Please see the attached screenshot, then this should make more sense. It is a simple setup of a pointinstancer where the primvar "color" of one instance gets overwritten. I do not understand why it works and was hoping someone can shed a light.

Without the instance, the mesh prim soccerball_IDcolor has a primvar "color" and is assigned a (redshift) shader, which reads this primvar to drive the material color. This primvar has to be on the same mesh that holds the shader (I think).

Now, when instancing, afaik only the top level primitive of an instance can hold modified parameters - I would expect this to be either Root here or soccerball - especially that the material variation node only points to /addpointinstancer1 (instad of /addpointinstancer/soccerball/soccerballAva/render/soccerball_IDwhite or something like that).

So it seems to me this new primvar, created by material variation is sitting on an Xform primitive somewhere "above" the actual mesh primitive. And yet - the color of the shader changes - how?

Thanks for any help - still learning more about USD every day.

Constant primvars can be inherited from a parent, so it's possible that the value attached to the instance root is getting picked up by the leaf mesh prims. I'm not sure if the details pane will list inherited primvars though.

Indeed, @antc is correct. If you scroll the SG Details pane down, inherited constant primvars are displayed at the bottom (see attached screenshot); they have an expandable + icon, which shows the parent prim where the primvar lives.

Inheritance - I see thanks.
If I change materialvariation2 to point to a primitive further up (like /soccerball) I can see the parameter being passed down and when rendering (before instancing) I see the color - great!

However after instancing this does not work anymore - the materialvariation acting on instances only seems to have an effect if there is a primvar of the same name already on the mesh prims (not inherited). Further the default value is reset: if materialvariation1 (after the instancer) is set to only affect a specific instance (with for example) all other soccer balls have black as a color (the default in the shader), NOT the color specified in materialvariation2.

Marco Dörner
Inheritance - I see thanks.
If I change materialvariation2 to point to a primitive further up (like /soccerball) I can see the parameter being passed down and when rendering (before instancing) I see the color - great!

However after instancing this does not work anymore - the materialvariation acting on instances only seems to have an effect if there is a primvar of the same name already on the mesh prims (not inherited). Further the default value is reset: if materialvariation1 (after the instancer) is set to only affect a specific instance (with for example) all other soccer balls have black as a color (the default in the shader), NOT the color specified in materialvariation2.

It should work, if I'm understanding what you want correctly. Here is an attached example.

goldleaf
It should work, if I'm understanding what you want correctly. Here is an attached example.

Was wanting to check this out, but this file won't load correctly for me in 18.5.596. Throws a bunch of errors about missing parameters.

Tim Crowson

goldleaf
It should work, if I'm understanding what you want correctly. Here is an attached example.

Was wanting to check this out, but this file won't load correctly for me in 18.5.596. Throws a bunch of errors about missing parameters.

looks like it was saved from 19. doh!

Sorry about that, fixed the link.

Thanks. I checked out your scene, but I think it's a little bit different as you use duplicates, instead of instancing.
I added a scene to demonstrate what I am going for. On the left there is the Karma approach - however I wasn't able to get that to work at all.
On the right side there is redshift. Redshift needs a "user data" node in the shader to pick up the primvars on primitives (from the documentation it sounds like Karma does that without an extra node). This userdata is set to look for the primvar "color". The pointinstancer creates instances. Then with materialvariation4 a primvar "color" is written to all instances. When I render here I see all soccer balls turn yellow - success! Then I want to overwrite a specific one which I can do with (materialvariation5) - also that works, success!
However when I now extract this particular instance (5) all the user data set before is lost. The color primvar doesn't exist at all.

So my original question is - is there a way to extract an instance with all the data on it?

EDIT: figured out how to show the problem with Karma (left side): when rendering materialvariation7 the 5th instance is blue, but after it is extracted with extractInstances it becomes grey and the primvar "basecolor" is nowhere on the primitives.

Marco Dörner
Further the default value is reset: if materialvariation1 (after the instancer) is set to only affect a specific instance (with for example) all other soccer balls have black as a color (the default in the shader), NOT the color specified in materialvariation2.

I have found this as well - specifying a single point instance in the Material Variation LOP will mean the override is obeyed by that particular instance, but all other instances get a value of zero (in my case I'm driving the Diffuse Gain or Albedo Multiplier value).