Trapping CMYK image to spot-ink background

[deaner]

Imagine a raster image (CMYK) with a very hard edge where the image meets the transparent background. Then imagine placing that on top of a spot-color blue background in Adobe Illustrator or InDesign. It looks fine on screen of course. But on press, you'll inevitably get misregistration and have a sliver of white on one side of where the image meets the spot-blue background. I've always used a technique where I open the image in Photoshop, select the transparent background, feather the selection a bit, then delete the selection. This technique seems to work okay, but I'm wondering if there are better techniques in use, or if there are guides or videos on best practices for this scenario.

Thanks in advance, everyone.

 

[keith1]

The simple thing to do would be to also print the border CMYK.
If for whatever reason that's not feasible, make the border slightly larger and overprint the spot colour. However that could be a problem if your blue border colour is very light.

 

[YourCastle]

depending on the output device. I don't bother trapping for digital and most platemaker software can trap on the fly.

if I had to do it manually, I'd set the spot color to overprint 0.25pt or whatever the pressman wants. If the color difference is too jarring, just build a whole new color that blends better between the two

EDIT: and you can build a new color using part of the spot color

 

[prepress labels]

You might try this (coming from a flexo environment where we do this stuff all the time):

Make a one-color vector outline of the image (multiple ways to do this--I usually start in PhotoShop, then Copy, then Paste in Illustrator and use Image Trace, then Expand).
Turn the vector object white, scale and position it to match the image as closely as possible.
Go to Effect/Path/Offset Path and type in a negative number for the amount of trap desired.
Cut the object and paste it behind the image.
Set the image's opacity to Darken.

This should result in a continuous overlap between the CMYK and the spot color, all around the image.