when I render an image larger than my screen, using OPENGL, I always seem to get a line half way through the image where there are some objects (circles for instance) that are somewhat transparent. I don't have this issue when I use JAVA2D or P2D and the image is larger than my screen. But I do have this issue when I use a Tiler (for instance  http://processing.org/discourse/yabb2/YaBB.pl?num=1248245155/0#11). And the line still appears half way of the image, not at the edges of the tile for instance. And of course, nowhere in my code do I draw a line, so it seems to be a side effect of using OPENGL and a large image?

Any idea what causes this, and more importantly, how to solve this?

I have placed an enlarged image of a fragment where this line appears on:  http://janwillemtulp.com/opengllineissue.png