In the post before we created leaf bitmap, either with embedded transparency or with alpha channel. Now I want to point out to some problems that might occur.
Halos or visible fringes
During rendering the leaf bitmaps are filtered and due to oversampling (antialiasing) the background color is mixed with the leaf color. This can lead to visible halos around the leaf edge. In our case with black background this would lead into a dark halo.
Most commercial render applications offer a special option for alpha channels that solves exactly this problem. This option is often called premultiplied or something similar….