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    <p>DNA decided to go for grayscale PNGs. The way he's creating them is in two steps:</p> <ol> <li><strong>Step 1:</strong> Convert a color PDF page (such as <a href="http://dl.dropbox.com/u/13223318/L_2lambda_max_1wl_E0_1_zg.pdf" rel="nofollow"><strong>this</strong></a>) to a grayscale PDF page, using Ghostscript's <code>pdfwrite</code> device and the settings<br> <code>-dColorConversionStrategy=/Gray</code> and<br> <code>-dProcessColorModel=/DeviceGray</code>.</li> <li><strong>Step 2:</strong> Convert the grayscale PDF page to a PNG, using Ghostscript's <code>pngalpha</code> device at a resolution of 300 dpi (<code>-r300</code> on the GS commandline).</li> </ol> <p>This reduces his initial file size of 1.4 MB to 0.7 MB.</p> <p>But this workflow has the following disadvantage:</p> <ul> <li><em>It looses all color info, without saving much disk space as compared to a <strong>color</strong> output written at the same resolution, directly from the PDF!</em></li> </ul> <p>There are 2 alternatives to DNA's workflow:</p> <ol> <li><p>A one-step conversion of (color) PDF -> (color) PNG, using Ghostscript's <code>pngalpha</code> device with the original PDF as input (same settings of 300 dpi resolution). This would have this advantage:</p> <ul> <li><em>It would keep the color information in the PNG output, requiring only a little more space on disk!</em> </li> </ul></li> <li><p>A one-step conversion of (color) PDF -> grayscale PNG, using Ghostscript's <code>pnggray</code> device with the original PDF as input (same settings of 300 dpi resolution), with this mix of advantage/disadvantage :</p> <ul> <li><em>It would loose the color information in the PNG output.</em></li> <li><em>It would loose the transparent background that was preserved in DNA's workflow.</em></li> <li><em>It would save <strong>lots</strong> of disk space, because the filesize would go down to about 20% of the output from DNA's workflow.</em></li> </ul></li> </ol> <p>So you can make up your mind and see the output sizes and quality side-by-side, here is a shell script to demonstrate the differences:</p> <pre> #!/bin/bash # # Copywrite (c) 2012 &lt;kurt.pfeifle@gmail.com&gt; # License: Creative Commons (CC BY-SA 3.0) function echo_do() { echo echo "Command: ${*}" echo "--------" echo "${@}" } [ -d out ] || mkdir out echo echo " We assume all PDF pages are 1-page PDFs!" echo " (otherwise we'd have to include something like '%03d'" echo " into the output filenames in order to get paged output)" echo echo ' # Convert Color PDF to Grayscale PDF. # If PDF has transparent background (most do), # this will remain transparent in output.) # ATTENTION: since we don't use a resolution, # pdfwrite will use its default value of '-r720'. # (However, this setting will only affect raster objects...) ' for i in *.pdf do echo_do gs \ -o "out/${i}---pdfwrite-devicegray-gs.pdf" \ -sDEVICE=pdfwrite \ -dColorConversionStrategy=/Gray \ -dProcessColorModel=/DeviceGray \ -dCompatibilityLevel=1.4 \ "${i}" done echo ' # Convert (previously generated) grayscale PDF to PNG using Alpha channel # (Alpha channel can make backgrounds transparent) ' for i in out/*pdfwrite-devicegray*.pdf do echo_do gs \ -o "out/$(basename "${i}")---pngalpha-from-pdfwrite-devicegray-gs.png" \ -sDEVICE=pngalpha \ -r300 \ "${i}" done echo ' # Convert (color) PDF to grayscale PNG using Alpha channel # (Alpha channel can make backgrounds transparent) ' for i in *.pdf do # Following only required for 'pdfwrite' output device, not for 'pngalpha'! # -dProcessColorModel=/DeviceGray echo_do gs \ -o "out/${i}---pngalphagray_gs.png" \ -sDEVICE=pngalpha \ -dColorConversionStrategy=/Gray \ -r300 \ "${i}" done echo ' # Convert (color) PDF to (color) PNG using Alpha channel # (Alpha channel can make backgrounds transparent) ' for i in *.pdf do echo_do gs \ -o "out/${i}---pngalphacolor_gs.png" \ -sDEVICE=pngalpha \ -r300 \ "${i}" done echo ' # Convert (color) PDF to grayscale PNG # (no Alpha channel here, therefor [mostly] white backgrounds) ' for i in *.pdf do echo_do gs \ -o "out/${i}---pnggray_gs.png" \ -sDEVICE=pnggray \ -r300 \ "${i}" done echo " All output to be found in ./out/ ..." echo </pre> <p>Run this script and compare the different outputs side by side.</p> <p>Yes, the 'direct-grayscale-PNG-from-color-PDF-using-pnggray-device' one may look a <em>bit</em> worse (and it doesn't sport the transparent background) than the other one -- but it is also only 20% of its file size. On the other hand, if you wan to buy a bit more quality by sacrificing a bit of disk space -- you could use <code>-r400</code> instead of <code>-r300</code>...</p>
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    1. POReducing PDF file size using Ghostscript on Linux didn't work
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    1. COThe bash script you provided errored out with: `Error: /undefinedfilename in (col*.pdf)` How should I go about changing the file name in the script?
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    2. CO@DNA: Sorry. Just change all occurrances of 'col*.pdf' to '*.pdf'. I did run 1 test, found a directory which had 200 PDFs, two of which started their name with 'col', and I used these to test them... :-) Fixed in script above.
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    3. COOh. Sure! `:)` I did try `pnggray` and like you said, there is an 80% reduction in file size from 1.2Mb to ~200KB. Thanks!
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