A packed source set is a packed primitive containing volumes that are to be sourced into a pyro simulation, along with instructions about how to apply the source to a simulation. You can feed source volumes to the Pyro Source Pack SOP to package them into a packed source set. At a low level, the core attributes of a packed source set are its name, frame length, and source rules that specify how to merge the contained volumes with simulation fields. These properties are stored in name, frame_length, and source_rules primitive attributes, respectively.

A spawn point is conceptually a recipe for an instance point that enables more intuitive control over the timing. In particular, spawn points store some basic animation data, such as when the source is to engage, when it should cease sourcing, how quickly it should play back, and how frequently it should repeat. These properties are stored in point attributes, which are listed below.

You can use the Pyro Spawn Sources node to generate spawn points.

An instance point is a point that identifies which packed source set to apply, which frame of the source to use, as well as where and how to merge the volumes with simulation fields. In contrast to spawn points, an instance point only exists when it is meant to be actively sourcing; additionally, each instance point stores the sourcing rules for it. Point attributes that affect sourcing behavior are given below, along with where they come from if the instance point is generated from a spawn point.

To drive a simulation using packed sets, you must wire the instance points into the Pyro Solver. You can use the Pyro Source Instance node to birth instance points from spawn points.

Although a packed set comes with its source rules, the actual rules used for sourcing will be read from instance points. This allows different instances for the packed set to vary how they apply the same source volumes. Imagine a simulation where you want to create hotter and cooler smoke from the same source, to influence how fast it rises; to achieve this, you would set different scale multipliers for rules that affect the temperature field.

To introduce such variations, you can specify overrides on both Pyro Spawn Sources and Pyro Source Instance SOPs. In either case, you can modify rules by identifying their target field; then, you can select the numerical rule property that you want to tweak, whose base value will be multiplied by the adjustment. When done on Pyro Spawn Sources, you can set the adjustment value for the spawn points directly. Since input to Pyro Source Instance consists of many unconnected spawn points, it doesn’t allow for direct adjustment value control; instead, you can register a point attribute to be used as the adjustment value on a per-instance basis. This also enables you quickly randomize rule adjustments by simply randomizing attribute values (using Attribute Adjust Float, for example).

When merging packed source sets into a library, it is very important that they are all available on the same frame. This can be ensured by setting the same value for the Reference Frame parameter on all Pyro Source Pack nodes that were used to create the packed source sets. Leaving this parameter at its default value of 1 is sufficient; this value indicates that the source sets will be timeshifted to start at frame 1. The timeshift does not affect the timing of these sources when they are applied to a pyro simulation: spawn points and instance points can be used to precisely time the activation of sources.

The library’s reference frame must also be matched by the Reference Frame parameters on the Pyro Source Instance and Pyro Solver nodes. As mentioned above, all these parameters default to the same value of 1, so everything should work if you simply ignore their existence.

However, this default value is not ideal if your timeline does not start at frame 1, in which case the packed sets will be timeshifted to a frame that’s hard to reach in the timeline. It’s recommended to save a new default value for these parameters if that is your situation. For instance, if your timeline starts on frame 1000, then it’s best to have Reference Frame parameters default to 1000. This will ensure that newly-created packed source sets will be visible on the first frame of the playbar.