Time parameters#

Time unit in the API#

The Omni Timeline API utilizes seconds as the standard unit of time. All methods within the API that accept or return time-related parameters do so using seconds as the base unit[1].

Relationship with USD time codes#

The core data format of Omniverse, USD, defines its own time representation called UsdTimeCode. UsdTimeCode is defined as

unitless, generic time measurement that serves as the ordinate for time-sampled data in USD files. A client of USD relies on the UsdStage (which in turn consults metadata authored in its root layer) to define the mapping of TimeCodes to units like seconds and frame.

The metadata of this mapping is the timeCodesPerSecond parameter of the USD stage, which is synchronized with the identically named parameter in Omni Timeline. To convert UsdTimeCode to seconds, divide the UsdTimeCode value by the timeCodesPerSecond value of the associated timeline.

Omni Timeline provides helper methods to facilitate conversions between time codes and seconds, based on the timeCodesPerSecond parameter:

import omni.timeline

# Obtain the timeline interface
timeline = omni.timeline.get_timeline_interface()

# Set the conversion factor
tcps = 30
timeline.set_time_codes_per_second(tcps)

# Note: wait a frame here for set_time_codes_per_second to take effect (omitted)

# Convert time between seconds and time codes
time_in_seconds = 2.5
time_in_timecodes = timeline.time_to_time_code(time_in_seconds)
print(f'Time in time codes: {time_in_timecodes}')  # Output: 75 (2.5 s * 30 tc/s)
time_in_seconds_again = timeline.time_code_to_time(time_in_timecodes)
print(f'Time in seconds: {time_in_seconds_again}')

Time bounds and current time#

In parallel with USD’s definition of start and end UsdTimeCode for a stage, Omni Timeline defines its start and end times in seconds. These bounds are synchronized with the main stage. Any changes to the start or end UsdTimeCode of the main USD stage are reflected in the main timeline’s start or end time, and vice versa.

As explained in the Time stepping documentation, the timeline is segmented into frames. The start time of a timeline defines the beginning of its first frame. Frames may end at or after the end time, but no frame is allowed to start after it. Note that end time does not need to fall on a frame border. When the timeline bounds define an integer number of frames, the end time is the end of the last frame.

The current time is defined as the beginning of the next frame. Note that during frame computation, Kit transitions from one point in time to another, making a frame a time interval. Thus, the current time is effectively the end of the ongoing frame (the point in time to which the frame is transitioning), and time is incremented before the invocation of callbacks registered with the timeline. This may differ from the current time concept in other software.

The following code example helps clarifying the interpretation of current time:

import omni.timeline

# Obtain the main timeline
timeline = omni.timeline.get_timeline_interface()

# Define a callback for time updates
def on_time_update(event):
	# Retrieve the end time of the current frame
	time = timeline.get_current_time()
	print(f'Current time from timeline: {time}')
	
	# Retrieve the end time of the current frame from the event payload
	time_from_event = event.payload['currentTime']
	print(f'Current time from event: {time}')
	
	# Calculate the start time of the current frame
	start_time_of_frame = time - event.payload['dt']
	print(f'Start time of the current frame: {start_time_of_frame}')

# Subscribe to the timeline event stream
timeline_sub = timeline.get_timeline_event_stream().create_subscription_to_pop_by_type(
	omni.timeline.TimelineEventType.CURRENT_TIME_TICKED, 
	on_time_update
)

# Display the current time, i.e. the start time of the next frame, before callbacks
print(f'Time before callbacks: {timeline.get_current_time()}')

# Advance to the next frame
timeline.forward_one_frame()
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