Bug 695925

Summary: Properly implement overprint simulation for non-separation devices
Product: Ghostscript Reporter: Ray Johnston <ray.johnston>
Component: Color ManagementAssignee: Michael Vrhel <michael.vrhel>
Status: NOTIFIED FIXED    
Severity: enhancement CC: customer531, henry.stiles
Priority: P1    
Version: master   
Hardware: All   
OS: All   
Customer: 531 Word Size: ---

Description Ray Johnston 2015-04-14 08:23:08 UTC 
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.
Comment 1 Ray Johnston 2015-09-17 11:02:22 UTC 
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.
Comment 2 Ray Johnston 2020-01-21 16:16:28 UTC 
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.
Comment 4 Ken Sharp 2021-10-09 08:40:15 UTC 
Since no customers are actually anticipating this feature (comment #2) I'm moving this to 'notified' without sending any emails to customers.