Ghostscript PDF quality downsample

gs-resample.sh

#!/bin/sh

# It seems it's very hard to set resample output quality with Ghostscript.
# So instead rely on `prepress` preset parameter to select a good /QFactor
# and override the options we don't want from there.

gs \
  -o resampled.pdf \
  -sDEVICE=pdfwrite \
  -dPDFSETTINGS=/prepress \
  `# font settings` \
  -dSubsetFonts=true \
  -dCompressFonts=true \
  `# color format` \
  -sProcessColorModel=DeviceRGB \
  -sColorConversionStrategy=sRGB \
  -sColorConversionStrategyForImages=sRGB \
  -dConvertCMYKImagesToRGB=true \
  `# image resample` \
  -dDetectDuplicateImages=true \
  -dDownsampleColorImages=true -dDownsampleGrayImages=true -dDownsampleMonoImages=true \
  -dColorImageResolution=150 -dGrayImageResolution=150 -dMonoImageResolution=150 \
  `# preset overrides` \
  -dDoThumbnails=false \
  -dCreateJobTicket=false \
  -dPreserveEPSInfo=false \
  -dPreserveOPIComments=false \
  -dPreserveOverprintSettings=false \
  -dUCRandBGInfo=/Remove \
  -f input.pdf

# Default settings for prepress profile:
# $ gs -v
# GPL Ghostscript 9.19 (2016-03-23)
# Copyright (C) 2016 Artifex Software, Inc.  All rights reserved.
# $ gs -q -dNODISPLAY -c ".distillersettings /prepress get {exch ==only ( ) print ===} forall quit" | sort
# /AutoRotatePages /None
# /CannotEmbedFontPolicy /Error
# /ColorACSImageDict << /ColorTransform 1 /QFactor 0.15 /Blend 1 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >>
# /ColorConversionStrategy /LeaveColorUnchanged
# /ColorImageDownsampleType /Bicubic
# /ColorImageResolution 300
# /CompatibilityLevel 1.5
# /CreateJobTicket true
# /DoThumbnails true
# /EmbedAllFonts true
# /GrayACSImageDict << /ColorTransform 1 /QFactor 0.15 /Blend 1 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >>
# /GrayImageDownsampleType /Bicubic
# /GrayImageResolution 300
# /MonoImageDownsampleType /Subsample
# /MonoImageResolution 1200
# /NeverEmbed []
# /PreserveEPSInfo true
# /PreserveOPIComments true
# /PreserveOverprintSettings true
# /UCRandBGInfo /Preserve