Export PDF with transparency & white channel

[Phif]

Hello there,

Gimp doesn't seem to retain neither alpha or channel information on PDF exports. 

I didn't find any way to export PDFs with transparency. I checked it by opening it after export on Gimp and Photoshop. 
I also would like to add a "white" channel (for undercoat printing) to the very same file and when exporting to PDF, the channel simply disappears when reopening it in Gimp or opening it in Photoshop.

Is there a solution to both of these problems?

Note: The end format MUST be PDF, I can't use any other file formats.

Thanks in advance for your answers,



EDIT: I would need to do this with TIF files too. TIF is OK with transparency but not channels

[PixLab]

PDF transparency seems to be a bug in 2.8 and 2.10
https://gitlab.gnome.org/GNOME/gimp/-/issues/5807

but > Jehan @Jehan changed milestone to %2.10.30 1 day ago
so this bug shall be fixed in GIMP 2.10.30 (2.10.28 shall be out very soon, like in few days)

[rich2005]

One problem with PDF format is finding a viewer that supports displaying an alpha channel. Nothing I use in linux, I believe Windows Ad*be viewer has settings for this. Then re-opening in either Gimp or PS and ...no transparency No solution I know of for that.

You can check the image properties with command line ImageMagick 

Code:

magick identify -verbose filename.pdf

This from a single Gimp layer

   

Quote:I also would like to add a "white" channel (for undercoat printing) to the very same file

This from two layers the bottom a white underlay. This I assume for printing where there is no "white" ink. Offset printing can create a white layer before the usual CMYK but this is expensive, you need to ask your printer about this.

   

Quote:..I would need to do this with TIF files too. TIF is OK with transparency but not channels

Tiff can save to 'pages' and use 'clipping paths' Have a look at Scribus (which makes the best PDF for publishing) maybe you can use something from here; https://wiki.scribus.net/canvas/How_to_I..._text_flow
Again the best ask your printing company.

[Phif]

Hi!
Thanks for your answers and sorry for the late reply

(09-16-2021, 01:38 PM)PixLab Wrote: PDF transparency seems to be a bug in 2.8 and 2.10
https://gitlab.gnome.org/GNOME/gimp/-/issues/5807

but > Jehan @Jehan changed milestone to %2.10.30 1 day ago
so this bug shall be fixed in GIMP 2.10.30 (2.10.28 shall be out very soon, like in few days)

I guess I'll have to wait or find another way then... Thanks though!

(09-16-2021, 05:04 PM)rich2005 Wrote: This from two layers the bottom a white underlay. This I assume for printing where there is no "white" ink. Offset printing can create a white layer before the usual CMYK but this is expensive, you need to ask your printer about this.

Actually my company doesn't print on paper, we print on reusable plastic cups, that's why we need a white undercoat! And that's also why I would need to have transparency.
I work directly with the printing operators and often visit them. That's why my demand is so precise
We usually use Photoshop for the orders, which gives us the kind of files we're looking for. But we're currently developing a customer-side 3D previewing system to allow the customers to make their visual themselves. This system uses GIMP, and that's why I have to get the same kind of file that Photoshop allows me to have, then do a script to make GIMP create those files automatically.

So far, I can't seem to get to that point. Thanks for your time and advice though
If anyone knows of a solution to get past those problems, your help will be welcome.

Regards

[rich2005]

"That's why my demand is so precise" = photoshop

Gimp is a RGB editor so all you get is RGB + an alpha channel. Other channels are selection masks. That is not going to transfer to a PDF.  Ir is RGB required and not CMYK ? Where you can add extra layer separations ? You can get CMYK PDF's but not from Gimp.

Looks like Gimp is not the only tool you will require. Maybe for your customers creating something with transparency, a png for example, but not for the PDF.

I knocked this out with Scribus, Same PDF, shows the transparency in PS but not Gimp.  https://i.imgur.com/4ALiluF.jpg