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Re: Making thumbnails efficiently with ghostscript

Cross-posted to 2 groups.

Followups directed to: comp.text.pdf

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["Followup-To:" comp.text.pdf]
bugbear wrote in <news:PKKdnck_eruzEibSnZ2dnUVZ8lSdnZ2d@brightview.co.uk>
> I would like to use ghostscript to make a thumbnail of the first page of a PDF file,
> or an EPSF file, to the following spec.
>
> * The thumbnail must have the SAME aspect ratio as the original
> * the size of the longest side of the thumbnail must be 128 pixels
>
> The correct pixels can be easily made, by simply rendering at a 
> "too-high" resolution, and then using ImageMagick:

Use the dimensions of the trimbox (or mediabox if missing), do some math 
to get the approriate resolution for an image with at least 256x256 
pixels and then use this as $MyRenderResolution. This

    gs -dUseTrimBox -q -dBATCH -dNOPAUSE -r${MyRenderResolution} \
    -sDEVICE=... -dDOINTERPOLATE -dTextAlphaBits=4 -dGraphicsAlphaBits=4 \
    -dMaxBitmap=100000000 -dMaxPatternBitmap=2000000 -sOutputFile=... \
    -c 100000000 setvmthreshold -f /path/to/pdf

will produce an image with correct aspect ratio (-dTextAlphaBits=2 and 
-dGraphicsAlphaBits=2 should work a bit faster without decreasing 
rendering quality since you get an image with twice the pixel dimensions 
as necessary).

When you scale down using ImageMagic in a second step you can then apply 
some slight sharping or unsharp masking which will produce superiour 
results.

Regards,

Thomas

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Making thumbnails efficiently with ghostscript bugbear <bugbear@trim_papermule.co.uk_trim> - 2012-05-22 14:05 +0100
  Re: Making thumbnails efficiently with ghostscript Thomas Kaiser <Thomas.Kaiser@phg-online.de> - 2012-05-22 14:11 +0000

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