BUG: overprint preview on working for white colors

[Lordrhavin]

Create a blue rectange, put white spot-color text onto it.
Change text to overprint.

-> in overprint-preview, text should now be some light blue, but it is invisible instead.
solution: spotcolors of p% opacity should change color to (1-p)*below_color + p*spot_color

[Eugene Tyson]

White is C0 M0 Y0 K0

Set to knockout you see below the blue rectangle - as it 'knocks out the background'

Set to overprint and you see nothing 0% ink.

If you want White - it's best to use an Off White to represent the White

Make it C20 M20 Y20 K20 - as a Spot colour

Then white ink can be applied at the printing stage 

You only really need this if printing on transparent stock - like acetate or labels. 

If you want to see the paper stock underneath - if it's white - then use Paper

If you use Paper on a transparent stock (like acetate or a label etc) you will see right through the label to your hand.

For this reason - on transparent stocks of substrate - it's best to use a Faux White colour to represent your white text or backgrounds that should be printed with white ink.

 

[Lordrhavin]

Its not helpful and wrong. First, white as a spot-color is not CMYK 0/0/0/0, that is just its screen representation in a CMYK-to-RGB setup. Actual white would be C0 M0 Y0 K0 W100 while in case of printing C0 M0 Y0 K0 is just "none".

The thing that is wrong here is the colormodel providing print preview. A Spot-Color is not a CMYK processing color and has to be handled different. As stated the correct way to put a White 80% color in a CMYK-model atop a 50/50/0/0 CMYK-blue would be:

80% * of 0/0/0/0 = 0/0/0/0
+
20% * of  50/50/0/0 = 10/10/0/0

= 10/10/0/0 in CMYK

[Lukas Engqvist]

I understand what you are saying, and how you would like for spot colours to be dispayed as opaque simulation… Spot colours are used also for metalicts, varnishing and embossing where these are not visible, but still need to be represented on screen, yes it is possible to render a simulation of print, but this is not what overprint preview is. 
I would suggest trying a software like Esko Studio. It is pricy but it does the kind of simulation you ask for.

[Eugene Tyson]

You have introduced a 5th colour into the CMYK model.

Adding W would make your workflow CMYK+W

This is your spot colour W

And the way colours are made in InDesign is for print using combinations of CMYK

No white

White in print is achieved by 0% ink.

Which is set to knockout and shows the underlying paper colour.

To get true white in print ink it needs to be a representable colour set to overprint.

So that white ink can be applied 

I've been doing this job for 25 years and I fully understand the process.

If you can provide some insight on what you want to achieve we can further assist.

I can tell you how it works, I cannot change that.

[Lukas Engqvist]

How is your white spot colour defined. You can define a spot colour to have any appearance. (I normally will make it Pink or some bright green depending on colours that do not appear in design).
If you are using a blue rectangle use a colour that will show on blue.
White will show as darker since it is overprinting. 

Spot colours may be defined any colour model, but must be displayed as "Overprint" so that you can judge trapping etc. Spot colours that overprint can not display as knocking out what is under.

[Lordrhavin]

My white is defined as a spotcolor with a CMYK-representation of  0/0/0/0. I know how I can circumvent the bug (give it some other representation), but a bug is still a bug and should be fixed. The internal color model is probably CMYKA (a=alpha), because you cant do tranparancy otherwise, so this is just a mathematical issue. I've doing colormath in multiple additive and subtractive colorspaces for a long time. I know what is going wrong here. You dont even need a fifth color, CMYKW can be translated to CMYK' by allowing K' to be -100 to +100 internally as they basically neutralize each other to K'=0 what then would be K=50 in a pure CMYK colorspace.

[Eugene Tyson]

If you can tell us what you're trying to achieve we can help you out and get it setup right.

[Lordrhavin]

I dont need help. I reported a bug.

[Eugene Tyson]

This is a user to user forum - for discussion and help with issues

Report bugs or make a feature request

https://www.adobe.com/products/wishform.html

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For the record it's not a bug.

But definitely a feature request.

Good luck - let me know the link to the bug/feature request and I'll support it.

Visually having white ink would be nice. 

[rob day]

The internal color model is probably CMYKA (a=alpha)

Hi @Lordrhavin , Spot colors in InDesign always represent an extra plate running a custom solid ink on a press running separations—e.g. an offset press. They are not related to alpha channels or transparency. Photoshop does not have spot colors, but represents spot separations with Spot Channels, which are not the same as Alpha Channels.

While you can define a Spot color with any Color Mode, the output is always to a separate plate and not a process CMYK color. For that reason InDesign started defining most Spot Color Libraries (eg. Pantone+ Solid Inks) as Lab, which allows the solid ink to be more accurately displayed.

There is an assumption that all offset inks are somewhat transparent, but as Lukas points out some inks like metallics might be more opaque, and if the separations are for something like screen printing—very opaque.

So the feature you are looking for would be to set the spot ink’s level of output solidity, which you can do in Photoshop. That task is easier to manage in Photoshop where the document has single color mode as opposed to InDesign where the Spot Color’s overprint might be overprinting a combination of four different process color modes blending together (Lab, RGB, CMYK, Gray).

But, even in Photoshop the accuracy of the soft proof is questionable. How do you know what the density of the solid ink actually is without going to the expense of a press proof?:

[Lukas Engqvist]

@rob day Thanks for your explenation with illustrative drawings.
The question is about how much of design is creation and mapping intent and how big a part of that is redering a realistic representation where when there still are a number of parameters that are needed to generate that, result… perhapps AI will help push technology to where that is possible, or perhapps the technologies in Substance could one day give us this kind of previews in future. I would love that. 
The ESKO studio plugin does this for Adobe Illustrator, along with 3D mapping which is usefull even for visualising matte and gloss varninshes as well as embosing, flourescents, metalics and foils.

[reproo2773183]

You'd need to define the opaqueness of the ink separate to its colour. There is no field available in PDF v1.x

Rob shows you Solidity with Photoshop, this won't work outside of Photoshop because the preview is generated differently. Trapping programs list the density but can also set Inks to Opaque, Dieline or Ignore, this is all great if you have the relevant plug-ins but doesn't affect the appearnce in Acrobat alone.

If I've got a gold foil on a black background I'd set it to overprint and it will disappear on my proof. It would be great to be able to set the colour to Opaque but I can only hope that it will be a PDF v2.x feature.

The issue seems to be that Adobe can make more money by focusing elsewhere, they don't seem much interested with print anymore.