An underbase can be created using either a new layer or a channel.
The way i work in photoshop, i usually make my spot color under base by using the selection tool. I select the color i want, then reduce the selection (depending on the dpi, usually 2pixels for 300dpi images) to create a choke. Save this selection as a channel, clear selection and invert the channel (so the ink areas you want are now the black in the channel). Go back to the original layer (select RGB channel if you are in RGB mode) select the next color you need to be underbased, reduce selection again by same amount to create a choke. Add that new selected area(s) to the under base channel you already created by selecting it and filling it in with 100% black. Repeat for all colors/areas needing an under base.
From there you can setup the base coat channel as you need it depending on how you are setting up the separation file.
if you just want the base coat as a separate file simply select the channel you created, right click for the option to 'duplicate channel', create new document from channel. Select the new document and convert it to a bitmap image (try to stay at the same dpi size, if you started with 300dpi, convert the bitmap to 300dpi also).
Save this new document and you can now import that into illustrator if needed.
Red, everything you described makes perfect sense. I can't wait to give it a try next time I am making a design in Photoshop (I usually use Illustrator.) Why did you mention Illustrator at the end? Do you use it to output your separations? Why not just stick with Photoshop altogether and output from there?
Just curious about the methods others use.
Thanks,
Jim
Ya i out put from illustrator always. I do this for a couple reasons, the main one tho is so i have complete control over info and registration marks (type/placement/line thickness). Using Illustrator also allows me to use vector art elements (ex: text) in the designs/separation, also allows me to mix different separation techniques into one job.
The magic wand tool
or using the selection functions depending on what i'm doing, i don't use the color range function very often unless doing simulated process seps.
Don't mean to hijack this thread but I have a question. I did a design in illustrator and created the seperations on different layers so that I can print. I also took the whole image and made as one solid image for a base but can't for the life of me figure out how to choke it. Red, you seem knowledgeable, what are the steps? Any help appreciated.
I have Illy CS3
Heidisp, feel free to pm me if you have questions.
creating a choke in AI is fairly easy. Combine all you objects to make one object that covers all the areas you want for your underbase.
Then add a stroke of 1point to that object. by default, strokes go half on and half off the object (they expand in thickness from the center of the path) so using 1 point means you're actually chocking by only 0.5 of a point. This should be enough of a choke for most designs/printers, i actually use 0.25 of a choke for critical work but this doesn't give the printer much play.
after that i often expand the stroke and then punch that from the object, using the path tool, so there's no more stroke. this is just an extra step i like to do.
Underbase in photoshop 7 
