Hi Jack, Is DPI the same as LPI ? I have always been confused on that point. If they are the same, why are there two seemingly different specs used for the same thing ?
I can do TIFF. How does that compare to the PNG ? thanks, Bill....WB6BNQ Jack Hudler wrote: > 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. > > Have fun! > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
