Re: Fix for 687591 Overprint Preview is not documented

Dan,

PLRM and the PDF specs mainly say that the effect is device dependent.

> 'overprinting' does not mean that the printer is
> backing up and making a second print pass

Where you took this from ?
In my world this sentence is false.

Igor.

----- Original Message ----- 
From: "Dan Coby" <[email protected]>
To: "Igor V. Melichev" <[email protected]>
Cc: "Gs-Code-Review" <[email protected]>
Sent: Friday, July 23, 2004 11:13 PM
Subject: RE: [gs-code-review] Fix for 687591 Overprint Preview is not documented (xefitra)


> 
> Igor,
> 
> >t looks like you and me live in different worlds, and
> >your terms in my world have no useful meaning.
> >Either you don't listen me at all.
> >Lets consider this :
> >
> >> >3.1. Assume I've got a printer, which can overprint physically.
> >> >Do we have a device driver for it ? Which one ?
> >> >Or can I force any driver to do it ?
> >> 
> >> Ghostscript currently supports overprinting for all of its CMYK
> >> output devices.  No special action is required by the user.
> >
> >pkmraw is a CMYK device, right ?
> >In same time it generates a plain raster.
> >Using that raster, how can I make a printer
> >to perform a physical overprint ?
> 
> I am using the term 'overprinting' in the same sense as it is
> used in the PLRM and the PDF spec.  If you have any questions
> then I would suggest that you check those documents.
> 
> Please note that 'overprinting' does not mean that the printer is
> backing up and making a second print pass.
> 
> 
> Dan
> 
> 
> -----Original Message-----
> From: Igor V. Melichev [mailto:[email protected]]
> Sent: Friday, July 23, 2004 11:31 AM
> To: [email protected]
> Cc: Gs-Code-Review
> Subject: Re: [gs-code-review] Fix for 687591 Overprint Preview is not
> documented (xefitra)
> 
> 
> Dan,
> 
> It looks like you and me live in different worlds, and
> your terms in my world have no useful meaning.
> Either you don't listen me at all.
> Lets consider this :
> 
> > >3.1. Assume I've got a printer, which can overprint physically.
> > >Do we have a device driver for it ? Which one ?
> > >Or can I force any driver to do it ?
> > 
> > Ghostscript currently supports overprinting for all of its CMYK
> > output devices.  No special action is required by the user.
> 
> pkmraw is a CMYK device, right ?
> In same time it generates a plain raster.
> Using that raster, how can I make a printer
> to perform a physical overprint ?
> 
> Igor.
> 
> 
> 
> ----- Original Message ----- 
> From: "Dan Coby" <[email protected]>
> To: "Igor V. Melichev" <[email protected]>
> Cc: "Gs-Code-Review" <[email protected]>
> Sent: Friday, July 23, 2004 9:11 PM
> Subject: RE: [gs-code-review] Fix for 687591 Overprint Preview is not documented (xefitra)
> 
> 
> > 
> > Igor,
> > 
> > >1. What is "spot colors" ? Where one can find a definition ?
> > >A month ago I heard that you consider a spot color as
> > >any tint besides Cyan, Magenta, Yellow, Black. 
> > >Is this correct ? Is this documented ?
> > 
> > I am not going to add a description of general printing related
> > terms to our documentation.  It is not currently the purpose of
> > our documentation to provide a primer on printing.
> > 
> > 
> > >2. Assuming the (1) is correct, I can guess what the rest of the
> > >text means. In my understanding it means : (1) the result of overprint
> > >depends on the device, 
> > 
> > Yes.  This is already explained in the documentation that I added.
> > 
> > 
> > > (2) overprint isn't supported for
> > >colors besides Cyan, Magenta, Yellow, Black.
> > >Is this correct ? If so, I wonder why other tints (for example, Silver)
> > >are considered to be worse than Cyan. 
> > 
> > Ghostscript does support overprinting for colors beside CMYK.
> > However it only does so for those devices which support spot colors.
> > Actually Ghostscript will support overprinting for any colorant that
> > a device says is known to the device if the device uses a subtractive
> > process color model.  (See the get_color_comp_index device proc description
> > for more details.)  This is part of the PLRM and PDF spec.  My
> > CP1700PS printer, which has Adobe Postscript but only CMYK, does not
> > support overprinting with spot colors since it also uses the tint
