[XeTeX] \font "<platform font>":color=FFFFFF produces black, not white glyphs \font "<platform font>":color=FFFFFF produces black, not white glyphs, re-visited

[Zdenek Wagner]

út 26. 5. 2020 v 1:50 odesílatel Ross Moore <ross.moore at mq.edu.au> napsal:

> Hi Zdenek,
>
> On 26 May 2020, at 9:31 am, Zdenek Wagner <zdenek.wagner at gmail.com> wrote:
>
> út 26. 5. 2020 v 0:59 odesílatel Ross Moore <ross.moore at mq.edu.au> napsal:
>
>> and at the time, that appeared to solve my problem. However, it would
>> appear that since then "xdvipdfmx" has been enhanced to support
>> transparency, as a result of which Khaled's suggested FFFFFF00 no longer
>> works (the text is invisible, see attached).  Could anyone tell me how,
>> short of using \specials, I can achieve 100% white with 100% opacity (= 0%
>> ink) in XeTeX ?
>>
>>
>> I’m sorry, but this just doesn’t make any sense to me — but see further
>> below.
>> Surely 100% opacity means that the blend between background and
>> foreground is 100% background, 0% foreground.
>> Thus your text will be invisible, whatever colour has been specified;
>> that this is white becomes irrelevant.
>>
>> The only way to get 100% white, over a coloured background, would be with
>> 100% ink, so 0% opacity.
>> Any other opacity level will allow some of the background colour to be
>> mixed in.
>> At least that is how I understand what colour mixing is all about.
>>
>> Sorry, correct me if my English is wrong but I would expect 100% ink =
> 100% opacity = 0% transparency
>
>
> You’re absolutely correct, my mistake.
> Certainly I meant 0% *transparency*, the opposite of what Phil was trying
> to do.
> It was he who said   100% opacity (= 0% ink)  which is where the error
> lies.
> Surely 100% ink = 100% opacity = 0% transparency,  as you say.
>
>
>> However, there is another PDF parameter called “knockout”.
>> See this link for an brief description of the issue:
>>
>>
>> https://www.enfocus.com/en/products/pitstop-pro/pdf-editing/knockout-white-text-in-a-pdf
>> <https://protect-au.mimecast.com/s/tuWYCBNqgBCvBNrAuzR3NV?domain=enfocus.com>
>>
>
> This is another topic. This addresses "black overprint" in printing.
>
>
> Sure, but it is about knocking out the colour in the background.
> Using black text is probably its most common usage.
> But if you want the natural paper to come through, then surely this is the
> only way to do it conveniently.
>
> Otherwise you would have to manually define regions outlining the letters,
> and make these
> boundary curves for your background. Totally impractical.
> PDF does this for you, if you have used the correct way to ask for it.
>
> Yes, I know, but knocking out works always while overprint requires CMYK.
The colour of an OpenType font can only be specified in RGBA.

>
> The idea is that process colours are printed in the following order:
> cyan-magenta-yellow-black. If you want to print a yellow text on a cyan
> background, RIP must erase the cyan plate to white where the characters
> will later appear on the yellow plate, otherwise the text would not be
> yellow. If the offset films are not precisely adjusted, you will see colour
> artifacts at the boarders of the characters. If you want to type a dark
> text (usually black) to a light background, it can just be overprinted. In
> order to make it work, both colours must be defined in CMYK (not RGB, not
> grayscale). Professional Acrobat since version 9 contains a prepress
> checking function which can verify whether overprint was really used. Black
> overprint is implemented in my zwpagelayout package. It was tested in
> xelatex, pdflatex, and latex + dvips. The package does not my test files.
> If you like, I can send them.
>
>
> Sure; I’d love to see these.
> I’m sure that this would most closely approach what Phil seems to want to
> do.
>
> I will send in in the next mail.

>
>
> Zdeněk Wagner
> http://ttsm.icpf.cas.cz/team/wagner.shtml
> http://icebearsoft.euweb.cz
>
>
>
>
>> How to achieve knockout using TeX+gs or pdfTeX or XeTeX?
>> I’m not at all sure. It must have a graphics state parameter.
>> The next image shows what I think is the relevant portion of the PDF
>> specs.
>>
>> <Screen Shot 2020-05-26 at 8.38.02 am.png>
>>
>> There’s a whole section on “Transparency Groups”, but mostly it is about
>> how different transparent objects
>> must combine to produce the final colour where objects overlap.
>>
>
> Transparency should not be used for prepress. It works fine on office
> printers but often come out as black on phototypesetters and CTP.
>
>
> Phil hasn’t said what is his application.
>
> After a cursory look, I think you need to use a Form X Object, which can
>> be done in pdftex using the  \pdfxform  primitive,
>> with appropriate attributes specified.
>> For XeTeX you would need to be using  \special  commands.
>> Someone here must have some experience with this.
>>
>>
> Zdeněk Wagner
> http://ttsm.icpf.cas.cz/team/wagner.shtml
> <https://protect-au.mimecast.com/s/TZFICD1vRkCX0ZzMs5cgKz?domain=ttsm.icpf.cas.cz>
> http://icebearsoft.euweb.cz
> <https://protect-au.mimecast.com/s/4H07CE8wlRCBMj2nhptby5?domain=icebearsoft.euweb.cz>
>
>
>
>
>>
>> Philip Taylor
>> <Demo.pdf><Demo.tex><Lighter ground.pdf>
>>
>>
>
> Sorry for my error adding to confusion.
>
> Cheers.
> Stay safe.
>
> Ross
>
>
> Dr Ross Moore
> Department of Mathematics and Statistics
> 12 Wally’s Walk, Level 7, Room 734
> Macquarie University, NSW 2109, Australia
> T: +61 2 9850 8955  |  F: +61 2 9850 8114
> M:+61 407 288 255  |  E: ross.moore at mq.edu.au
> http://www.maths.mq.edu.au
>
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