Re: scan dpi for photos

Don Stauffer added these comments in the current discussion du
jour ...

Could we say that high resolution is a necessary but not
sufficient requirement for good results?

A digital photograph is essentially an array, a grid of columns
and rows. Each element in the array is called a pixel, short
for picture element. There is a brightness value (and color
values in a color image) for each pixel.

"Dot" can mean a lot of different things. A halftone dot on a
printing press is the same as a "pixel". The dot in an inkjet,
dot matrix, or laser printer is NOT the same as a pixel. In
inkjet, DM, or laser printers all the dots are the same size.
Lets talk about black and white, because it is easier to see,
but color is done with basically the same idea.

Yes to the first sentence, but no to the second sentence. In half-
tone printing, not only are more "dots" typically laid down than
digital pixels, the size and color(s) of the dots also vary. This
can be visually verified by looking at a typical printed photo in
various publications such as newspapers, magazines, calendars, and
books of varying quality with at least a 2x magnifier. Even more
can be discerned with a 4X-8X magnifier which allows you to see the
much smaller dots of a quality photo print.

An inkjet printer prints various brightness pixels by printing
an array- say a nine by nine array of dots for each pixel. If
it prints 81 black dots the resulting pixel is very black. For
the brightest pixels it prints nothing. One black dot is the
shade of grey just darker than white, two dots is the shade two
steps darker than white, etc.

How a given printer actually works has as much to do with it's
image processing system and number of ink colors available as it
does for the complex "dithering" you speak. For example, black
black can be simulated by over printing multiple CMY colors or by
true black ink or both. And, the overall brightness and contrast
across the image can be attenuated by mixing all of the ink colors
available with black. I can't speak to the specific grid that my
printer or any other actually uses, but I'd be quite surprised if
it were always a simple X by X array of dots.

Now, the pattern within the nine by nine square is something
else, lots of various algorithms around to optimize the "shape"
of the pattern to make the most pleasing picture. Google
dithering and error diffusion.

Good idea. The common defintion of dithering has become blurred -
no pun intended - over the years since it was originally
mathematically defined to mean the mixing of various inks to
simulate not only as broad a spectrum of colors as possible but
also attempts at anti-aliasing which may be part of the process or
optional. Dithering used to be a faux printing idea in reality
since printers in the 1970s through 1990s weren't capable of even
approaching 24 bit color to their print engine, much less the non-
noise bits of 48 bit color.

I am aware of the fundamentals of error diffusion but I expect that
you're technical and mathematical knowledge is much superior to
mine. So, I'll just end by saying that "dithering" is a very good
analogy to describe the process in a way that most people can
understand even if it is used much differently by modern photo
printers.

And, I won't even attempt to get into the very expensive commercial
printer technologies as I have essentially zero knowledge in those
areas.

--
HP, aka Jerry

"Laid off yet? Keep buying foreign and you soon will be!" - popular
bumper sticker


.

Relevant Pages

  • Re: Confused
    ... Certainly not dpi. ... When it comes to printing or displaying an image, the size it prints/displays at is set by the pixels-per-inch (ppi) parameter. ... But a Canon printer will print at 4,800 x 2400 dpi - dots per inch. ... because the printer lays down many dots to make up one pixel - in Canon's case, 32 ink dots in an 8 x 4 matrix to make one pixel of the image. ...
    (alt.graphics.photoshop)
  • Re: dumb question
    ... used for scanning where the hardware scans X dots per linear ... use of dithering to turn CMYK into millions of colors. ... "Pixel" is a term coined a relatively short time ago for digital displays, ... There are also advances in halftones for printing, ...
    (rec.photo.digital.slr-systems)
  • Re: D3 vs flatbed scanner
    ... terms of the number of separate dots in can pick up or put down. ... You can have an image scanned at 2400 dpi, ... 24,000 dots, which could be saved as a 6,400 x 8,000 pixel image ...
    (rec.photo.digital.slr-systems)
  • Re: Canon printer - MP resolution
    ... to how much detail it prints, can it give full resolution of a 23mp ... camera at 8.5" x 11"  Or is it more or less. ... Keep in mind that printer dots are not pixels. ...  Each pixel in the image ...
    (rec.photo.digital)
  • Re: I wonder how Dpreview will handle....
    ... printers, a couple of which I owned that could do color by ... were the last of the one-to-one mapping of pixels to ... "dots". ... sophisticated dithering engines ...
    (rec.photo.digital.slr-systems)
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