Multiply combined with Overprint giving odd results in InDesign

[Gregg]

I ran into this problem while preparing an interior picture book file for print.

This specific project had both live type and hand-lettered text (either grayscale.tifs or bitmap.tifs) overprinting 4/c art. All of the text (vector and raster) is assigned to TextBlack (which is just a spot 100K). This is done to allow for co-edition printings. All TextBlack elements are set to overprint. The hand-lettered text used for this project was grayscale tifs that were set to Multiply (in InDesign CS2) to remove the white background. What I noticed was, when looking at the Overprint preview, the bounding box of the grayscale tif were darkening the 4/c art below, and I pinpointed it to the fact that the grayscale.tif was set to Multiply and to Overprint. If I set it only to Multiply it was fine, and the same was true if I set it only to Overprint. I just can't figure out why this is.

A test:
This is what I did to test it out. I created a 50% cyan P'shop file (saved a TIF, EPS, and PSD version). Created a grayscale.tif with some squares with varying densities, and some empty areas (white). I brought all of the cyan squares into ID (CS2), placed the grayscale.tif file on top of each one. Set the grayscale to Multiply, then assigned it the color TextBlack and set the fill to Overprint (Under Attributes). Then I would look at my separations preview, and the 50%c would change to 75%c wherever the bounding box of the grayscale.tif sat on it. (Note: this also happened when I ripped a PDF, both flattened and live).

I have my workaround, which is to just set the grayscale file to Multiply (that way it is overpinting, and the white box is not intruding on the designer's needs). But I just can't figure out why the area of the grayscale image with no information in it is causing the underlying colors to darken.

Hope this is clear, and thanks in advance for any feedback.

Gregg

 

[Tech]

Re: Multiply combined with Overprint giving odd results in InDesign

Hi Gregg,
We used CS3 and I have seen something similar, but on a 100K box without overprint nor any effect mode applied. Under Separation Preview, this box displays darker than surrounding box that doesn't overlay images.

Our vendor has no problem with printing the cover correctly. It doesn't make any sense, which leads me to think, it's an InDesign issue... come to think of it now, the file is probably updated from CS2 to CS3.

 

[digi-digi]

Re: Multiply combined with Overprint giving odd results in InDesign

this is correct behaviour. i pulled this quote from the pdf reference, should answer your question?

+“If the current blend mode when CompatibleOverprint is invoked is any mode other than Normal, the object being painted is implicitly treated as if it were defined in a non-isolated, non-knockout transparency group and painted using the CompatibleOverprint blend mode. The group’s results are then painted using the current blend mode in the graphics state.”+

the last sentence is what is giving you the odd effects.

 

[Gregg]

Re: Multiply combined with Overprint giving odd results in InDesign

Thanks, digi-digi!