in reading it!
no of tones = (printer resolution in dpi/line screen frequency in dpi)^2 +1
(For any unused to maths on computers, the symbol ^ is used to mean 'to the
power of' - in this case squared)
Thus in my case of a 400 dpi printer and a 50 lpi screen, this gives
(400/50)^2 + 1
= 8^2 + 1
= 64 + 1 =65
The +1 is for the paper colour - the printer can actually print 64 grey
shades (one of which is black). Actually I used the equation rearranged to
work out the desired lpi from the given maximum tones and dpi of the printer.
To get 64 greys, the printer uses an 8x8 grid of dots, printing any number
from 0 to all of them. So it isn't surprising that it starts with 400 dpi and
ends with 400/8 = 50 lpi!
However a clever printer driver can make this actual 50 lpi have the dot size
of 100 dpi by the way it prints the 8x8 grid representing each dot which is
needed for 64 shades - splitting it into 4 4x4 grids.
Even cleverer solutions are possible......When you get away from normal
straightforward half-toning things are altered even more by the way the
driver works.
Peter Marshall
On Fixing Shadows, Dragonfire and elsewhere:
http://faraday.clas.virginia.edu/~ds8s/
Family Pictures & Gay Pride: http://www.dragonfire.net/~gallery/
and: http://www.speltlib.demon.co.uk/