Lineart scans: Multiply or overprint?

While i’m not sure about the behind-the-scenes technical differences between the two, but it would appear that there is a feature of multiply on lineart images which isn’t so great once a PDF is flattened.

Using the following demonstration, i’ve placed the same two images side by side – one greyscale, one lineart, then one applied with multiply and then one applied with overprint.

From here, i’ve exported to PDF with two major considerations: using Acrobat 4 compatibility and  downsampling CMYK and greyscale images.

Now, once the PDF is analysed using a third party plug-in Enfocus Pitstop, I can see things which Acrobat Professional would not normally tell me. In this instance, the greyscale images have been unaffected and essentially appear the same. However, the lineart image which had the multiply effect on it has dropped in resolution from 1200 DPI to 350 DPI; and it has gone from being a lineart image (1 bit per channel) to being a greyscale image (8 bits per channel).

So multiply has effectively flattened the lineart as a greyscale and compressed it based upon the compression settings for greyscale images. The lineart image set to overprint on the right is unchanged – it is still 1-bit at 1200 DPI.

What this means is that sharp, crisp lineart which has had a multiply effect applied to it (presumably to behave as an overprint) can render as not so sharp greyscales. This also means that rather than outputting as lineart, the image will be rendered as halftone dots.

If linearts have to appear as if they are overprinting, use the overprint function from the attributes panel.

Further to this, if a PDF is made using higher Acrobat compatibility settings and no downsampling is applied, it may appear that the PDF is fine in Enfocus Pitstop, but to print a PDF to final output it must ultimately be flattened at the RIP and the same thing will happen as in the example above.

“Spot” the difference of soft light with overprint preview

I recently found myself being the “bad guy” after having to instruct a customer to resupply their artwork given that many of the effects applied to the pictures in InDesign would not print as desired.

In short, the artwork was an annual report printed in full colour plus a metallic silver spot colour. Originally supplied PDF only, everything looked fine on first glance with the overprint preview off. However, while the document was being manually preflighted using Acrobat’s Output Preview, I had noticed that a greyscale-like effect on the silver had disappeared once I had entered the Output Preview. Concerned, I restarted Acrobat to make sure the glitch was not software related, but again the same thing appeared. This happened on several machines and it soon became apparent that the artwork would in fact print as it appeared in Output Preview rather than the normal preview.

The customer was then contacted and informed of the situation. After replying that the artwork looked fine on his screen, the customer was then instructed to turn the overprint preview on within InDesign, and lo and behold… he began to see what I saw. He then told me he had used the soft light effect.

To demonstrate the phenomenon, I have created a new InDesign file with five elements: a rectangle coloured with Pantone 871C; a rectangle coloured with the default green which ships with InDesign; a stock photo with the soft light effect applied , and two captions of the colours in the rectangles. In the before image, the Separations preview is turned off.

and this is how the InDesign file looked after the separations preview was turned on:

resulting in the image disappearing from Pantone 871C rectangle. However, the image still appears over the process green rectangle.

Ultimately, this means that the effect is only reproducable over process colour, and not spot colour, regardless whether it is metallic or not.

Interestingly as well was the fact that in Live preflight, there was no error warning of this particular feature of the soft light effect, so if I was purely to obey the live preflight and not check my file with the separations preview or overprint preview, this would have been completely missed.

The lesson here? Always check artwork using the separations preview to make sure the artwork will appear as designed, and that some effects will work in process only.

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