For a particle you’ve created with the Particle Tool, a runtime expression typically executes in each frame after the frame that appears upon rewinding, before or after dynamics calculation.
For an emitted particle, a runtime expression typically executes in each frame after the first one where the particle was emitted. More specifically, a runtime expression executes once for each particle whose age is greater than 0, each time Maya evaluates dynamics.
Maya evaluates dynamics whenever the Time Slider time changes and the time is greater than or equal to the particle object’s Start Frame value. To set the particle object’s Start Frame, select the particle object and set the Start Frame in the Attribute Editor. Time changes when you rewind, play, or otherwise change the current frame displayed.
A runtime expression executes once per oversample level per frame as you play or otherwise change the animation time. For example, if the oversample level is 4, Maya executes a particle shape expression four times per frame for each particle in the object.
From the Dynamics menu set, use to set the Over Samples level. Maya’s default setting is 1.
In addition to executing when animation time changes, a runtime expression executes when the value of an attribute it reads changes, and when either of these actions occurs for an attribute the expression writes to:
In this context, the predefined variables time and frame are also considered attributes the expression reads.
For an nParticle shape node, you can write only one runtime expression for all its attributes. You don’t need to select an attribute from the Attributes list. You can create only one runtime expression per nParticle shape.