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From: Helge Blischke <h.blischke@acm.org>
Newsgroups: comp.lang.postscript
Subject: Re: downsampling images
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Date: Tue, 04 Dec 2012 22:32:39 +0100
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Michael Unger wrote:

> I want to control the downsampling parameters of (JPEG) images from
> within each PostScript program individually instead of modifying the
> Distiller parameters ("joboptions" file) globally. The following lines
> from the "joboptions" file seem to be relevant here:
> 
> |  << /DownsampleColorImages true
> |     /ColorImageDownsampleType /Bicubic
> |     /ColorImageResolution 300
>                             ^^^ (1)
> |     /ColorImageDepth -1
> |     /ColorImageDownsampleThreshold 2.00000
>                                      ^^^^^^^ (2)
> |     /EncodeColorImages true
> |     /ColorImageFilter /DCTEncode
> |     /AutoFilterColorImages true
> |     /ColorImageAutoFilterStrategy /JPEG
> |  >> setdistillerparams
> 
> Am I correct assuming the "Resolution" (1) is the desired final image
> resolution (dpi) to be embedded into the final PDF document and
> "Threshold" (2) is a multiplier indicationg a resolution ratio above
> which the downsampling will take place? (The JPEG iamges are converted
> to EPS using the freeware utility "jpeg2ps" [1] and included via the
> "run" command into the PostScript program.)
> 
> Michael
> 
> 
> [1] <http://www.pdflib.com/download/free-software/jpeg2ps/>
> 


Ad (1):
---------
The resolution is the lower bound for the downsampling procedure; the actual 
resolution achieved is dependent on the downsampling algorithm (subsampling, 
average downsampling, or bicubid downsampling).

Ad (2):
---------
If the original image resolution is already below the "destination" 
resolution times this factor, no downsampling/subsampling takes place

For details see the documents on distiller parameters (or the "Adobe PDF 
Creation Settings).

Helge