> > transform to convert the spot colors.  Ghostscript presents the same
> > appearance as my CP1700PS printer with Ghostscript's CMYK only output
> > devices.
> > 
> > 
> > >If someone just replaces cartridges on the 
> > >physical printer, will Ghostscript crash or fail ?
> > 
> > As you know, Ghostscript does not actually know any details about
> > a printer that may being driven by a Ghostscript output device.
> > If Ghostscript were being used inside an a printer, and one changed
> > the ink cartridges on the printer while a print was being processed,
> > then one would probably get a strange result.  Probably, in this
> > situation, the printer should either ask the operator to reinsert
> > the original ink color or else restart the processing of the document.
> > 
> > 
> > >3. It is still unclear how overprint works with Ghostscript :
> > 
> > Please read the PLRM and the PDF 1.5 spec. for a description of
> > overprinting.  We are compliant with the specifications.
> > 
> > 
> > >3.1. Assume I've got a printer, which can overprint physically.
> > >Do we have a device driver for it ? Which one ?
> > >Or can I force any driver to do it ?
> > 
> > Ghostscript currently supports overprinting for all of its CMYK
> > output devices.  No special action is required by the user.  The
> > documentation that I added, describes which devices support spot
> > colors.
> > 
> > 
> > >3.2. Assume I've got a printer, which cannot overprint physically.
> > >I need to print a document, which uses an overprint.
> > >I guess that can Ghostscript somehow convert overprinted
> > >objects into "plain" colors. What should I specify to Ghostscript 
> > >to get this effect ? I guess, the effect is a composition of colors with
> > >some math formula. Which formula is that ? 
> > >Can I customize the formula ?
> > 
> > Ghostscript uses the rules specified in the PRLM and the PDF 1.5 spec.
> > for handling both overprinting and spot colors.  (This is plainly
> > described in the documentation that I added.)  Ghostscript makes no
> > special attempt to simulate the appearance of overprinted data with a
> > device which uses an additive process color model.
> > 
> > 
> > >3.4. Assume I want to view an overprinted document on a 
> > >display device (i.e. with additive colorants). I guess that 
> > >Ghostscript can somehow emulate that view, using a
> > >composition of colors with some math.
> > 
> > The display device in its RGB mode does not properly display the
> > effects of overprinting.  As is described in the documentation that
> > I added, the handling of overprinting is based upon the color model
> > of the device.  The display device does simulate CMYK, and CMYK plus
> > spot color models.  The documentation that I added gives a link to
> > the display device.  This location gives examples of how to specify
> > different modes for the display device.
> > 
> > 
> > >What should I specify to Ghostscript to view an emulation of 
> > >the overprinting on the display ? I know what DisplayFormat is
> > >relevant, but I still don't know what to specify exactly.
> > 
> > The documentation that I added gives a link for the display device.
> > That location gives specific examples of how to specify different
> > modes for the display device.
> > 
> > 
> > >IMO answers for all these questions to be added 
> > >into GS documentation. If an answer is long and may be found
> > >is a publicly accessible source, a reference should be added.
> > 
> > As I said previously, I do not think that our documentation should
> > attempt to be a primer on the art of printing.  If you have publicly
> > available URLs for the PRLM 3rd and the PDF 1.5 spec  then I will
> > add them to Use.htm.  Obviously it is also possible to add links
> > to other documents that may be relevant.
> > 
> > 
> > Dan
> > 
> > 
> > -----Original Message-----
> > From: Igor V. Melichev [mailto:[email protected]]
> > Sent: Friday, July 23, 2004 7:02 AM
> > To: [email protected]
> > Cc: tech
> > Subject: Re: [gs-code-review] Fix for 687591 Overprint Preview is not
> > documented
> > 
> > 
> > Dan,
> > 
> > Rather this change clarifies something, 
> > IMO the main information is still missing.
> > 
> > 1. What is "spot colors" ? Where one can find a definition ?
> > A month ago I heard that you consider a spot color as
