An xpMetaGroup is a special kind of group because it enables the user to specify the criteria for membership of that group.

With a conventional group, the particle is either a member of it or it is not.

You can use the xpChangeGroup modifier or an action to change from one group to another depending on criteria used in a Question object, but if you have a lot of groups with frequent changes, the scene can become complex, very quickly.

In addition, metagroups have some advantages over conventional groups.

These include:

• a particle can belong to more than one metagroup at a time (or to no metagroup at all)
• particles can be automatically removed from a metagroup if they no longer meet the criteria for membership
• metagroups behave like a particle modifier in the scene and can use falloffs as modifiers do
• each metagroup can specify multiple criteria for membership of the group without using Question objects.

You can use a metagroup almost anywhere you would use a conventional group, such as in the Groups list of modifiers and actions, the Question object, etc.

One thing you cannot do is specify that an emitter should emit particles already in a metagroup, since membership of the group depends on the particle meeting the required criteria.

xpMetaGroup_01.mp4

In both of these scenes, the particles are being emitted in a ring, with a gradient of black to white. The xpGravity Modifier is linked to the xpMetaGroup in its Groups Affected tab. In this first animation, the Criteria has the Parameter set to Color, with increasing color brightness values.

xpMetaGroup_02_v02.mp4

In this second animation, an additional Criteria is added (the Combine Mode is set to AND), with the Parameter setting of Radius, **between 3cm and 6cm

Object Properties

xpMetaGroup, Object tab menu settings.