[Vincent Rosati]

It sounds like you want to take B&W line art and turn the ‘line’ into a negative shape that will allow a color background to show through?
As wuzelwazel described, there are many ways to achieve this in Photoshop.

You might describe the following as ‘destructive’ methods, as you’ll actually delete or permanently alter the layer.
Masking, on the other hand, can be non-destructive, as you retain all of the image data – but it’s more complicated.

Delete to achieve composite
-Open the line art in Photoshop, duplicate the layer, and hide the original background layer as a backup.
-To delete the ‘line’ you will use the Wand tool. Enable the wand tool and deselect ‘Contiguous’, this will select all relevant pixels in the document instead of only the connected shapes. Otherwise you would need to Wand+Shift click to add all of the disconnected line parts into your live selection.
-Delete the selection
-Paste your color background into a new layer, beneath the line-art layer.

I think the ‘Contiguous’ setting was your original issue?
Once you have created your live wand selection, you might want to look at the main menu under ‘Select’, and experiment with modifying your selection using options such as; Feather, Modify, Grow, etc.

OR \\\

Multiply to achieve composite (very easy with nice photographic qualities)
-Prepare your Photoshop file with you color background on the bottom layer and you B&W line art on the top layer.
-Click the line-art layer and Select/All. Next, click-Image/Adjust/Invert.
-Finally, back to the Layers palette. With the inverted line art layer, change the blend mode from Normal to Multiply.

[Vincent Rosati]

Ideally (here), you’d want 2 blue objects and four red objects, created by the Divide operation.
It looks like you’ll need to chop off the the three pointed corners with the Knife Tool, if Divide didn’t perform the cut.
Once your corners are removed and you have the final form, you’ll select all of the components and press Alt+Pathfinder/Add to Shape Area – or, Add to Shape Area than Expand.
This will make a unified shape.

[Vincent Rosati]

Could you create a Symbol with your object, and use instances of this symbol as the duplicates?
I don’t think this really “flattens” the vectors, as you can always Expand a Symbol. But, it does put your object in a nice container that will scale strokes and effects regardless of your preferences. And, if you need to make a change to your master object, you can easily apply changes to all instances.
I’m unsure if this would make any difference with your heavy object/memory issues.

You could also optimize your object by..
Removing absolutely anything that isn’t necessary.
Expand filters and effects, and outline type.
Use Pathfinder tools to simplify objects.
Use stepped Blend/Gradients instead of Smooth Color.
This can be very tedious.

If the graphic will not be rotated, you could snap it to a pixel grid and rasterize it.
Setting your Document and Raster Effect settings to 300 or whatever your spec’s are.

[Vincent Rosati]

That’s the best Photoshop action I have. I work mostly in Illustrator, and use mostly simple actions. My most used actions are simple Reflect Horiz. and Reflect Vert. set to my F2 and F3 keys.

Here’s a little set of illustrator action toys..
https://www.newinelabs.com/Rosati/stuff.htm

[Vincent Rosati]

I think you need to combine all of the paths into a compound path. Select all of the paths and press Ctrl+8 (Object/Compound Path/Make).

[Vincent Rosati]

I’ve been trying to use those as much as I can. I’ve found that 3D modeling effects, in Illustrator, can sometimes cause a complex document to corrupt.
But, since you lose your original line-art when you Object/Expand the object anyway, I’ll sometimes move the 3D effects into a new document and paste the expanded object back into the print file.
Hope that makes sense.
All of Illustrator’s effects generators are great. And, having the ability to craft complex effects and save them as Graphic Styles is nice too.
These effects can be pushed really far – I’ve just been messing with the Map Artwork option in the 3D effects options. Quite nice.
I’m sure there are plugins that greatly expand Illustrator’s extrusion tools. I’ve see them marketed to architectural and real-estate related businesses.
Or, you could jump into Cinema 4D, but if you need vectors for print than stick with Illustrator (for now?).

[Vincent Rosati]

With the text object selected, click on the main menu..
Type/Create Outlines

[Vincent Rosati]

This is a nice comic-type Photoshop action. It’s about a year old.
https://www.newinelabs.com/Rosati/stuff.htm

[Vincent Rosati]

Ideally, I would want to create a horizontally tileable vector of this “shape”, with sides that match seamlessly when connected. Than drag this shape into the Brushes palette. You’d probably want to select an Art brush, and set the size to Proportional. The ‘Proportional’ setting tells the brush to stretch the image, rather than repeat it.
Turning your image into a brush will allow the image to follow the curves and angles of a path.
Now you can select your path and apply the brush to it. You can continue to modify the settings of the brush by clicking the Options button in the Brushes palette, with the brushed object selected.
If you are trying to do this to a raster image, you could try using the same technique.

[Vincent Rosati]

If I understand correctly.. You could create a new document in Photoshop, choose the preset for A4, than paste your image into this file three times and position/nest accordingly. You would now have a “3-up” print file of your slip.
You could do the same in Illustrator in File/Document Setup/Setup/Size:A4. Than, duplicate or copy/paste the image and position/nest accordingly.