logical-pivot-position#

Code	VG.026
Validator	Manual
Compatibility	core usd
Tags	🔑

Summary#

Transforms and pivots should be positioned at intuitive points on the asset to ensure expected behavior during placement and rotation.

Description#

Asset origins and internal objects should be positioned to enable intuitive transformation behavior based on the object’s real-world function and placement. This requirement establishes specific guidelines for common object types:

Ground plane objects: Objects that sit on a surface should have their origin at the center of their base
Rotating objects: Objects designed to rotate around a specific point should have their origin at that rotation center
Hinged objects: Doors, lids, and similar objects should have origins at their hinge points
Attached objects: Objects which are intended to be attached to other objects should have their origin at (one of) their intended attachement points

Proper origin placement ensures predictable behavior when positioning, rotating, and scaling assets in scenes.

Why is it required?#

Enables intuitive manipulation of assets in 3D applications
Provides predictable transformation behavior for different object types
Supports realistic animation and simulation workflows
Facilitates automated placement and layout tools
Ensures consistent user experience across different assets

Examples#

#usda 1.0

# Valid: Table with pivot at center of base
def Xform "Table" ()
{
    float3 xformOp:translate = (0, 0, 0)
    float3 xformOp:pivot = (0, 0, 0)  # Pivot at base center - table sits correctly on ground
    uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:pivot", "!invert!xformOp:pivot"]
    
    def Cube "TableTop" {
        float3 xformOp:translate = (0, 0.4, 0)  # Table surface above pivot
        float3 xformOp:scale = (2, 0.1, 1)
        uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:scale"]
    }
    
    def Cube "TableLeg" {
        float3 xformOp:translate = (0, 0.2, 0)  # Leg extends from pivot upward
        float3 xformOp:scale = (0.1, 0.4, 0.1)
        uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:scale"]
    }
}

# Valid: Door with pivot at hinge point
def Xform "Door" ()
{
    float3 xformOp:translate = (0, 0, 0)
    float3 xformOp:rotateXYZ = (0, 0, 0)  # Door rotation around hinge
    float3 xformOp:pivot = (0, 0, 0)  # Pivot at hinge location
    uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:pivot", "xformOp:rotateXYZ", "!invert!xformOp:pivot"]
    
    def Cube "DoorPanel" {
        float3 xformOp:translate = (0.5, 1, 0)  # Door panel offset from hinge
        float3 xformOp:scale = (1, 2, 0.05)
        uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:scale"]
    }
}

# Valid: Ceiling fan with pivot at rotation center
def Xform "CeilingFan" ()
{
    float3 xformOp:translate = (0, 2.5, 0)  # Fan mounted on ceiling
    float3 xformOp:rotateXYZ = (0, 0, 0)  # Fan blade rotation
    float3 xformOp:pivot = (0, 0, 0)  # Pivot at motor center
    uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:pivot", "xformOp:rotateXYZ", "!invert!xformOp:pivot"]
    
    def Cube "FanBlade" {
        float3 xformOp:translate = (0.5, 0, 0)  # Blade extends from center
        float3 xformOp:scale = (1, 0.02, 0.1)
        uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:scale"]
    }
}

# Invalid: Chair with pivot at arbitrary location
def Xform "Chair_Invalid" ()
{
    float3 xformOp:translate = (0, 0, 0)
    float3 xformOp:pivot = (0, 1, 0)  # Pivot in mid-air above chair
    uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:pivot", "!invert!xformOp:pivot"]
    
    def Cube "ChairSeat" {
        float3 xformOp:translate = (0, -0.5, 0)  # Chair appears to float when placed
        float3 xformOp:scale = (1, 0.1, 1)
        uniform token[] xformOpOrder = ["xformOp:translate", "xformOp:scale"]
    }
}

How to comply#

For Ground Plane Objects:#

Position origin at the center of the object’s base (bottom surface)
Ensure the object sits correctly on a ground plane when pivot is at Y=0
Examples: chairs, tables, lamps, trees, buildings

For Rotating Objects:#

Position origin at the mechanical center of rotation
Align pivot with the axis around which the object naturally rotates
Examples: doors (at hinge), wheels (at axle), ceiling fans (at motor)

For General Objects:#

Consider the object’s primary function and typical manipulation
Place origin where users would intuitively expect the transformation center
Test by rotating the object to ensure natural behavior
Use the snapping function of a USD editor to position the asset