pdfbitmapghostscript

How to apply custom halftone to ghostscript conversion of PDF file to bitmap

I have a PDF file with a grayscale image that I'm trying to convert to monochromatic bitmap (1 bit per pixel) using ghostscript. I can get everything to work fine, but I don't like the way the default grayscale conversion looks with coarse lines going through it. My understanding is that I can customize the halftone algorithm to my liking, but I can't seem to get the postscript commands to have an effect on the output. Am I using the 'sethalftone' command incorrectly? Is there a different approach I should consider?

gs -sDEVICE=bmpmono -sOutputFile=test.bmp -dBATCH -dNOPAUSE -r300 -c "<< /HalftoneType 1 /Frequency 40 /Angle 0 /SpotFunction {pop}>> sethalftone" -sPageList=1 input.pdf

I can completely remove the "-c" command line parameter and it makes no difference.

This is what the current mono conversion looks like that I'm trying to improve upon:

Sample

Solution

  • Using the default halftone built into Ghost Script has a limited working range and a tendency to provide an appearance leaning to one side, (it is deliberate design not accidental) there is a dither control but that tends to work effectively at r/20 to r/5 in this case showing -dDITHERPPI=25 to -dDITHERPPI=60 thus reducing the blocks in size. However there are other potential controls.

    enter image description here

    I am no expert on how these settings are best used (plagiarized) we would need to read the PS manual, or play with their values, but they seem to fare better.

    enter image description here

    gswin32c -sDEVICE=bmpmono -r300 -o out.bmp -c "<< /Frequency 133 /Angle 45 /SuperCellSize 36 /Levels 255 >> .genordered /Default exch /Halftone defineresource { } settransfer 0.003 setsmoothness"  -f ../examples/text_graph_image_cmyk_rgb.pdf

    (note you may wish to include -dUseFastColor before -c "....)

    Changing values

    /Angle common values may be 0 (0=GS default) 22 30 45 52 60 67 however /Angle 45 is most common for this mono conversion, and variations coupled with a range of /Frequency (Line Screen Frequency ~= LPI) and SuperCell size may produce unachievable or sub normal result.

    /Frequency (75=GS default) can be set to different values but a common ratio is 4/3 = 133 (%) another is 170 however a rule of thumb is r/2 thus if your using 300dpi /Frequency 150 may be better in this case. The correct choice is down to a complex consideration of factors, so your grid at 45 degrees is spaced at sqrt(2)=1.41421356237 at an angle. Thus the horizontal & vertical effect is 0.7071....(in terms of page dpi) thus to match 300 dpi the LPI frequency needs to be possibly made relative so consider 212

    /SuperCellSize eludes me Integer; default value = 1 -- actual cell size determined by Frequency, Angle, H/V Resolution. A larger value will allow more levels to be attained. Unclear as to what may be optimum without sample of target, so try a few values but some will trigger error feedback try around 32 or 64 as a reasonable max start point. note example uses 36 ?

    To prevent the undesirable moire patterns, Peter Fink proposed the Supercell Method named the Adobe Accurate Screen to realize the screen angle having RT with the required accuracy.3 The supercell designed in the image domain including m × m (m: integer) identical halftone cells is one solution to approximate accurately the required screen angle and screen ruling. https://www.imaging.org/site/PDFS/Papers/1998/PICS-0-43/668.pdf

    You may also wish to try
    /DotShape Integer; default value = 0 (CIRCLE). Other shapes available are: 1=REDBOOK, 2=INVERTED, 3=RHOMBOID, 4=LINE_X, 5=LINE_Y, 6=DIAMOND1, 7=DIAMOND2, 8=ROUNDSPOT,

    These values and more can be found at https://ghostscript.readthedocs.io/en/latest/Language.html

    Upper Left is a mono area dithered in a graphics app where depending on resolution and density "worms" or "moire" patterns start to appear. To regulate such effects a deliberate screen is applied and at smallest units aberration's in the linework pattern will not be noticeable, unless zoomed beyond intended scrutiny.

    gswin32c -sDEVICE=bmpmono -r300 -o out.bmp -c "<< /Frequency 106 /Angle 45 /SuperCellSize 64 /Levels 255 >> .genordered /Default exch /Halftone defineresource { } "  -f test.pdf

    enter image description hereenter image description here