Copyright Blue Sky Studios.
This effect is a modification of the fluid splash rig used in previous films. A plugin is added to "interrupt and freeze" the ballistic motion in order to direct the individual points to form an expanding semisphere before the bursting event. At burst time, each horizontal row collapses in cascading fashion, and each point within resumes its ballistic motion. The timing of the burst event, the "build up" time leading to the event, and the force & direction of the burst are all available to the animator in the channel editor via slideable channels.
Creating the bursting bubble was only the first part of the equation. It then became necessary to devise a scheme to populate the surface with burst instances. To that end I created a script which procedurally builds a library of bursting bubbles of various scales & forces. The surface patch was then populated randomly from items in the library and the result could be edited by the user if desired. To wrap things up, I wrote a script to automatically create an animating materials signal for each event, essentially an extruded cocoon around the pre-bursted semisphere.
This clip first displays some of the many renders that were created during testing of the burst, then continues with working progressions of 2 of the tarpit shots- standins, bare geometry, playblast, render & final. Next is a small clip demonstrating the procedural random placement, scale & forces applied to the surface patch, followed by working progressions of a few of the lava shots.