> > any tint besides Cyan, Magenta, Yellow, Black. 
> > Is this correct ? Is this documented ?
> > 
> > 2. Assuming the (1) is correct, I can guess what the rest of the
> > text means. In my understanding it means : (1) the result of overprint
> > depends on the device, (2) overprint isn't supported for
> > colors besides Cyan, Magenta, Yellow, Black.
> > Is this correct ? If so, I wonder why other tints (for example, Silver)
> > are considered to be worse than Cyan. 
> > If someone just replaces cartridges on the 
> > physical printer, will Ghostscript crash or fail ?
> > 
> > 3. It is still unclear how overprint works with Ghostscript :
> > 
> > 3.1. Assume I've got a printer, which can overprint physically.
> > Do we have a device driver for it ? Which one ?
> > Or can I force any driver to do it ?
> > 
> > 3.2. Assume I've got a printer, which cannot overprint physically.
> > I need to print a document, which uses an overprint.
> > I guess that can Ghostscript somehow convert overprinted
> > objects into "plain" colors. What should I specify to Ghostscript 
> > to get this effect ? I guess, the effect is a composition of colors with
> > some math formula. Which formula is that ? 
> > Can I customize the formula ?
> > 
> > 3.4. Assume I want to view an overprinted document on a 
> > display device (i.e. with additive colorants). I guess that 
> > Ghostscript can somehow emulate that view, using a
> > composition of colors with some math.
> > What should I specify to Ghostscript to view an emulation of 
> > the overprinting on the display ? I know what DisplayFormat is
> > relevant, but I still don't know what to specify exactly.
> > 
> > IMO answers for all these questions to be added 
> > into GS documentation. If an answer is long and may be found
> > is a publicly accessible source, a reference should be added.
> > 
> > Igor.
> > 
> > 
> > ----- Original Message ----- 
> > From: "Dan Coby" <[email protected]>
> > To: "Gs-Code-Review" <[email protected]>
> > Sent: Friday, July 23, 2004 10:13 AM
> > Subject: [gs-code-review] Fix for 687591 Overprint Preview is not documented
> > 
> > 
> > > 
> > > Fix for 687591 Overprint Preview is not documented.  This change adds
> > > documentaion for how Ghostscript handles overprinting and spot colors
> > > to Use.htm.
> > > 
> > > Index: doc/Use.htm
> > > ===================================================================
> > > RCS file: /cvs/ghostscript/gs/doc/Use.htm,v
> > > retrieving revision 1.109
> > > diff -u -r1.109 Use.htm
> > > --- doc/Use.htm 29 May 2004 01:21:54 -0000      1.109
> > > +++ doc/Use.htm 23 Jul 2004 06:05:05 -0000
> > > @@ -45,6 +45,7 @@
> > >  </ul>
> > > 
> > >  <li><a href="#EPS">Using Ghostscript with EPS files</a>
> > > +<li><a href="#SPOT">Using Ghostscript with overprinting and spot colors</a>
> > >  <li><a href="#Finding_files">How Ghostscript finds files</a>
> > >  <ul>
> > >  <li><a href="#PS_resources">Finding PostScript Level 2 resources</a>
> > > @@ -144,7 +145,7 @@
> > >  Ghostscript on unix-like systems type:
> > > 
> > >  <blockquote>
> > > -       <b><tt><pre>gs</tt></b> [options] {filename 1} ... [options] {filename <em>N</em>} ...
> > > +       <pre><b><tt>gs</tt></b> [options] {filename 1} ... [options] {filename <em>N</em>} ...
> > >  </pre></blockquote>
> > > 
> > >  <p>Here are some basic examples. The details of how these work are described below.
> > > @@ -625,6 +626,79 @@
> > >  class="offsite">
> > >  http://partners.adobe.com/asn/developer/technotes/postscript.html</a>
> > > 
> > > +<h2><a name="SPOT"></a>Using Ghostscript with overprinting and spot colors</h2>
> > > +
> > > +<p>
> > > +In general with PostScript and PDF interpreters, the handling of
> > > +<b>overprinting</b> and <b>spot colors</b> depends upon the
> > > +process color model of the <a href="#Output_device">output device</a>. Devices
> > > +that produce gray or RGB output have an <b>additive</b> process color model.
> > > +Devices which produce CMYK output have a <b>subtractive</b> process color model.
> > > +Devices may, or may not, have support for spot colors.
> > > +
> > > +<blockquote><i>
> > > +Note: The differences in appearance of files with overprinting and spot colors
> > > +caused by the differences in the color model of the output device are part of the
