overprint and transparency in Illustrator 7 - help

[Liz_Day@adobeforums.com]

Hello Illustrator pros....

I'm having a problem preparing art for press.

Mac OS 8, Illustrator 7, Photoshop 5 and 6.

I and my collaborator have some images in Illustrator, which will be printed in a book (offset printing). These are layered EPS files. The top layer is line art (black and white bitmap) at 1200 dpi. The lower layers are patches of solid color (CMYK) at 300 dpi.

(A sample image, flattened and jpegged):
<http://www.kiva.net/~daylight/colorbee.jpg>

We want the two layers to print separately: the top will be printed as line art and the bottom will be printed as halftone screened color.

I thought that to accomplish this, the file (Object) needed to be set to "Overprint" in Illustrator. But I can't get it to do that. I think I'm doing something wrong.

When we start, you can only see the top layer of the file.
We select the object, go to
Window > Attributes, and check Overprint Fill.

But this seems to change nothing. You can still only see the top layer, not those underneath. And when printed on a laser printer as a test, only the top layer prints. Yet a friend with Illustrator 10 says this method works properly on his system.

So we tried something else. We gave the top layer a transparent background in Photoshop before importing it into Illustrator 7.
This SEEMS to work, because you can now see the layers below it, and because it prints on the laser printer correctly. It is not possible to use Overprint in this case; Overprint is greyed out.

I'm worried whether we're doing this right.
Do I need to use "Overprint" for what I'm trying to accomplish?
Do Illustrator 7 and 10 handle this differently?
If we set up these files in 7 using the transparent background, will a press be able to correctly print them?
What do I need to do to make this work?

Help. :-(

Thanks,
Liz Day
Indianapolis, Indiana, central USA
[email]beebuzz@kiva.net[/email]

(In case you're wondering why the design house isn't doing this, instead of two digital-ignorant newbies, the answer is that there is no design house. This is a non-profit scientific publication, with no budget, so the author and artist also get to do much of the production work. The university has Illustrator 7, so that's what we're using.)

[Scott_Weichert@adobeforums.com]

Objects have to be on the same layer I believe for overprint to be utilized. (I'm trying to remember AI7 so I could be wrong)

Also, what you see on screen may not be correct for output. An imported black and white raster image set to overprint, should only print the black even though on screen the white will obscure other objects.

[Liz_Day@adobeforums.com]

Hi Scott,

OK, back up a second here. What exactly does Overprint do, and what is it for? I don't understand how it could even affect two objects that are on the same layer. Help.

(I got a book "Illustrator 7 Bible", but I can't find this in there. Nobody at the university seems to know Illustrator either.)

Thanks,
Liz

[Douglas_Habben@adobeforums.com]

I think if you try this, you'll be OK. It's been awhile since I did much in AI 7. Be sure the black art is saved as a bitmap TIFF. After placing it in Illustrator (it shouldn't matter if it is linked or not) select the image and click on the C=0 M=0 Y=0 K=100 swatch to assign it that color in Illustrator. The Overprint Fill checkbox should then be activated. Check it. Print your job out as separations to make sure everything is correct. Layers don't matter.

[Liz_Day@adobeforums.com]

OK, will try.

What exactly does Overprint DO? Or, to ask another way, what happens if the overprint feature is *off*? I don't really understand what this feature does in terms of printing.

(I don't have the computer right here to test this, it's at Purdue, an hour away.)

Thanks,
Liz Day
Indianapolis, IN USA

[Scott_Weichert@adobeforums.com]

overprint essentially places one ink on top of another. if Overprint is off, the bottom ink is knocked out (or erased) and the two colors must butt up against each other or overlap just a hair (trap) in order to appear correct on press.

Think of a blue circle and a yellow square.

If the blue circle is on top of the yellow square, overprint will result in a green circle (blue + yellow= green) or a built third color. If Overprint is off, the yellow square will be knocked out (erased) where the circle is, resulting in a yellow square with a blue circle in it.

[Liz_Day@adobeforums.com]

Wow. (Yes, I know what trapping is.) OK.... so in effect, if a printing press sees "overprint", it prints one layer, then in a second pass prints the other layer in the same place, without regard to their contents. (I mean it doesn't work exactly that way mechanically, but that's the result.) Right?

(Refer to
<http://www.kiva.net/~daylight/colorbee.jpg>

So I'm still confused.

I do see how Overprint will make the white background of the top layer print as though it were transparent. Because every part of the underneath layer will be printed in a separate pass.

But am I correct in thinking that, without overprint, all the layers will be flattened together and (since some are color) screened? I want the upper, black layer, to NOT be screened; I want it printed as line art, while the under layers get treated as 4-color process and printed with little dots. One must use overprint for this to happen - true? Because if we just make the top layer's white background transparent, so the bottom layer shows through, then they'll all be flattened into one image and screened. Right?

Thanks,
Liz
the benighted