I have a large number of elements (triangles) that together make up a bigger object. Each triangle has a slightly different fill color and no edge color. A part of the surface is shown below: enter image description here

The problem is that although zooming in to 6400% still doesn't show any disconnection or overlap (see below), the thin edge lines are still visible. enter image description here

Does anybody know what is causing these lines and, for me more importantly, how I can get a smooth transition between the different triangular elements?

Since the images above are with transparency I tested without transparency as well. Without transparency the lines are still there, but now they are white. enter image description here

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    Well, first you need to determine why there's the darker line to begin with. If there is no stroke applied, are you using transparency which overlays? What do the triangles look like in Outline mode? Properly aligned paths shouldn't show a dark edge unless there's transparency in play. – Scott Oct 2 '13 at 7:11
  • Indeed I have set the transparency of the elements to 30% in this case, but even without that there are visible lines (white ones) as you can see in the edited post. I will check out the Outline mode later today (I don't have Illustrator access at the moment) – Michiel Oct 2 '13 at 7:59
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    do these lines persist when you export the file to *.pjg or print it? Illustrator's preview mode is not perfect, and can show some gaps that aren't there, just like a *.pdf viewer will. – Vincent Oct 2 '13 at 11:05
  • I don't know of a guaranteed solution, but I've had this before. You could try googling "illustrator anti-alias seams", there are many suggestions. – noio Oct 2 '13 at 13:02
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    @Michiel Add a single solid filled rectangle behind everything set to a base blue color. I'd be willing to wager the lines then disappear. – Scott Oct 2 '13 at 18:03