The whole series stands or falls on the our ability to fool the viewer into thinking that they are looking at something real

Mike Milne, Framestore

"All of those elements have to be animated," he said. "So, the number of animation controls the animators have to work with is vast compared with dinosaurs."

Before the animators got to work on a beast's face, they had to create a plausible model of the entire animal.

A 30-centimetre (one foot) clay model was created using photographs of fossil bones as well as reconstructions of the beast created by other artists.

Palaeontologists were also on hand to advise about the look of the creature.

While most palaeontologists agree about the general shape of some of these ancient beasts, there is still great debate about their individual markings and behaviour.

Second skin

Once a clay model was finished, it was laser scanned to create a three-dimensional, digital still-life of the creature.

This digital skin is used to put flesh on the movements that animators create for a beast.

Different creatures need different techniques to bring them to life

These movements are generated using a computerised skeleton developed from the same set of photographs and drawings of fossils used to create the clay model.

But bringing life to a computerised skeleton is no easy task.

"It has to be done by really experienced animators because the way that the bones move and walk and run is the character of the creature," said Mr Milne.

The Framestore animators faced particular problems when working out how the distant cousins of humans in the Walking With Beasts series should walk.

"How does an ape-man walk?" asked Mr Milne. "Obviously, it has got to be a more advanced walk than an ape, but at the same time you must realise that it's not fully human so it must retain some ape-like character."

Expert aid

Again palaeontologists proved invaluable. Studies of fossil hips and hip joints of proto-humans have shown just what forces the arrangement of bones could bear.

These suggest that instead of pushing from the ball of the foot like modern humans, our distant cousins swung their legs from the hip. It is the sort of movement seen today in great apes such as orang utans.

The series mixes live action and animation

Armed with this knowledge, the animators could get the skeletons moving convincingly. A repertoire of movements was created for each beast in the programme that could be stitched together to create entire behaviours.

The laser-scanned skin was superimposed on the skeleton to get a basic beast.

Next skilled artists developed the markings and colourations for the skin of the beast.

Because there is precious little evidence about the surface markings of these ancient creatures, Mr Milne said the artists chose colour schemes like those seen on contemporary animals with similar lifestyles.

"For the sabre-tooth cats, we've gone for a colouration that is like a leopard," he said.

"We felt that this would give immediately the shorthand for speed and strength that we wanted to get across."

Fooling the viewer

Finally, the creatures were superimposed on the backgrounds filmed on location for the series.

For every real world sequence, images were taken of how the ambient light of that scene illuminated a sample object with a known reflective quality. In Framestore's case this was a volleyball.

Mike Milne: Up to viewer to decide

An analogue of the volleyball was created on computer and the animators tried to match the light falling on this in the virtual world with what was captured during the real-life shoot.

This was then applied to the animated creature along with shadows and other shading effects to bring the beasts to life.

The final judge is the viewer.

"The whole series stands or falls on our ability to fool the viewer into thinking that they are looking at something real," said Mr Milne.

Walking With Beasts is broadcast in the UK on BBC One on Thursdays at 2030 GMT.