Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!chinacat!woody
From: woody@chinacat.Unicom.COM (Woody Baker @ Eagle Signal)
Newsgroups: comp.fonts
Subject: Re: PostScript vs TrueType?
Summary: black versus white.
Message-ID: <1468@chinacat.Unicom.COM>
Date: 8 Aug 90 02:49:46 GMT
References: <1100.26af57d3@waikato.ac.nz> <1990Jul26.135834.9874@tsa.co.uk> <3880@bwdls58.UUCP>
Organization: a guest of Unicom Systems Development, Austin
Lines: 105

In article <3880@bwdls58.UUCP>, hwt@.bnr.ca (Henry Troup) writes:
> information in systemdict (PostScript) to know which way a laser printer is
> set up?

Nope, but there is a way to tell.  Print Times Roman at 6, 7 and 8 point
and look at the tops of the o's e's etc.  Try this on an Apple, then a Ricoh
based laser like the TI.  On a white writer, the tops of those letters will
tend to be broken.  Why?  see below.
> 
> Exp. Note: quoting Rebenstein here:
> 	"One striking difference among laser printers is that some write a
> 	black image on the drum during formation of the image, and others
> 	write the white part of the image.  Because of the way the toner is
> 	attaracted to the resulting charged image, one image will be bolder 

This is close, but not *exactly* what happens.  Here is what happens
I have been thinking about writing a PS program to demonstrate this, but 
haven't.  
1. Lasers use a round laser beam to write the graphics to the page.  If you
try to clip the beam to a square beam you get fringed effect around the spots.
There are 2 ways to effect the drum. A short course in laser printers:

The drum in a laser has a photo sensitive layer.  This layer acts like
a capacitor.  The drum passes a highvoltage corona wire, from when comes
the smell of ozone.  The corona charges the surface of the drum.  Depending
on the type of photo surface, and the polarity of the corona, it applies
either a + or - charge.  That part has nothing to do with write black
or write white.  The laserbeam hitting the drum, causes the charge to leak
through the photo sensitive layer, and disapate into the metal drum.  The
result, is a series of regions on the drum with either a + or - charge
depending on the polarity.  toner is formulated for either a + or - charge.
The toner carries a charge that is opposite to the drum charge WHERE THE
BLACK IS TO GO. Now, if it is a white writer, the beam will neutralize the
charge to the same as the toner.  If it is a white writer, the beam will
cause the toner to NOT be attracted to the drum.  If it is a black writer
the beam will cause the toner to BE attracted every where it hits.  The
charged drum with the latent image, moves through the toner.  There are
2 types of toners.  Mono-component, and dual component.  Canon uses mono
component toner.  This means that the toner particles are magnetic.  A
Dual component system on the other hand, has a "carrier" called a "developer"
that is magnetic.  The toner is static sensitive, and sticks to the sharps
points of the carrier.  The carrier in turn sticks to the magnet and forms
sort of a brush.  This lightly touches the drum.  The mono component toner
sticks to the magnet forming a brush as well, but it does not touch the
drum.  It merely jumps the gap to the drum.  It has the distinct advantage
of being a simpler system, but the disadvantage of not covering as well.

The drum with the latent image, then continues rolling, and comes to
a point when it is in contact with the paper.  On the other side of the th
paper, is another corona wire that has a voltage on it that is opposite
of the charge holding the toner to the drum.  The toner leaves the drum,
adheres to the paper and is melted into the paper under heat and pressure.

Now:  Since we are working conceptually with square pixels, and we have a
round laser beam, we have a choice to make. How to fit a round peg in a
square hole.  There are 2 choices.  We can fan the beam out such that it
just completely compases our 300th inch pixel.  This means that the pixel
is ENTIRELY within the beam, and the beam extends outside of the sides of
the pixel.  We can focus the beam so that it is completly contained WITHIN
the 300th inch square pixel, or we can go in between, and make it slightly
bulge out the 4 sides, but stay inside the corners.  Now, consider a write
BLACK engine.  If we choose the first method, we get a line a bit bigger than
1/300th of an inch.  If we choose the second method we will get LARGE white
diamonds between ajacent rows of dots.  If we pick the third method, we
will get SMALL diamonds.  IF we design the toner correctly, the charging
system correctly, and the laser power correctly, we can arrange things
such that the charge is just a tad bit larger than the pixel, such that the
4 pixels surrounding the diamond, will bleed enough charge to allow the
toner to cover the diamond.  The original laser writers (CX engine) attempted
this, but was not designed quite right, and thus you had VERY fine rows of
white diamonds in the black areas, resulting in washed out blacks.  The
SX engines solved the problem with diffrent toner, particle size, chargin
system, and spot size.  It should be immediatly apparent that for a WHITE
writer, it just would not do to have rows of tiny black diamonds all over
your white areas, so the white writers HAVE to use the first technique.
Now, the simplest way to visualize this, is to get some graph paper, and
draw a line of dots that COMPLETELY enclose each square.  SKIP a line
(this will be our black line) and draw another line.  Then mark out a
2x2 square and draw circles completely around the borders.  Go to the upper
left and lower right corners of teh 2x2 square and draw a style 1 dot around
each.  Now set back and look.  You will notice that the single black line
is NO LONGER 1/300th of an inch.  It has little bitty nibbles along the
edges.  You will also note that the 2 black pixels in our 2x2 gray shade
box are smaller than 1/300th of an inch.  This is why, on fine details
such as 1 pixel lines, and the tops of small o's (which get down to 1
pixel) that often times the charge is not enough to make the toner stick,
thus resulting in broken lines, and missing pixels.  However, the BLACKS
ARE BLACK (there are no white or black diamonds, the drum was never touched
by the laser here).  To sum things up, White writers are bad on fine gray
shades, and small letters, but good on solid blacks, so If you are say, making
silkscreen masters, the White writer would be the printer of choice, the
resolution of the silk is less than 1/300th of an inch, and you need the
blocking power of solid solid black.  On the other hand, for doing fine
typesetting, etc, the black writer is superior because the pixels don't
drop out.

Now, in some cases, attempts are made to correct the white writer problem,
by always doing 2 rows of pixes, never one row.

I know that this probably should be over in comp.lasers, since it has
no real bearing on postscript, except for the output, but it sure helped
me sell a lot of CANON black writers against Ricoh white writers.

Cheers
Woody