From BlenderWiki

Explode Modifier

Description

The Explode modifier is used to alter the mesh geometry (by moving/rotating its faces) in a way that (roughly) tracks underlying emitted particles that makes it look as if the mesh is being exploded (broken apart and pushed outward).

For the Explode modifier to have a visible effect on the underlying mesh it has to be applied to a mesh which has a particle system on it, in other words it has to be a mesh which outputs particles. This is because the particle system on the mesh is what controls how a mesh will be exploded, and therefore without the particle system the mesh wont appear to alter. Also both the number of emitted particles and number of faces determine how granular the Explode Modifier will be. With default settings the more faces and particles the more detailed the mesh exploding will be, because there are more faces and particles to affect detachment/movement of faces.

Here is a link to an Ogg Theora video showing a cube with a particle system and Explode modifier applied:

Here is a link to the original Blender file which has an Exploding cube setup, just free the particle cache by pressing the Free Bake button in the Bake panel and then press the Animate button to see the animation:

Options

Explode modifier panel with ParticleSystem modifier above it.

Stacking Order Importance

This modifier is highly affected by its position within the modifier stacking order. If it is applied before a ParticleSystem modifier it will not be affected by particles and therefore appear to do nothing. The ParticleSystem modifier must appear before the Explode modifier, because the first one has the information needed to drive the last one.

Explode modifier panel with “Protect this vertex group” field highlighted in yellow.

Protect this vertex group

If a mesh that has an Explode modifier on it also has vertex groups assigned to it, then this field will allow the selection of one of those vertex groups. This will force the Explode modifier to take into account the weight values assigned to the vertices of this group: it then must protect the faces from explosion, depending on the weights assigned their vertices (full protection for a weight of 1.0, no protection for a weight of 0.0).

It is also possible to have in between face protection: just assign them intermediate weights, between 0.0 and 1.0.

Weight painted plane before Explode modifier is run.

To the right is a plane which has been weight painted in two different weight paints, red which represents a weight of 1.0, and blue which represents a weight of 0.0. The reason that weight painting is used is because whenever you weight paint something it automatically makes a new vertex group (if the plane doesn’t already have one), which can then be used with the “Protect this vertex group” field.

Weight painted plane after Explode modifier is run. Showing the red area unmodified and the blue area exploded.

To the right is the same plane while the Explode modifier is in the process of exploding the plane. As can be seen the weight painting means that the red (weight of 1.0) faces are left unmodified, and the blue (weight of 0.0) faces are modified.

Remember that by default weight painting makes a vertex group, that is what needs to be selected in the “Protect this vertex group” field for the weight painting to be taken notice of.

Here is a link to an Ogg Theora video showing a weight painted plane, painted with minimum and maximum weights to be animated with the Explode modifier:

And here is a link to the original Blender file which the above movie was made from, just free the particle cache by pressing the Free Bake button in the Bake panel and then press the Animate button to see the animation:

Blended weight painted plane before the Explode modifier is run.

To the right is a plane which has got a blended weight paint on it, with red being a weight of 1.0 through to other lower weighted colors all the way to blue which is a weight of 0.0.

Blended weight painted plane after the Explode modifier is run.

And now is the same blended weight painted plane after it has had the Explode modifier running on it.

As can be seen the top part of the blended plane is highly affected by the Explode modifier, whereas, lower down the plane, where the weight painting gets higher in weight, is affected less and less by the Explode modifier.

Here is a link to an Ogg Theora movie showing a blended weight painted plane, painted with multiple weights animated with the Explode modifier:

And here is a link to the original Blender file which the above movie was made from, just free the particle cache by pressing the Free Bake button in the Bake panel and then press the Animate button to see the animation:

The “Clean vertex group edges” numeric slider field highlighted in yellow.

Clean vertex group edges

This numeric slider field appears to erode the exploded edges around a vertex groups edges (making edges less jaggied). The slider can have a value between 0.0 (no eroding of edges) and 1.0 maximum erosion of edges.

Below are some screenshots of a mesh that has had an Explode modifier applied to it with various values for “Clean vertex group edges”:

“Clean vertex group edges” to 0.0.	“Clean vertex group edges” to 0.2.	“Clean vertex group edges” to 0.4.
“Clean vertex group edges” to 0.6.	“Clean vertex group edges” to 0.8.

Refresh (Recalculate faces assigned to particles)

When certain changes are made to the underlying Explode modifier mesh (such as changing the vertex group weightings of faces), the faces displayed and the particles that influence certain faces can get out of sync. If this happens pressing the Refresh button will tell the Explode modifier to update all its calculations to take into account new settings.

As an example, if you have a mesh with faces that get modified by the end of an Explode modifier run, and you then alter those faces vertex group weights so they have a value of 1.0 (meaning they can no more get altered in future – when the “Protect this vertex group” option is active), the Explode modifier will still show the old altered state of the faces until you click the Refresh button.

Split Edges

When this button is selected, faces which are exploded are split making them smaller and more fragmented, giving more faces for the Explode modifier to work with and usually giving better explode results.

Below are two screenshots of an Explode modifier in progress, one with no Split Edges option active and one with Split Edges activated:

Uvsphere without Split Edges activated on the Explode modifier.

Uvsphere with Split Edges activated on the Explode modifier.

Unborn

This button controls whether mesh faces will be visible or not before a particle for it has been created/born.

It means that, if you have for example an Explode modifier on a mesh and certain faces on that mesh do not currently have any created/born particles on them, then those faces will not be visible unless the Unborn button is selected, making faces appear to pop into visibility when particles are created/born.

Alive

This button controls whether mesh faces will be visible or not while a particle for it is alive/active.

It means that, if for example you have an Explode modifier on a mesh and certain faces on that mesh have particles that are alive/active on them, then those faces will be visible only if the Alive button is selected.

Dead

This button controls whether mesh faces will be visible or not when the particle associated with the face is dead.

It means that, if for example you have an Explode modifier on a mesh and certain faces on that mesh have particles that are dead on them, then those faces will be visible only if the Dead button is selected.

Examples

Exploding Cube

Weight painted object with SplitEdge option not selected

Note that in the image to the right, the faces are not jagged.

Start of simulation.

Middle of simulation.

End of simulation.

Weight painted object with SplitEdge option selected

Note that in the image to the right, the edges are jagged.

Start of simulation.

Middle of simulation.

End of simulation.

Download .blend file

Another explosion

Using the Explode modifier with a particle system to break meshes apart: