Illustrator Outline Problem

About a month ago, a classmate of mine had pointed out that illustrator sometimes had a mysterious hairline around shapes of objects, even when there wasn’t a border or outline defined on the shape. Furthermore, this outline when exported out into raster format, retained and translated that outline into the pixels of the image!

See the following image close up to see what is happening. Also described here some six years ago but with no solution.

Well as you can see in the image, the problem has to do with the CYMK specifically, and the depth of the colour values for not only K, but cyan, yellow and magenta as well.

For some reason illustrator puts a thin outline around the image.

I really don’t know know if this was planned by Adobe, but I do know how to fix it – simply Set all CMYK values to 100%, or if you want, set all of the values of C, M & Y to those of the background colour, and let the K (black) fill the rest of the shape as the black normally.