RGB to CMYK : 2 different cases with 2 different results when printed

[Jack03]

Hello,

I have an image (a drawing) in RGB.

For printing purposes, I have to convert it to CMYK.

So here is what I did:

Case n °1
I open the RGB file and go to Image > Mode > and switch to CMYK.
I print it.

Case n °2
I create a blank CMYK file and paste the layers of the RGB drawing into it.

And I print.

While the 2 documents are now in CMYK, I have huge differences when printed.

Converting directly to CMYK (case 1) darkens the design a lot when printed, whereas importing the RGB layers into a blank CMYK document (case 2) leads to nearly the same color as the printed RGB.

I'm attaching 2 images:
- 1st one is to see the CMYK differences when printed (case 1 and case 2)
- the 2nd one is to see that RBG and case 2 printed out are very similar.

Can someone help me to understand those differences, and what should I do have perfect printed colors ?

Many thanks !

[Typo in subject line corrected from "RGB to CMJN" to "RGB to CMYK" by moderator.]

[Jack03]

And I should add that this Japan profile is only a guess, having received the file the printer designer will send to production... and I can't have a precise reply on that, so I'm only assuming it could be the right one...

[rob day]

Problem is also that some RGB files have colored designs, and when converting it to CMYK, the colors change a lot, so I'm asking my designer to redo colored design in CMYK...

When you edit in RGB turn on Proof Colors with the Proof Setup set to Document CMYK, that will give you a CMYK preview while you are in RGB mode.

And I should add that this Japan profile is only a guess,

If the designer’s AI file has Japan Color Coated assigned (Edit>Assign Profile) then that is the profile she is using and you don’t want to provide a file with a conflicting CMYK profile.

In the end an unknown output profile is the reason for not making CMYK conversions in Photoshop. Adobe applications use the same color management system, so the conversion to CMYK can happen anywhere

[Jack03]

Thanks.

When you edit in RGB turn on Proof Colors with the Proof Setup set to Document CMYK, that will give you a CMYK preview while you are in RGB mode.

=> Where can I set that ?

If the designer’s AI file has Japan Color Coated assigned (Edit>Assign Profile) then that is the profile she is using and you don’t want to provide a file with a conflicting CMYK profile.

=> Alright, so for futur designs, should I ask my illustrator to use the Japan Color Coated too ?

In the end an unknown output profile is the reason for not making CMYK conversions in Photoshop. Adobe applications use the same color management system, so the conversion to CMYK can happen anywhere

=> hum, this part I don't understand

[rob day]

=> Where can I set that ?

View>Proof Setup>Working CMYK and then choose Proof Colors.

=> Alright, so for futur designs, should I ask my illustrator to use the Japan Color Coated too ?

No, you told us your designer is assigning Japan Coated, so I assumed he knows what the press profile is Japan Coated and assigned it to his AI file.

=> hum, this part I don't understand

Your designer can make the conversion from Illustrator:

The exported PDF

[Jack03]

View>Proof Setup>Working CMYK and then choose Proof Colors.
=> Is it accurate, always, so I can base myself on this to estimate the printed file ?

No, you told us your designer is assigning Japan Coated, so I assumed he knows what the press profile is Japan Coated and assigned it to his AI file.
=> Let me precise
The printer has a designer that puts my design on his .AI file. He seems to use the Japan profile.
I have my own designer here, so should I ask him to always pick the Japan profile too when he creates new designs ?

Your designer can make the conversion from Illustrator
=> Alright, but what is the difference converting it from Illustrator and not Photoshop ?
Basically, my designer send me PDF files with the design on it (1 or multiple layers).

Thanks.

[Jack03]

Alright, now the last problem:

If I flatten the file before CMYK color conversion, it looks great but it removes the transparency which I need to keep.

If I don't flatter it, color conversion will darken the design quite a lot, but I still have the transparency.



To show you, I made this screenshot (before I placed a white background to all images to be able to compare):

Left: RGB
Middle: CMYK with merged layers before CMYK conversion
Right: CMYK with flatten image before CMYK conversion

See, if flatten image, the colors are perfect as RGB (but no transparency anymore).

What should I do here ?

Thanks

[rob day]

The display of transparency depends on the pixel color below the transparency. If there are no pixels underneath the transparent objects (0% OP) the color will be different than when there is a layer of white pixels (100% OP or 0|0|0|0 CMYK) below.

To see the final transpaency effect, you would have to either insert the desired background into the Photoshop file, or place the transparent Photoshop file over the background in Illustrator or InDesign and turn on Overprint Preview. Also blending modes set in Photoshop will not get applied to Illustrator or InDesign objects below the placed PS file.

