I'm applying a simple white/black opacity map onto the shader of some rectangular shaped particles, using ice... but for some reason roughly half of the particles are not opaque like they should be, while the other half renders fine... I'm assuming this has to do with which side of the particle is facing the camera, but if so I can't figure out how to render correctly.
The only way I can get it to work is if I use a constant shader, which I would really prefer not to use as I need some specular, so any help on how to use opacity maps with particles on standard blinn/phong ect.. shaders would be much appreciated.

Sorry for the probably painfully obvious question,
but where does your opacity map come into play?
In the ICE Tree or in the Render Tree?
And are you trying to use the map per particle or per cloud?

lol np, not so obvious here.
I've applied the opacity in the render tree onto the particle cloud.
I've been keeping it as simple as possible for now, a default blinn shader, with an opacity map plugged into its transparency slot.

its working here. can you post a scene?

Sure here are a few quick renders:

Phong Shader.. maybe not the best frame to capture, but some render fine, and some as you see here show the full rectangle.


Constant Shader (Which works right but I don't want constant)

This 'rectangle' is probably specular reflection. You must mask out specular as well. Or use arch shader with cut_off - It could be faster than phong/blinn.

I tried disabling specular on the shader awhile back... and it still would not render correctly, so assumed it must be something else. But I just tried plugging my opacity map into the specular channel as well, and it renders fine now!
Thanks for that.

The arch shader cut_off feature as origin mentions, or using the Sprite shader is a much simpler and predictable approach. You should lean towards one of those in the future.