[time-nuts] Scanning darkly

[WB6BNQ]

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 Nyquist–Shannon (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
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts