Edge line between partially transparent shapes in Illustrator

Question

I need to create a series of shapes that have a gradient applied to only part of the shape (many of these shapes wrap around, so I can’t just put the gradient on part of it). To do this, I’ve cut the shapes, and put a gradient on the part that I want. This is fine, until some of the shapes need to have the opacity set to less than 100%. For whatever reason, a line shows up at the edge between the gradient part and the non-gradient part.

I’ve checked to see if there’s any overlap between the parts (since they have opacity set to 50% here, I thought maybe they were slightly overlapping,) but dividing the shapes further does nothing, so it seems there’s no overlap. Is this a glitch in Illustrator, or am I missing something?

The line is still there when I save the image as a jpg, png, etc.

I tried cutting the shapes by math a path on top of the shape and using "divide", and I also tried by using the scissor tool. Same edge either way.

Picture is below, the edge line is very faint but visible.

Answer 1

It's possible to add a linear gradient along a stroke in Illustrator and thus avoid such problems altogether.

Basically, make a stroke, suitably thick. Select the path in the Appearanace panel, and open the Gradients panel, and add it to the stroke - select Linear, and click the icon for "Apply Gradient Along Stroke".

Answer 2

It's a really irritating one. The 'fix' is to group the objects first and then apply the transparency to the group (rather than have two independent transparent objects).

Likely cause: The cause appears to be antialiasing. Illustrator antialiases each of the objects separately creating a thin line where the semi-transparent antialiased edge pixels overlap each other. This isn't an issue in (most) opaque objects because the edge pixels are likely the same colour as the object itself.