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I'm attempting to draw a basic "armorial achievement" (aka a "coat of arms"). I have a shield outlined with a bold stroke and there is a diagonal white stripe that passes through the shield.

I used the shield outline to create a clipping mask that I want to use for the entire shield (by expanding the outline and using that as the mask, which very nearly works). I'm applying the clipping mask by grouping everything first and then adding the mask to the group overall...

enter image description here I am seeing artifacts of the white diagonal (with red outline) appearing outside the shield outline. Zooming in does show that the clipping algorithm handles the shield outline differently to the white diagonal (in other words the red and white areas are more clearly visible at higher zoom factors).

I'm convinced that nothing has been nudged out of place (the fact that both ends of the white part are having he same problem supports this) and that the shield outline thickness has not changed since I made the mask.

Is this a real problem or is it just a graphical / floating point error in my graphics engine telling me lies? If it's a real problem what can I do about it?

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  • It's probably just a flaw in the screen render as you suspect. If you want it to look cleaner on screen, you can apply a thin stroke to the mask that has the same gradient as border. This should cover the hairline gap issue on screen while making minimal impact on your design.
    – 13ruce
    Feb 10, 2020 at 20:14
  • So, that thick outline is actually on the clipping mask?? Or is the thick outline an additional object? Strokes on clipping masks have always been a bit "hinky". Best to use an additional object for the stroke.
    – Scott
    Feb 10, 2020 at 23:02
  • @Scott, sorry I didn't describe it very well. The grey outline is not the CM itself, but the CM was greated from a duplicate of the grey outline. The CM itself has no outline, is Filled white and (in theory) exactly matches the outer perimeter of the grey outline.
    – Wossname
    Feb 11, 2020 at 7:51

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It appears as though the clipping mask didn't act quite as precisely as you would have hoped. Having the colors behind the gray stroke line up so closely to the edge is never a good idea, in my opinion - leaving a little breathing room between the edges would be better. My suggestion would be to simply scale up the size of the gray outline. With the gray outline selected, use Alt+Shift to scale it larger proportionally. OR, with gray outline selected, go to Object > Transform > Scale > Check Preview, and check Uniform. Then experiment with the percentage (starting with 101% and working up) until you have a safe gap between the edge of the shapes. That will ensure that no shapes behind the outline are visible. Hope that helped!

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  • I see. Is it normal practice to tweak a CM after creating it? Is it an imprecise tool in general?
    – Wossname
    Feb 11, 2020 at 7:59
  • I've just tried a simplified version of this drawing (that does not use bezier curves, just a rectangle with one corner radius applied) and this problem isn't nearly as bad. In fact at higher zoom levels there is no problem at all. At 100% view there is a slight artifact though, but I'm happy that's just a rendering issue. Perhaps offsetting a bezier is prone to floating point errors.
    – Wossname
    Feb 11, 2020 at 8:09
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    In my experience, it's relatively normal to have to tweak things as you go along, especially if you're a perfectionist! And the more research I do on this particular issue, the more I see other artists suggest to always have some overlap between the edges of your objects. It can also depend on how you're saving the file once your done. Here's an interesting related question you might find useful as well.
    – gina
    Feb 11, 2020 at 16:15

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