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downsampling images

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  downsampling images Michael Unger <spam.to.unger@spamgourmet.com> - 2012-12-04 19:48 +0100
    Re: downsampling images Helge Blischke <h.blischke@acm.org> - 2012-12-04 22:32 +0100

#1144 — downsampling images

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/>

-- 
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My e-mail account at DECUS Munich is no longer valid.

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#1146

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

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