Render an object so that it is pure black with zero transparency

Question

I have rendered images that I'm going to post-process. Some objects in the scene are supposed to act as markers that the post-processing script will try to locate based on the pixel value being pure black (ie argb: #ff000000).

I created a principal BSDF material with zero specular etc, and applied it to a small cube object, but when it's rendered, you can see the output pixels around the edges are of varying shades / opacities:

Is there any way to make a material that'll render pure black with zero opacity?

Thanks

Answer 1

Use no shader at all. Empty socket on surface.

You could also use Cryptomatte to generate a pixel perfect mask if the anti-aliasing around the edges is troublesome.

You can clearly see the staircase here representing every pixel where the object could be sampled.

Answer 2

I would recommend trying a Diffuse shader instead of a Principled BSDF. A diffuse shader essentially adds 'pigment' to a material, without any kind of gloss, specularity, opacity, etc.

However judging by your screenshot I believe the grey variations you are getting are a result of anti-aliasing related to the resolution of your render (and the relative size of the black object), not the shader. The grey variations appear to be a blend of the pure black from your shader, and the grey background.

I would maybe try increasing the resolution of your render (try 200/400%?) and maybe turning off denoising to see if that improves matters.