RE: Fix for 687591 Overprint Preview is not documented

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>
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