MaterialX OpenPBR Surface node definition natural logarithm (ln) function could be undefined

[nvpamand]

We noticed an issue with the MaterialX OpenPBR Surface node definition:

OpenPBR/reference/open_pbr_surface.mtlx

Line 221 in b212785

<ln name="transmission_color_ln" type="vector3">

If one of the components of transmission_color_vector is <= 0, then the result of the natural logarithm (ln) function is mathematically undefined:

   <ln name="transmission_color_ln" type="vector3">
      <input name="in" type="vector3" nodename="transmission_color_vector" />
    </ln>

You might want to prevent such a case by using:

  <max name="safe_input" type="vector3">
     <input name="in1" type="vector3" nodename="transmission_color_vector" />
     <input name="in2" type="vector3" value="0.0001, 0.0001, 0.0001" />
  </max>
   <ln name="transmission_color_ln" type="vector3">
      <input name="in" type="vector3" nodename="safe_input" />
    </ln>

A condition could do, too. It might also be okay as is if you want to rely on the renderer to handle NaN/Inf on a per sample basis.