I'd like to have aliasing against background in RTT

Hello there
Sorry, i posted this also in the public forum because i hadn’t the full access.

Is there a way to have no anti-aliasing in the render to texture ?

I want just my shells border to be aliased and not the interior of course.

I tried to save with straight alpha .tga but the pixel border color is wrong (too bright) :

I want this to combine the baked maps of different objects and make a dilatation with Xnormal filter.

With premultiplied alpha, i got the black outline. So the dilatation is all black.If i use the divide mode in photoshop, i get the same result as with straight alpha (logical!) the outline is too bright, so the dilatation is also too bright.
I tried photoshop defringe option, which is great, but it gets funky if my uv shells are small (like 3 pixels)

Thank you !