Randomizing an attribute from another one

[Lucas Layeul]

Hello there,

I'm looking for some way to quickly isolate curve primitives.

For that purpose I noticed the deleteCurves node which deletes curves based on an input attribute.

I'd like, within gaffer to generate a temporary attribute based on the id of the curves so that the user could select a percentage of the curves to display.

In Houdini, this is pretty straightforward by randomizing a new attribute based on the id attribute and then using a threshold to split the curves between the ones to keep and the ones to get rid of. But I'm still quite new to Gaffer and can't yet understand how I could replicate a similar behavior (possibly using the Random/RandomChoice node and querying the existing attribute).

I found some information about how to have that kind of behavior in shaders, but not directly in the geometry's primvars/gaffer attributes.

Do you have any ideas about how this could be achieved ?

Thanks in advance and have a good day.

Lucas.

[John Haddon]

Hi Lucas,

You can do this fairly easily in Gaffer by using the OSLObject node to create a new primitive variable to control what is deleted. I've attached an example which is hopefully fairly self-explanatory - on the OSLCode node you'll find a `percentage` setting that controls how many of the curves to keep.

Cheers...

John

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[Lucas Layeul]

Hey John,

Thanks a lot for your quick answer, that's actually what I found out while digging more into it, glad to see I got it the right way!

Also very nice to see the getattribute() function, I couldn't find something like that in the documentation and used a InInt instead pointing to the shading:index, though I didn't manage to get my custom "hairid" primvar this way, this really helps!

Is there anywhere I can find more information about those context variables like the shading one ? In the documentation about Context Vars I could find information about the scene context to get the current frame or path for example, as well as some but nothing related to this "shading:" one.

Anyway, thanks again for the help, I really appreciate it. Have a good one.

Lucas.

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