Full Cover Margin Issues

The red border is the outer border of the "page". This is what InDesign or PhotoShop would care about, so it would print everything within that border. It doesn’t care where things overlap or do not overlap within that.
Now look at the inner part. The blue box is the "paper size" of the actual finished book - but I never stop an image AT that blue box. That then risks too much if the printer is off by a bit. The printer only has to be slightly off for white borders to show up. Instead, I always have all four sides of my image go past that "artificial margin". Hopefully you can see that overlap if you look at the edges of the blue box - that the imagery goes past it on all four sides. That way even if the printer is off, there is still color on that side of the printing when they physically cut off the edges of the paper.
Make sure when you create the PDFs you’re making for the printer, that you have your design go past that artificial margin. Do not have the images stop right at the hairy edge of what that inner artificial margin was set at. If you try the hairy edge technique, you're sure to have random books produced where the cutting equipment wasn't exactly on line, and you end up with white margins.
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