[time-nuts] Scanning darkly
jack at hudler.org
Mon May 14 00:47:41 EDT 2007
Try scanning at 300 DPI grayscale (400 max, anything more is a
waste). Do not use the histogram or descreening functions in your scanner
software unless you spent some serious dollars. All "descreening" really
does is increase the actual scanning resolution up to 2 times what you asked
for (or fake it), then do some post processing (which you have no control
over), in order to give you the image you hope you asked for. I prefer to
get the raw data and do my own post processing because it can be different
from page to page, and is never the same from manual to manual. Besides the
guys that write the scanner software that come with your home scanner; don't
really know what they are doing.
Why 300 DPI? Most manuals are printed at 135 LPI (or 150) therefore
scanning at 300 DPI satisfies NyquistShannon (sampling theorem >2*LPI)
which reduces halftone moiré.
Do not use lossy image compression such as jpg. Lossy image
compression screws up the spatial frequency of halftone images, making it
almost impossible to do any effective post processing. I use PNG because it
supports 16 bit grayscale but most scanners won't give you that and acrobat
really likes it.
Moiré can be eliminated after scanning by using a Gaussian blur of
~1.5 pixels (see Photoshop). Some scanner software actually do this as part
of "descreening" and some high end software ($$$$) may do Fourier analysis
to calculate the correct Gaussian distribution.
Failure to do this step will just reintroduce the Moiré when you
later downsample the image.
At this point you want to do a histogram stretch chopping off the
highs and lows as needed to remove noise. If you have any version of
Photoshop 6 or greater this can be set up as a batch process.
Normally I downsample all final text to 150 and leave the schematics
at 300 or higher. If you want to OCR then do not downsample prior to OCR.
If you're plagued by bleed through then you may have to use a duplex
scanner or get aggressive in the histogram phase.
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