PHOTOSHOP - New trapping suggestion for lines with variable width

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Hi, I have a suggestion to improve Photoshop. Most illustrators and designers, who work for print with lineart, need to trap colours under the black lines to avoid white edges next to black lines when printed. I guess that the most common way to trap lines in Photoshop is to make a selection and then expand it so that selection goes under the lines. Unfortunately this doesn't work too well when working with lines made width Pressure for size activated or scanned brush strokes, because you can't find the middle point of the strokes with variable width by simply expanding selection. That's why some illustrators are using another programs such as Wintopo to make trapping faster. If you are not familiar with Wintopo, it is a program that changes all strokes to 1px wide. When changed the width of the whole line to one pixel, you can copy this one pixel wide line under your original lineart in Photoshop and by colouring this new layer, your colours get always maximum trapping, because this line is always in the centre of the original line. If this option would be possible in Photoshop by making 1 pixel wide layers from the whole original lineart layers or from selected are directly in to your photoshop-document, it would save a lot of time and make trapping really fast and simple in every situation.
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Janne Toriseva

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Posted 3 years ago

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Chris Cox

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Why are you not using the Trap command in Photoshop?
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Janne Toriseva

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Trap doesn't work in every situation. First of all, you have to flatten your image when using trap. But in print, you want to use a different resolution for your black lineart and for your colours because if you make a bitmap of your lineart you can use a much higher resolution for it. For example, you are making an illustration for a magazine which is published in 300 ppi. First you make an image for example in 600ppi or even in 1200 ppi. Then you make bitmapped tif or pdf of your lineart with that resolution and resample your colours to 300 ppi and then make a pdf where the bitmap lineart with higher resolution is placed over the 300ppi colour tif/pdf.

The second problem is if you are using lineart that has lines with variable width. Let's say you have a line that is only 4 pixels wide in the edges but 40 pixels wide in the middle and there is red on the other side of the line and blue on the other side. If you use maximum trapping 10 pixels the red and blue will spread over the lineart and there will be mixed colours in both sides of it, which will not look nice. But in the middle of the line will be just white under the black. You see that a lot in Donald Duck magazines and that doesn't look too nice. But if you are making the  1 pixel wide conversion of the lineart with Wintopo, for example, put that under the lineart and colour the ares now the blue will always get in the middle of the line as will the red and there will be no white gaps in the black or colours spreading to the wrong side of the lineart.  As an attachment, I have an example of the second problem of using Photoshop trapping in some situations (photoshop trapping.png, the first image) and an example of the same situation when used wintopo trapping (wintopo trapping.png, the second image). I decreased the opacity of lineart in that example so that you can see what's goind under the black line.

Edit: There are also some other problems when using trapping. For example, you have an illustration with white snowflakes. If you use wide trapping in that image and snow is just white dots, not circled with any lineart, the trapping would ruin the snowflakes if they have been made before trapping. When using this Wintopo-example there wouldn't be any problem.
 
(Edited)
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Chris Cox

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OK, so you're not doing traditional trapping - you're just trying to spread colors under the line art (black plate/channel).  Generally you need to trap to compensate for the lack of print placement precision (i.e.: offsets), and that applies to all channels, not just under the black channel.  With more information about what you are doing and your print workflow, we might be able to create a "trap under black only" command.

BTW - most folks print the CMY and K channels at the same resolution.
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Janne Toriseva

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Yes, this is not traditional trapping, but quite general workflow with cartoonists and illustrators, who are working with lineart and colours. Here is the standard procedure with this method: 
1. Lineart, inked or computer-made, is in it's own layer with high resolution (600-1200 dpi).

2. Colours are made in their own layer(s) so that there won't be any gaps between them.

3. Lineart will be transformed into bitmap with 50% threshold with the original, high resolution and then saved in tif format.
4. Colours will be flattened if they are in separate layers, turned into the CMYK-profile that the publisher is using, resampled into the resolution that will be used in the publication (for example 300ppi) and then saved in tif format.
5. Tif with lineart will be placed on top of the tif with colours in InDesign or in some other program that can handle tifs with different resolution together. PDF will be exported in overprint-format. Now we have lineart in high-resolution and colours in the resolution that publisher is using. Lineart will remain it's high resolution, because bitmap won't get rasterized.

If there could be a way to spread colours under the lineart easily in Photoshop it would be a huge advantage for most cartoonists and many illustrators who are working with print. I think that your suggestion "trap under black only" command would work just fine if it could be used without flattening lineart with colours. Or maybe "trap under selection only" would do it also and maybe be useful in many other circumstances too.
Best, Janne