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Old December 30th 03, 07:15 PM
J C
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Default how to put the border around a headshot?

On Tue, 30 Dec 2003 01:16:36 -0700, Marc 182
wrote:



You shouldn't top-post, that's for email. The effective resolution of
photo paper is around 200-250 ppi. By keeping the resolution of your
digital images above 300 ppi you ensure that no pixilation will be
detectable on your final output.

Marc


For digital printing on an inkjet or laser printer connected to a
computer the resolution recommendations have little to do with the
paper. It has everything to do with the print engines used by output
devices. And in fact, for inkjet printing generally a 150 ppi image is
more than enough for a photographic output

With real printing, on a printing press, the paper does play a small
role (because of dot gain, which is explained last).

As a simple example, lets take a one color laser printer and a one
color TIF file at 300 dpi (the explanation for inkjet printing is a
bit more complex).

Also, and this is very important, lets start with the concept that no
printer on the face of the planet prints a one-to-one correspondence
between the information in the digital file and the output device. To
put it simply the print engines of all output device process the image
into something that it can print.

Now this gets complex... In order to output that one color digital
file on a one color laser printer the print engine must interpret the
colors. The digital file can contain 256 colors of grey, but a laser
printer has only one color of toner (black). Therefore to simulate the
grey scale image the laser printer must break it down into halftone
dots, the size of these dots then determines the grey value that the
eye perceives. Take a look at a laser print out of a digital image and
you'll see the dots.

Now further, a laser printer capable of 600 or 1200 dpi printing does
not actually print 600 or 1200 individual dots that can be discerned
in the printed halftone, though the dots that do show up on paper are
composed of several toner dots joined together.

The reason that the RIP (or "raster image processor") in the printer
restricts its halftone line screen output is because a higher output
would require much more processing time.

So, instead the laser printer spits out a halftone with a maximum of
approximately 127 halftone line screen (also known as lpi, lines per
inch) [And note here that even though the print driver says the laser
is capable of a 150 or 200 line screen for halftones it is NOT.]

So what does all this processing mean... Well the printer's RIP
samples the image information and creates an appropriate halfone dot.
Since there is not a one-to-one correspondence between the colors in
the image and the resulting halftone dots, the RIP samples image
information ("pixels" in the image) and determines the size of the
halftone dot. The finer the resolution of the starting image (within
limits) the better the halftone output. Lower resolution images will
look fuzzy because of this sampling process.

AND NOW to answer why a 300 dpi image is a standard... because in high
quality printing (on a printing press) photographs will be output at
anywhere from 133 lines per inch to 200 lines per inch -- 133 is used
in most magazines, 150 used in technical publications (medical xrays
for example) and sometimes 200 in art books (and many times art books
will be printed in duotone, but I'll not confuse you here and just
leave that at that).

So in the 300 image resolution printed with a 150 line halftone, this
means that 4 image "pixels" will be converted into one halftone dot
(but the raster image processor also considers other surrounding
pixels in the process as well) and this resolution gets a very sharp
image for a printing press.

If you want to test this out then try this: Create an image that is
half white and half 100% black. Now create different resolutions of
this file and print this on your laser printer using different
halftone line screens. Then look closely at the border area between
black and white. The lower resolution images printed with lower
halftone line screen values will have more indistinct (aka fuzzy)
borders, which will make the image look out of focus.

However once you reach about 127 lines per inch in the halftone,
you've not reached the compromise point between the size of the
halftone dot and the size of the lasers toner splatter -- there's
unwanted toner dots in the white areas which degrade the image.
Similarly on a real printing press another effect occurs... the wet
ink spreads out because of the capillary action of the fibers in the
paper and this makes the halftone does slightly larger and thus the
image darker. There's a point at which making the halftone screen
smaller will not increase the image quality (neither black and white
nor color). [And FYI, because of this dot gain, the images produced
for most publications are tweaked so that their saturation when viewed
on screen looks washed out... which is done by "Adjusting... Curves"
in Photoshop.]

There's a lot that I've not covered and that I've simplified, but
that's basically how it works.


-- JC