Most 3D applications write images where the rgb-image is premultiplied with the alpha channel. The result canīt be used in Photoshop, since photoshop wants a clean (not-premultiplied) edge and cuts it with the alpha on its own. The result is, that if the 3D element is rendered against black background and you put a white background behind the object in photoshop, you end up in dark edges around the object. Compositing tools like AfterEffects handle this with ease (they automatically unpremultiply).
So .. in formula, what 3D delivers (using finalrender, actually no straight mode available):
RGB(premultiplied) = RGB(rendered) * Alpha
What i need:
RGB(rendered) = RGB(premultiplied) / Alpha
quite easy formula, but how can this be done in Photoshop? Or is there a filter available that calculates this?