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These nodes correspond to the native “Constant” nodes.
However, they have optional evaluation modes controlling how their values are retrieved if they are animated via an envelope:
You can use the output of these nodes to, for example, change the position of a texture and have that change in position be motion blurred.
Interpolated is the preferred method to use, Per Sample should only be used if the result of Interpolated isn't accurate enough.
As of Version 2019.01.02, the nodes also have a Time input and a corresponding Time Range control.
If the Time input is connected, the Time Range control will be available. In this mode, the scalar connected to the Time input will be used directly as a current time when evaluating an enveloped Value of the node.
The Time Range control determines which range of the envelope is equivalent to a range of 0..1 of the Time input scalar. I.e. if the Time Range is 4.0 (s), in a 30fps scene, then a range of 0..1 on the Time input will be equivalent to 0..120 frames on the envelope.
This allows you to use any enveloped value as a gradient, by connecting the Time input to something that would normally drive a gradient.