Re: overprint and currentcolorname in mkiv

[Peter Rolf <indiego@gmx.net>]

Hans Hagen schrieb:
> Peter Rolf wrote:
> 
>> Well, it seems that my formulation was too fuzzy (its out of question
>> that overprinting is implemented). Anyhow, its time to test it again.
>> This code fails here and at ConTeXt live (online).
>>
>>
>> \setupcolors[state=start]
>>
>> \starttext
>>
>> No overprinting.
>>
>> \startoverprint
>> Overprinting is active.
>> \stopoverprint
>>
>> \stoptext
> 
> run ok here, but the question is .. what exactly do you expect to see
>

to be exact: some orange text :)

Acrobat8 menu 'Advanced'-->'Print Production'-->'Output Preview...'

[Output Preview Window]
- select 'Color Warnings' entry in the 'Preview' listbox
- check the now visible 'Show Overprinting' checkbox

Now all overprinting stuff is shown in the selected color (orange).
Well, at least should...

After comparing the pdftex and the luatex pdf files, I found a
difference in the used overprinting modes. pdftex uses the 'Standard
Overprinting Mode' (OPM 0) here and luatex the 'Illustrator Overprinting
Mode' (OPM 1). This is probably the reason why the overprinting is not
shown in orange in the prior described output preview. So its no ConTeXt
related problem (overprinting attribute is set), but more a bug or at
least missing feature in Acrobat8.

I have to read more about the differences between the two modes. What I
have found out so far (no guarantee):

- 'OPM 0' overprints only on gray and spot colors (which makes it nearly
useless for my needs)
- 'OPM 1' can overprint on CMYK, but only if the RIP is capable of
Postscript Level 3. If any color channel of the overprinting color is at
0%, the overprinting is done channelwise (which means the corresponding
channel of the underlying color is visible in case of the 0% channel; so
knockout should be used here instead of overprinting)
- Also overprinting in general only works if no ICC-profile is attached
to the object


Sigh, prepress problems. Sorry for the noise!

Peter

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