I'm trying to write a particular shader “from the ground up” as a learning exercise. I'm an old-school graphics programmer but just learning VEX and VOPs. Using Houdini 16.5.

My question is this: the BSDF is an opaque type. There's a few VOPs/VEX functions to create one, “diffuse” and a few others. I'm looking for the “noop” BSDF, that just always returns the incident ray – i.e. fully transparent, no refraction or opacity or anything. I'd like it to be really fast.

The reason I'm asking is I want to write a shader which is emissive-only. I know how to export Ce, but I'm stuck on creating the “noop” F. Right now I'm using pbrglass with ior set to 1 which works, but when I look at the VEX code I can see it's doing a lot more work than I need (i.e. it's calling get_bsdf(“phong”)).

Any advice appreciated!

the no op bsdf is
check out shaderlayer.h for an example of a noop bsdf

Edit:
To elaborate, a bsdf in mantra does not determine opacity or emissivity. To do that you need to make a complete surface shader. The Of export sets opacity, an the Ce export sets emission.

jsmack
the no op bsdf is
bsdf()

check out shaderlayer.h for an example of a noop bsdf

True – I found that. But that bsdf reflects all incoming energy randomly in a hemisphere around the normal, as far as I can tell; i.e. it's perfectly diffuse with zero transmission. I'm looking for a (fast) bsdf that takes all incoming energy and continues it (“refracts” it) through the object in the same direction, i.e. the ideal transparent object. I understand I can set Of to 0 but that's not what the glass shader does, for instance. (Again, as far as I can tell – I could very well be wrong.) I would expect (hope?) such a bsdf to be even faster than the bsdf() one you mention above, since it doesn't need to randomize anything.

garyo
… I understand I can set Of to 0 but that's not what the glass shader does…

why would you want to do what glass shader does? Setting Of to 0 is much more efficient than using refractive bsdf
so empty bsdf() with Of to 0 and whatever emission you want should be very efficient as no light or geometry sampling will be invoked from such shader

tamte
Setting Of to 0 is much more efficient than using refractive bsdf
so empty bsdf() with Of to 0 and whatever emission you want should be very efficient as no light or geometry sampling will be invoked from such shader

Ah, I see! Thanks very much for the help. If Of>0, then the bsdf will get sampled, but not if it's exactly zero. That's helpful.

Also, I haven't tested it thoroughly (but have used it…), doesn't multiplying a bsdf by zero set its probability of sampling to zero, hence no sampling occurs?

garyo
Ah, I see! Thanks very much for the help. If Of>0, then the bsdf will get sampled, but not if it's exactly zero. That's helpful.

empty bsdf() should already not cause any sampling
Of to 0 is just to get it fully transparent without using any raytracing if all you want to do is just send the ray through