dotted lines in the quads

The thing is I drawed some quads and texture them, I enabled the blending and anti-aliase, but there were
always some dotted lines( with the background color) acrossing the non-adjacent two points in the quad, there is only
one dotted line each quads, when I turned off the anti-aliase, it went away.

Any ideas ?

Thanks,

[Glenn Upton]

Check backface culling and your windings.

[MikeC]

I've seen this as well; it's a side-effect of enabling GL_POLYGON_SMOOTH. The quad is being drawn as two triangles, and all the edges of the triangles (including the shared edge through the middle of your quad) get blended with the background colour.