Transparent pixels over 0% opacity (no CMYK or RGB value)

Transparent pixels over a solid color background set to 100% opacity 0|0|0|0 CMYK changes the preview and output numbers. The color difference happens in either RGB or CMYK mode.

[rob day]

Also not sure if this helps, you could flatten the Photoshop transparency and have the designer set the image to Multiply in Illustrator to change its background color:

[rob day]

=> Is it accurate, always, so I can base myself on this to estimate the printed file ?

Ideally the Proof Setup should be the final CMYK profile. If the proof Setup is a CMYK profile it will  show how out-of-gamut (unprintable) colors will convert.

Alright, but what is the difference converting it from Illustrator and not Photoshop ?

If the destination profile and color intent are the same there would be no difference. The conversion should only be made when the press profile is known. That could be the designer preparing the final PDF where the profile is chosen at Export to PDF, or by the printer at output.

[Jack03]

Hello,

Alright, however the printer asked for CMYK files.

So should I convert to Japan profile directly ?

I guess I should.

thanks

[Jack03]

Hello,

Does anyone know what I can do  regarding this last problem:

To show you, I made this screenshot (before I placed a white background to all images to be able to compare):

Left: RGB
Middle: CMYK with merged layers before CMYK conversion
Right: CMYK with flatten image before CMYK conversion

If I flatten the file before CMYK color conversion, it looks great but it removes the transparency which I need to keep.

If I don't flatten it and just merge layers, color conversion will darken the design (even change a bit the color), but I will keep the transparency.

Many thanks

[Test Screen Name]

Transparency WILL be different between CMYK and RGB. What aspect of the transparency do you need to keep? 

[Jack03]

Hello,

I need to convert RGB to CMYK.

If I flatten the file before CMYK color conversion, it looks great but it removes the transparency which I need to keep.
If I don't flatten it and just merge layers, color conversion will darken the design (even change a bit the color itself), but I will keep the transparency.

I would need the converted colors of the flatten image (which is very close from RGB color), but I also need transparency so I don't know what to do here.

[Test Screen Name]

I asked what aspects of the transparency you wanted to keep - or maybe why you wanted to keep it would be a better question. Bear in mind you must keep the RGB file for all future edits - the CMYK file is an end product, never to  be edited for any purpose.

[Jack03]

Well, normally I send files with transparent background to the printing company.

The printing company can print or not the white, depending of my wishes.

If I send the design with white background (colors are good) asking the printing company NOT to print the white... do you think it will exactly be the same final look as if I printed myself the RGB on my own printer ?

I mean will I have the same transparency on the printed CMYK with with background ?

Many thanks.

[Test Screen Name]

If you mean complete transparency (either completely opaque or completely invisible) there is surely a way to preserve that. What you cannot keep is transparency that mixes colours.

I'm not sure what you mean by "print or not the white". While white ink does exist it is rarely used. Usually something is white only because it isn't printed. Do the printing company do more with this work than just print it, on a page, by itself, with nothing else?

[Jack03]

If you mean complete transparency (either completely opaque or completely invisible) there is surely a way to preserve that. What you cannot keep is transparency that mixes colours.

=> hum do you mean it could be 100% transparent, or 50% transparent, etc ?

I'm not sure what you mean by "print or not the white". While white ink does exist it is rarely used. Usually something is white only because it isn't printed. Do the printing company do more with this work than just print it, on a page, by itself, with nothing else?

=> Yes this company can print white if needed
Yes they only print on temporay tattoo sheets

[Test Screen Name]

I mean there should be a way (someone can hopefully say) to keep 0% or 100% transparency in the result. Everything must be flattened, because you want to convert to CMYK and keep colours. Because converting anything except 0% and 100% will change colour in going from RGB to CMYK (or back).

[Test Screen Name]

Working with white, by the way, is a very complex art. Normal CMYK has NO WAY to print white at all (it can print Cyan, Magenta, Yellow or Black, and that's all; it is not CMYKW). The usual way to print white is a spot colour, but you have to liaise with the specific printer to find out exactly how they will do it. Some will do production work for you and convert regular white to spot. Don't assume anything if you change printers.

The truly baffling thing here is why you are being expected to deliver CMYK, when they clearly have prepress production people who are much better placed to do this conversion. Of course, you can't answer that, but it's still baffling!

[Jack03]

If I want to print white, then I would need to indicate where I want to print white.
That's the printer answer, and they will arrange that for me.

Well, it was probably my mistake not to have precise my designer to draw on a CMYK basis, so now it's difficult with the RGB...

I would prefer not to be 100% blind if I give the RGB file to the printer 🙂 

[Jack03]

Hopefully yes 🙂