Hello! I'm looking into something that I'm assuming most game studios have (outside of USD) already in their pipelines and trying to see if this would map well or less well into USD:
To assemble a character, we have usually three file types:
A character file with the base skeleton with the relevant bones/joints required by the default animation system (root, hips, legs, arms, head)
N number of attachment files, these usually contain Geometry and the part of the base skeleton that they skin to, as well as sometimes additional bones/joints, that can be driven by secondary animations.
Some sort of manifest file that tells the engine to load the base skeleton with a number of attachments.
The process in the engine (at runtime) then takes the manifest file and creates an assembled character from the character file with the different attachment files merged together into one.
Ideally I'd like to achieve something similar in USD in order to give proper preview to Animators in the DCC of their choice. We have done this in the past in different DCCs with either
loading the different (FBX at the time) files and constraining them together (bone by bone) (maintaining the individual pieces, but slow to evaluate in the DCC) or
having a pre-processor merge all (FBX again at the time) files together and then load the FBX data into the DCC in a lossy process (but faster to work with in DCC)
With USD's referencing and opinion workflows, I was hoping that there would be some clever way to achieve this result, but so far, I'm struggling to put something coherent together - hence my message here: Has anyone already looked at such a workflow and what was your approach?

Charles Flèche
  19 days ago
As far I know you can't list list-edit an attributes, so generating the full set of bones can't be done with USD composition itself.
Maybe that could be done with additionnal logic ?
Variants per bone set contains contains a prim that has an attribute: a list of bones
When a variant is activated it also add the relationship a custom rel attribute to your Skeleton
on recomposition the attribute that contains the rel to Prim that contain bones is notified to have changed
a script rebuilds the full skeleton
(edited)
:+1:
1

Charles Flèche
  19 days ago
I am not super clear...

Charles Flèche
  19 days ago
Let me see if I can make a quick example

Charles Flèche
  19 days ago
#usda 1.0
def SkelRoot "SkelRoot" (
  variants = {
    string tool = "hammer"
    string shoe = "trainer"
  }
  prepend variantSets = ["tool", "shoe"]
)
{
  def Skeleton "Skeleton"
  {
    uniform token[] joints = []
    custom token[] main_joints = ["root", "root/hand", "root/foot"]
    custom rel addons
  }
  
  variantSet "tool" = {
    "hammer" {
      over "Skeleton" {
        append rel addons = </SkelRoot/Skeleton/Hammer>
        
        def "Hammer" {
          custom token[] addon_joints = ["root/hand/hammerbase"]
        }
      }
    }
    "compass" {
      over "Skeleton" {
        append custom rel addons = </SkelRoot/Skeleton/Compass>
        
        def "Compass" {
          custom token[] addon_joints = ["root/hand/hinge", "root/hand/hinge/left", "root/hand/hinge/right"]
        }
      }
    }
  }
  
  variantSet "shoe" = {
    "trainer" {
      over "Skeleton" {
        append rel addons = </SkelRoot/Skeleton/Trainer>
        
        def "Trainer" {
          custom token[] addon_joints = ["root/foot/trainer"]
        }
      }
    }
    "robotBoot" {
      over "Skeleton" {
        append custom rel addons = </SkelRoot/Skeleton/RobotBoot>
        
        def "RobotBoot" {
          custom token[] addon_joints = ["root/foot/boot", "root/foot/boot/hinge", "root/foot/boot/hinge/tip"]
        }
      }
    }
  }
}

Charles Flèche
  19 days ago
image.png
 
image.png

Charles Flèche
  19 days ago
Don't know if it is clearer, but at list if you open the file above in usdview you should be able to switch variants and see /SkelRoot/Skeleton.addons to be update with the list of addons prim. Those prim contains a list of addon_joints , so with a notification handler you should be able to recompose /SkelRoot/Skeleton.joins  on top of the base skeleton bones defined in /SkelRoot/Skeleton.main_joinrs

Charles Flèche
  19 days ago
IIRC there was a discussion on the previous Google List on allowing list-editing uniform (non time sampled) Attributes. That definitely could be useful for this kind of workflow, as Skeleton.joints is uniform, so we could leverage's USD list-editing operations directly.

Koen Vroeijenstijn
  18 days ago
When I tried to represent attachments elegantly, I ran into the issue that there is no parenting to bones. As all the joints are represented as a single prim, you cannot parent the attachment to a specific joint ( connect the neck joint of the body to the neck joint of the body for example to swap out heads). For static models, I just apply the transform of the "attach to" joint  to the attachment, if you then convert to native geometry, you can easily create a parent contraint, but it's not ideal. I played around with merging the skeletons, but then updating all the skin weights for the meshes gets messy. Maybe we can have a very simple "parent joint constraint" or perhaps a more expensive representation of a usdskel which does unroll into the regular scene graph. Hope I remember right, it was a while ago I looked at this. Curious if Charles' suggestion works for you, please keep us posted. (edited) 
:+1:
1

Alan Kent
  15 days ago
A standard way to do parenting to bones, or a "UsdSkel as prims" would be nice. E.g. how to put an arbitrary hat on a head?