Opacity Culling In Renderman

[grasshopper]

I'm rendering many particles that are semi-transparent. The following option is listed in Pixar's documentation to enable opacity culling:

RtColor thres = {0.995, 0.995, 0.995};
RtOption("limits", "othreshold", (RtPointer)thres, RI_NULL);

Is there a way I can insert this option into the RenderMan ROP so it gets passed to the RIB? I tried adding these options to the Pre-Frame incude but that didn't seem to work. Will I have to render out ribs so that I can add this in directly?

Thanks,

john.

[thekenny]

did you try to add the lines on the camera's preInclude line?

that or the object with the particels..... depends on what it is doing I guess.

-k

[edward]

That looks like it's adding the option via the Pixar's C SDK when outputting RIB. A google reveals this RIB syntax (via 3Delight's manual):

Option "limits" "color othreshold" [0.995 0.995 0.995]

BTW, as with any of Houdini's output, just turn on Generate Script File in the ROP to see what Houdini is sending to the renderer.

[grasshopper]

Thanks for the help guys. I got it working. This is the link to the manual that

Ed is talking about which includes a nice example demonstrating why opacity

culling is a good thing.

3Delight User's Manual 4.1 Options

So, yes, I mixed up my C and RIB syntax!!! Should have been obvious if I'd

looked more closely. Ed's syntax is what I needed. Turns out the option

needs to be added to the pre-world include in the RenderMan ROP rather

than the pre-frame include as I'd been trying as this is an option that applies

to the entire scene.

In short, if you are rendering lots of transparent layers in RenderMan it

is worth adding the following to the pre-world include parameter of your RenderMan

ROP to see if you can get big speed-ups without a drop off in quality:

Option "limits" "color othreshold" [0.94 0.94 0.94]

john.