what is your alpha threshold?

If it's 50 (default) your code won't work..

set it to 0 or 1:
Quote:
d3d_alpharef
Determines the transparency threshold for overlay textures. Pixels below this threshold are completely transparent. By adjusting this variable, seams around the transparent parts of overlay images can be minimized.

Range:
0..255 (default: 127 = 50%)


Hope this helps..