How to print halftone from Ghostscript - ghostscript

Question

I have a PostScript file with image and text. I want to print it to a laser printer so that the images print in a halftone round pattern.
I am trying to print it from the command line. I would prefer a PDF output first and then I'll print to the laser printer.
I have an HP Laserjet P2015 printer installed in Windows 10.
gswin64c.exe -dQUIET -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=output.pdf test.ps -c "<< /HalftoneType 1 /Frequency 37 /Angle 45 /SpotFunction {180 mul cos exch 180 mul cos add 2 div} >> sethalftone"
The PDF file is generated. However, the images does not appear to be in Round Halftone format. I see no change in the image.
This is the original Image
I want the printout to look like this: Required output
For some reason the output is the same as the original.

Answer 1

pdfwrite doesn't create monochrome output, so no, the output won't be halftoned. The whole point of the pdfwrite device is to maintain the output as close in quality as possible to the input.
There is no way to get monochrome output from pdfwrite unless the monochrome is in the input.
If you instead use a monochrome output device (e.g., tiffg3 or tiffg4) you should see a difference, unless the input file (which you haven't supplied) also sets a screen, in which case the last one encountered will take effect. That is, if the PostScript input file specifies a halftone screen, that's the one that will get used.
What you are asking for is what I might generally describe as a hard problem, probably much harder than you expect.
Your best bet is to send the PostScript directly to the printer, since it supports PostScript.
Problem 1; your PostScript file may already contain a halftone in it which overrides the one you want to use. You can usually prevent that by redefining the sethalftone and setscreen operators:
/sethalftone {pop} bind def
/setscreen {pop pop pop} bind def
You do that after you'[ve defined your own halftone, obviously. Or you can just edit the PostScript file and remove the halftone definition, its usually not that hard to find.
Problem 2 is that printer manufacturers sometimes 'tweak' the interpreter to always use their preferred halftone and prevent you overriding it in PostScript. The only way to find out if that's happening is to try a quick test; print a simple image with the default screen, then set a really coarse screen, say 10 lpi and print the same image. If there's no difference then you know that's not a viable option. If there is, then you can just set the screen you want.
Now, if the printer won't let you change the halftone then the only solution is to render the PostScript to an image using Ghostscript (which will let you change the halftone) and then send the resulting image to the printer. You will need to use a 1 bpp device, something like tiffg3 or tiffg4 should work fine.
The problem here is that you need to ensure that the image fits on the media of the printer, because if it doesn't then the likelihood is that the printer will slightly scale down the image so that it does fit, thereby squashing the halftone cells.
Even more subtly, the printable area of the media in the printer may not be the same as the size of the media. Paper handling can mean that there are some areas of the paper that can't be printed on, and the variability in the feeding of the paper can mean that the printer will refuse to print right to the edges anyway. Because if it did, and there was, say, a 1 mm deviation in the paper handling, then 1 mm of the edge would 'fall off' and there would be a 1 mm white gap at the other edge...