> > > +PostScript and PDF specifications. They are not due to a limitation in the
> > > +implementation of Ghostscript or its output devices.
> > > +</i></blockquote>
> > > +
> > > +<p>
> > > +With devices which use a subtractive process color model, both PostScript
> > > +and PDF allow the drawing of objects using colorants (inks) for one or more planes
> > > +without affecting the data for the remaining colorants. Thus the inks for one
> > > +object may <b><tt>overprint</tt></b> the inks for another object. In some cases
> > > +this produces a transparency like effect. (The effects of overprinting should
> > > +not be confused with the PDF 1.4 blending operations which are supported for
> > > +all output devices.) Overprinting is not allowed for devices with an additive
> > > +process color model. With files that use overprinting, the appearance of the
> > > +resulting image can differ between devices which produce RGB output versus devices
> > > +which produce CMYK output. Ghostscript automatically overprints (if needed)
> > > +when the output device uses a subtractive process color model. For example,
> > > +if the file is using overprinting, differences can be seen in the appearance
> > > +of the output from the <a href="Devices.htm#TIFF">tiff24nc and tiff32nc devices</a>
> > > +which use an RGB and a CMYK process color models.
> > > +
> > > +<p>
> > > +Most of the Ghostscript <a href="Devices.htm">output devices</a> do not have
> > > +file formats which support spot colors. Instead spot colors are converted using
> > > +the tint transform function contained within the color space definition.. However
> > > +there are several devices which have support for spot colors. The PSD format
> > > +(Adobe Photoshop) produced by the <a href="Devices.htm#PSD">psdcmyk device</a>
> > > +contains both the raster data plus an equivalent CMYK color for each spot color.
> > > +This allows Photoshop to simulate the appearance of the spot colors. The <a href="De
> > > vices.htm#display_device">display
> > > +device (MS Windows, OS/2, gtk+)</a> can be used with different color models:
> > > +Gray, RGB, CMYK only, or CMYK plus spot colors (separation). The display device,
> > > +when using its CMYK plus spot color (separation) mode, also uses an equivalent
> > > +CMYK color to simulate the appearance of the spot color. The
> > > +<a href="Devices.htm#TIFF">tiffsep
> > > +device</a> creates output files for each separation (CMYK and any spot colors
> > > +present). It also creates a composite CMYK file using an equivalent CMYK color
> > > +to simulate the appearance of spot colors. The
> > > +<a href="Devices.htm#XCF">xcfcmyk device</a>
> > > +creates output files with spot colors placed in separate alpha channels. (The
> > > +XCF file format does not currently directly support spot colors.)
> > > +
> > > +<p>
> > > +Overprinting with spot colors is not allowed if the tint transform function
> > > +is being used to convert spot colors. Thus if spot colors are used with overprinting
> > > ,
> > > +then the appearance of the result can differ between output devices. One result
> > > +would be obtained with a CMYK only device and another would be obtained with
> > > +a CMYK plus spot color device. In a worst case situation where a file has overprinti
> > > ng
> > > +with both process (CMYK) and spot colors, it is possible to get three different
> > > +appearances for the same input file using the
> > > +<a href="Devices.htm#TIFF">tiff24nc</a> (RGB),
> > > +<a href="Devices.htm#TIFF">tiff32nc</a> (CMYK), and
> > > +<a href="Devices.htm#TIFF">tiffsep</a> (CMYK plus spot colors) devices.
> > > +
> > > +<blockquote><i>
> > > +In Adobe Acrobat, viewing of the effects of overprinting is enabled by the
> > > +'Overprint Preview' item in the 'Advanced' menu. This feature is not available
> > > +in the free Acrobat Reader. The free Acrobat Reader also uses the tint transform
> > > +functions to convert spot colors to the appropriate alternate color space.
> > > +</i></blockquote>
> > > +
> > > +
> > >  <hr>
> > > 
> > >  <h2><a name="Finding_files"></a>How Ghostscript finds files</h2>
> > > _______________________________________________
> > > gs-code-review mailing list
> > > [email protected]
> > > http://www.ghostscript.com/mailman/listinfo/gs-code-review
> > > 
> > 
> > 
> 
>