Xform Queries

As mentioned in our transforms section, we can batch query transforms via the UsdGeom.XformCache().

It caches ancestor parent xforms, so that when we query leaf prims under the same parent hierarchy, the lookup retrieves the cached parent xforms. The cache is managed per time code, so if we use XformCache.SetTime(Usd.TimeCode(<someOtherFrame>)), it clears the cache and re-populates it on the next query for the new time code.

Checkout the official API docs for more info.

import math
from pxr import Gf,  Sdf, Usd, UsdGeom
stage = Usd.Stage.CreateInMemory()
root_prim_path = Sdf.Path("/root")
root_prim = stage.DefinePrim(root_prim_path, "Xform")
cone_prim_path = Sdf.Path("/root/cone")
cone_prim = stage.DefinePrim(cone_prim_path, "Cone")
# Set local transform of leaf prim
cone_xformable = UsdGeom.Xformable(cone_prim)
cone_translate_op = cone_xformable.AddTranslateOp(opSuffix="upAndDown")
for frame in range(1, 100):
    cone_translate_op.Set(Gf.Vec3h([5, math.sin(frame * 0.1) * 3, 0]), frame)
# A transform is combined with its parent prims' transforms
root_xformable = UsdGeom.Xformable(root_prim)
root_rotate_op = root_xformable.AddRotateZOp(opSuffix= "spinMeRound")
for frame in range(1, 100):
    root_rotate_op.Set(frame * 15, frame)
# For single queries we can use the xformable API
print(cone_xformable.ComputeLocalToWorldTransform(Usd.TimeCode(15)))
    
## Xform Cache
# For batched queries, we should always use the xform cache, to avoid recomputing parent xforms.
# Get: 'GetTime', 'ComputeRelativeTransform', 'GetLocalToWorldTransform', 'GetLocalTransformation', 'GetParentToWorldTransform'
# Set: 'SetTime'
# Clear: 'Clear'
xform_cache = UsdGeom.XformCache(Usd.TimeCode(1))
for prim in stage.Traverse():
    print("Worldspace Transform", xform_cache.GetLocalToWorldTransform(prim))
    print("Localspace Transform", xform_cache.GetLocalTransformation(prim))

Usd Survival Guide

Xform Queries