graphicsmanip Posted: 12 Jan 2004 01:08 am Post subject:
--------------------------------------------------------------------------------
I know this is petty so please ignore this post if you desire...
Printers output is measured in dpi,(dots per inch).
Scanners output is measured in ppi,(pixels per inch).
Just a little fetish I have...
This is not petty... DPI is not the same as PPI. Dots are only one colors worth of information. Pixels are multiple colors of information.
I have this definition in my clips so I am going to quote it here:
(It is not from PBASE but from another board)
Andy Piper , oct 28, 2003; 05:07 p.m.
There is some confusion here: Pixel resolution (in the image) and printer dot resolution are completely different things.
Seth: you are basically right - a 2400x1200 printer prints 1200 dots (droplets of ink) per inch in the direction the print head travels back and forth. It also prints 1200 dots per inch in the other dimension (in terms of the holes in the inkjet printer head), but moves the paper only a half-dot at a time, so it can cram in 2400 overlapping circular dots in the same inch in that direction. The 'true' resolution is always the smaller number.
You need at least TWO dots to resolve a line or other fine detail point. One dot for the line/detail, and one for the white background. (Think about printing black type - to make a letter "a" you need to be able to see the hole in the "a".) So the detail resolution will never be more than half the 'dot' resolution. And that's for pure black/white images (no grays/no colors).
To get a range of colors or grays, you need multiple printer dots to provide tonality. If you want, say, 36 possible grays in an image, each pixel in the scanned file must be represented on paper by a 6x6 matrix of printer dots - with some or all of the dots 'missing' to make the matrix contain any number of ink dots from 0 to 36. (I'm using a single ink for example purposes - multiple inks will improve things).
So the detail resolution of the printer will be 1/4 or 1/6th or 1/8th the advertised 'dot' resolution. Thus a 1440 x 1440 Epson printer can resolve smoothly-toned photographic detail only out to about 1440 divided by 6 (or whatever the matrix size is) per pixel. Which works out to 240 pixels per inch. That's what someone meant by saying the image is resampled on the fly by the printer software to 240 or 360 or so. It is. The 360 is made possible by the multiple inks, which extend the tonal range to 'photographic' even with only 16 dots per ink per pixel.
The point is: don't confuse pixels-per-inch in your image with dots-per-inch in the printer. They are not the same thing at all, and there isn't even much of a relationship between the two - any more that the size of the sand grains on a beach (printer dots) determines how many cars can be parked on it (image pixels).
A minor note: the above is for inkjet type printers. Die Sublimination printers are different animals that print in layers and actually print in dpi.
With all that said I would still scan at the maximum that you can. If nothing you may find out later you want to crop the picture down.
You might want to try some other formats to get a little extra space. TIFF is standard but other formats are newer and may give better results.