alpha issue with Arabic text

Hi,

I use TLF to render Arabic text and this looks fine. But when I set the alpha (using textAlpha) then I can see where Arabic characters overlap at the point where they join. At the joins the characters are rendered on top of each other and this results in a join that is less transparent than the other parts of the character. (I've attached a screen grab of an example of this situation.)

I've played around with the ligature level and kerning but this doesn't solve the overlapping parts of the joins. Is thre a way to fix this?

Thanks Marco.