Several customers would like Ghostscript to do the same as Adobe's Acrobat when Advanced/Print Production/Output Preview has "Simulate Overprint" checked. We don't do this, and the previous SimulateOverprint was doomed to failure since it did not maintain the planes for the separations. The concept is to use the pdf14 compositor to simulate overprint since it has the capability of maintaining buffers for the colors (CMYK + spot colors) and then to map the image to the target device color space when the page is sent to the target via "put_image". This final step would be invisible to devices that need the conversion, and would need to maintain the page level alpha plane for devices like pngalpha that implement put_image and want the alpha information.
The front end work to analyze the PDF to determine if we need to push a compositor in order to "correctly" render a page that uses OP/op when the device cannot support overprint. This is the case for RGB or Gray devices (since the page might use CMYK colors with overprint), or when a DeviceCMYK device is used and the page has SpotColors. Rather than using the pdf14 compositor device, which would maintain larger buffers than needed (overprint doesn't need the alpha channel) and without the color model complexity reliance on the target device at many places, I am implementing and overprint simulation (opsim) compositor device.
Note, no customer is currently urgently needing this, but it has come up with 531 and 532 as a desire. It has also come up occasionally with potential printer customers.
Committed with https://git.ghostscript.com/?p=ghostpdl.git;a=commit;h=813b5f48e845d528d3070d9168aa51035a614c1c
Since no customers are actually anticipating this feature (comment #2) I'm moving this to 'notified' without sending any emails to customers.