When a print product’s manufacturing process includes a cutting step, you usually need bleed. Something many creators of print layouts are unaware of. For this reason, PDF prepress workflows often include a step that adds bleed if missing. Several approaches exist for deriving bleed content from page content. All of thesemethods need to determine where the bleed is created. Normally you would use the trim box to derive bleed from there. In this blog we want to demonstrate that this approach is not always ideal and talk about a new automation feature in pdfToolbox 15 that uses rendering to determine the best location.

pdfToolbox knows several methods to create bleed. In this blog we will not discuss these methods, instead we concentrate on two of them: mirroring and pixel repetition. Normally you would use the trim box to mirror page content or to repeat the last pixel inside of it to create bleed. However, using the trim box is not ideal in two cases:

• The page content does not fully reach to the edge of the trim box, instead there is a small white area that has (probably) not been recognized by the creator.
• The page content already extends beyond the trim box, but does not completely fill the bleed area.

To address this, pdfToolbox allows for specifying an offset to the trim box which can be positive (outside to address existing bleed) or negative (inside to address white areas). However, in an automated prepress world – and that is where pdfToolbox lives – it would be nice if the engine could determine the best offset automatically: You can render small stripes along the trim box (inside and outside) to find out whether there is content that is an extension of the next inner line of pixels.

Starting with pdfToolbox 15 exactly that is possible. The best way to demonstrate how that works is with examples:
The first example deals with the problem that the page content does not reach all the way to the edge of the trim box.

The file has a thin white area between content and trim box at the left and top edges. Since it is so thin it is almost invisible.
When we now create bleed for that file without using an offset the results are unusable:

With mirroring, the thin white area is twice as large as before and with pixel repetition it is even extended into the bleed.

When we let pdfToolbox determine the offset automatically, the results are much better:

The next example deals with the other issue - where there already is some bleed, but not enough:

When we now create bleed for that file without using an offset the results are not optimal:

Since the existing bleed ends very close to some of the small balls and to the puddle neither of the two bleed creation methods creates very good results.

Again, when we let pdfToolbox determine the offset automatically the results are much better:

pdfToolbox now takes the existing bleed into account and automatically adds the missing bleed after the existing one.

You may now want to know how you can activate that in pdfToolbox?

Here is the answer: Generate bleed from page content. Watch out for "Derive offset from page"!