Following the eye-popping Elvis and Celine Dion Impossible Duet on American Idol last month, viewers around the world were buzzing with the question: "How'd they do that?"

Thanks to the talented pool of VFX artists at Time-Slice Films and some new rotoscoping tools from Imagineer Systems, the impossible was not only quite possible, but an opportunity that raised the bar for dazzling, realistic rotoscoping projects.

Bath, UK-based Time-Slice Films, innovators in the field of film and television frozen-time effects, full digital image capture, interpolation and post production techniques, was invited to participate in this project, as they had previously worked on a similar TV show in the UK called Duet Impossible. Also, as this episode of American Idol was for charity, Time-Slice was keen to get involved, and consequently, almost everyone who worked on the show worked for free.

The VFX team faced a range of challenges on the project, not the least of which was the 12-day deadline - which consisted of rotoscoping nearly 3,500 frames for two minutes worth of footage. As the team set out on this newest challenge, they would discover this was just one of many challenges that lay ahead.

Visual effects supervisor for Time-Slice Films, Callum Macmillan, notes, "Adding to the challenge, the source footage was not shot on film, which would have provided us with good resolution to work with. However, Elvis's 1968 Come Back Special performance was one of the first items ever to be recorded to videotape. As such, Time-Slice received footage that was in 29.97 drop frame with quite extreme interlace artifacts. Considering the demands for a pristine final product, the source material quality was not very good. From the outset, we had to decide exactly how much careful smoothing and field blending we were going to apply across the board as this would dictate what kind of edge we would be rotoscoping to."

Along with colleague Graham 'Gee' Clarkin, Macmillan put together a team of eight roto artists who formed quite a broad range of skill sets.

"We had Shake artists who came from a feature film background and then Combustion and After Effects artists who came more from a motion graphics and television background. Having beta tested Imagineer Systems' motor on the UK Duet Impossible show the previous year, Gee and I knew its potential and were keen to integrate this software alongside all our other applications, especially as we were facing such a tight deadline," states Macmillan.

To tackle the complex roto requirements, the team looked to Motor from Imagineer Systems. Macmillan continues, " One of the main areas where I felt Motor did well was to hold a consistent shape around the back of Elvis's head. There was also this misconception that it was very straightforward for us to extract Elvis from the original footage via some basic luma keying. This was not the case. While keying of different sorts enabled us to get some edges, Elvis's black hair on a black background provided no edge whatsoever to roto with - we were literally guessing in the dark. However, we found that the planar tracker in Motor was able to pick up enough of the small, twitchy movement of such an organic object that once we had defined our shape, we could leave Motor's transform, scale and shear functions to keep the line of the head, thus pretty much eliminating any edge boil normally associated with such guess work."

Once all the roto and matte work was completed, Macmillan upconverted and retimed the footage to 720p 59.94fps using further smoothing effects and an optical flow retiming plug-in.

"The final product was then FTP'd to the team at Chainsaw Edit and  American Idol, who did the amazing job of compositing and editing our matte work into the final sequence. From start to finish, I feel everybody involved in the project did their absolute best. I'm pleased with our creative team and the innovative tools from Imagineer Systems that enabled us to play a key role in raising the bar for this type of effect to be used in television."