disabling GL_Blend at the 2dfilter drawing routine makes the trick here. there is not a clear function invoked before the 2dfilter drawing routine. Therefore I found better to disable alpha blending while we are setting the other OpenGl attributes/matrixes.
We are not re-enabling GL_BLEND after we disable it. We could and it wouldn't hurt but I can't see why to. open to suggestions here of course.