Bathing suits and magic bubbles: Behind the scenes of Sabrina Down Under

By David Hickey and Trent Shumway

On September 26, ABC aired Sabrina Down Under, the third telefilm staring Melissa Joan Hart as America's favorite teenage witch. Directed by Ken Koch on location in Australia, the film was the most technically ambitious in the series so far, featuring logistical hurdles such as underwater and aerial photography. The visual effects crews at Modern Videofilm and LightPoint Entertainment were asked to deliver shots ranging from a magically conjured typhoon to motion-captured 3D character animation.

One of the most difficult sequences in the movie was when Sabrina and her best friend Gwen (Tara Charendoff) were scuba diving on the Great Barrier Reef. Sabrina tries to talk to Gwen but can't because they are underwater. To solve the problem, Sabrina blows air bubbles out of her mouth, then uses magic to enlarge one of them until it is big enough for the girls to swim inside. With their heads poking inside the bubble, the girls can talk while watching the fish swim along the reef.

The shots were completed using several components. The production crew provided plates shot with the actresses swimming on the reef, along with matching bluescreen footage shot by Australian effects supervisor Lynne Cartwright. The second unit crew provided footage of fish swimming on the reef, and air bubbles shot against black. Modern Videofilm's staff artist David Hickey created the "magic" bubble using 3D animation.

"The bubble effect was accomplished in Alias|Wavefront's Maya, using a combination of keyframe animation, dynamics, soft bodies and hard bodies as collision objects," said Hickey. The overall sequence of creating the bubble effect was to first create a primitive sphere with increased subdivisions for greater detail. The sphere was animated in reference to the backplates, then converted into a soft body. All primary physical animation is performed on the hard body objects. Secondary dynamic animation is performed on the soft body.

The animation of the hard body was accomplished by animating both their transform node and also in some sequences, animating the control vertices of the sphere. Once the animation was matched in reference to the backplates, the hard body was converted to a soft body. A turbulence field was added to the soft body to add some arbitrary secondary movement. Interaction between the live actors and the 3D bubble was created using modified invisible primitive spheres as collision objects.

In pre-production , Executive producer Paula Hart explained that she wanted the magic bubble to look different then other air bubbles in the water. She wanted it to look like a soap bubble floating in the air. To create this look, the bubble was textured using various maps and original backplates. The main shader was a dark blue color at ninety-five percent transparency. An animated water map was used as a bump map to add additional ripples throughout the bubble. A rainbow hued noise ramp was applied to the secularity to enhance the prism effects of the highlights. And finally, the scene backplates were used as an animated reflection map.

For the shots where the actresses were within the bubble, Hickey had to render the effect for each shot in six different pieces, because the actresses needed to be sandwiched between the layers of the bubble. This gave the illusion of distortion and reflection in front and behind them, and gave Henry artist Trent Shumway the extra control he needed in compositing. Additive keying seemed to retain the transparent nature of the bubble while allowing the different soap like colors and keeping the highlights brilliant and opaque.

"The thing that we were bumping into was the amount of black that was used in the wardrobe," Shumway said. "The solution seemed to be to render it over blue; combining the luminance and additive keys." This process retained all of the girl's reflections. Mattes were also needed for the front and back pieces. These mattes allowed Shumway to distort and apply a slight color correction to the back -plates. The idea was that whatever was inside the bubble would be slightly warped and less murky then the surrounding environment. Obviously, whatever was supposed to pass behind the bubble had to have these same adjustments. It became easier to comp those elements first then apply the setups overall.

A couple of fish and bubble elements were selected and then composited. Shumway then simply plotted a point right in the middle and pushed it back in Z space. When elements traveled from beside the bubble to behind it there was a slight offset, giving the movement a three-dimensional look.

In most of the shots, effects supervisor Steve Kullback wanted to exaggerate the motion of the actresses floating in the water. This floating effect had to be animated by hand. The shots with both actresses side-by-side were especially difficult. The girls had to be independent of one another, which would have been easily accomplished with simple garbage mattes if the actresses had been split apart.

However, the actresses crossed each other and broke frame. Shumway rotoscoped where it was necessary to handle the crossing, but the motion was limited in X and Y because of the way the actresses broke frame. To sell the illusion, it became necessary to animate in Z space, as well.

Since the girls are entering the air bubble to talk, they naturally take their regulators out of their mouths. The regulators passed from inside the bubble wall and back out again. Once the regulators pierced the wall of the magic bubble, quick blasts of normal air sprayed out and up towards the surface. These were added simply by taking the bubbles shot by production, pulling a luminance key and hand tracking a group or two for each respirator. A little rotoscoping was needed for clean up.

The bubble surrounding the girls was supposed to be fairly large, extending toward the camera a foot or two past the actresses. To enhance this illusion, Shumway used the bubble's matte to reveal the same groups of bubbles offset by a couple of frames. This made it appear as if the normal air bubbles burst out underneath the magic bubble, then came around and floated up out in front and along the sides of the magic bubble. A warp was added to enhance the illusion of depth.

The final icing on the cake was to create an airtight seal near the actresses' waists and a subtle reflection in their face masks. The seal was accomplished by taking the difference between the girl's mattes and the 3D bubble mattes. The Henry has a great emboss feature which rendered a matte that Shumway ran a watery texture through, thus keeping the soapy-like movement of the bubble.

The reflection in the masks needed to be so subtle that a circle garbage matte was hand tracked for each mask. The original plates allowed some room for error, because the rims of the mask were metal and would reflect as well. Shumway then selected some footage of fish on the reef, and composited it all together. In the final shot of the sequence, Sabrina pops the bubble, and transforms herself into an animated fish.

Modern Videofilm's Elastic Reality artist Marlan Harris morphed Sabrina from her human form into the cartoon fish character animation created by LightPoint Entertainment. Rick Cortes added particle animation on a Discreet Logic Flint. The entire sequence of five shots required more than 150 artist-hours to complete.

Sabrina Down Under was produced by Hartbreak Films and Viacom Productions. Modern Videofilm also provides the digital effects for the weekly Sabrina television series, which is now entering its fourth season. Supervisor Steve Kullback and effects producer Mark Spatny manage a team of artists that provided up to 60 effects per half-hour episode.