[at-l] Scanning Question

[saunterer@jimbullard.org (saunterer@jimbullard.org)]

At 08:05 AM 2/23/02 -0600, L. Parker wrote:
>A word of caution here, for all of you folks out there who own ink jet or
>laser printers that tout 600 dpi or even 1200 dpi resolution, don't bother.
>The machine's actual maximum physical resolution is typically only 300 dpi

An interesting point to consider is that the average human eye cannot 
distinguish resolution finer than 330 dpi.  The Epson printer resolutions 
are not directly related to the image resolution that is being 
printed.  They are using a smaller drop size and overlapping the ink dots 
in a stochastic pattern to smooth the output.  A file scanned to 300 dpi at 
8×10 will result in just as good a print as one scanned at 1200 dpi and 
printed at 8×10 if all other things are equal.  The important point is to 
set scan resolution for output size, not original size.  If you are 
scanning a smaller original to be printed at a larger size then you need 
higher resolution to achieve 300 dpi in the print.

>You probably want to look at storage format also. .tif had some compression
>available, but it is not as "lossless" as .jpeg. JPEG will compress far
>better with less pixel loss, but may not be supported by some commercial
>printer RIPS. GIF format should only be considered for screen images.

TIFF is essentially lossless but it's compression cannot reduce file size 
much.  JPG on the other hand is a lossy format.  It achieves compression by 
dumping colors not pixels.  If you are scanning a map with a total of 10-12 
colors losing most of the 16 million colors available with modern monitors 
isn't a problem.  Pixelation of JPG images begins where the remaining color 
set is less than what is needed to reproduce the original.  This is 
compounded in web use because although your monitor and video board may be 
capable of 16 million colors browsers are not.  There are only 216 "web 
safe" colors (colors that a browser will display).  If the original colors 
do not fall into those colors, the browser will try to approximate the 
color by dithering.  It is possible to convert to browser safe colors at 
the same time you are reducing the size/resolution of the image.  This will 
allow you to compress fairly heavily.  Last point: JPG loses each time it 
is saved (50% - 50% of that - 50% of that, etc.) so don't save to JPG until 
you are finished manipulating your image.

sAunTerer