Creating the visual effects for an entire series of Merlin would be a challenge for any company. With its otherworldly mixture of creatures, environments and wizardry – both literal and digital – the BBC's Arthurian fantasy is broadcast to millions of viewers in over 180 countries worldwide. But for Michael Illingworth, founder of boutique London facility Vine, the challenge was doubled. Before he could create the effects, he had to build a studio.

"Before we started work, there were five of us," says Illingworth, a veteran digital artist and VFX supervisor, who had handled the matte painting for the previous series of Merlin. "When we finished, there were 20. We had to get the whole company up and running – down to buying the desks and chairs – in three months. And of course, we had to decide what software to use."

Fortunately, Illingworth had an ace up his sleeve: Solid Angle's Arnold renderer. Arguably the visual effects industry's best-kept secret, Arnold was developed by Solid Angle founder Marcos Fajardo. Since 1997, this fast, physically accurate raytracing renderer has steadily built a user base that includes some of the world's major VFX studios. Sony Pictures Imageworks, with whom Solid Angle has a long-established development partnership, has used it as its primary renderer for years. Other clients include Digital Domain and Framestore. But go to the company's website, and you will be greeted only by a single, enigmatic equation: the formula for light transport through a differential solid angle of space.

"All we knew about Arnold before we got started was that big studios were using it and liked it," admits Illingworth. "But we were pleasantly surprised with the support we got from the developers and management team – and how easy it was to get beautiful images out."

Rendering quality, particularly in complex 3D scenes, was a prime factor in Vine's choice of Arnold as its render engine. For the first two episodes of Merlin alone, the studio was asked to create two entirely new all-digital creatures: the white dragon Aithusa and the Euchdag – a 400-year-old humanoid creature who is reputed to hold the key to all earthly knowledge.

The Euchdag, a photorealistic humanoid character driven by actress Josette Simon's motion-captured performance, was to prove the show's key technical challenge.

"She's very slight, and withered and skinny," says Illingworth. "But she also has a translucent quality. Her skin refracts the light, and you can see her internal organs glowing inside. We were trying to achieve a look like a jellyfish."

To replicate the luminous, gelatinous transparency of a sea creature, Vine needed to mimic a range of complex real-world light effects, including subsurface scattering, soft shadows and multiple internal refractions. Moreover, it needed to do so in no more than 40 minutes per frame.

"It's one thing to make shots look great, but we had to prevent render times from getting out of hand," says Illingworth. "We had a colossal amount of work to do."

All told, the studio had 204 VFX shots to deliver for the first two episodes, all of which had to be completed inside six months. From starting to build assets to delivering the first completed renders took just three months. And again, Arnold played a key role.

"The great thing about Arnold is that you can get really nice renders right off," says Ivor Middleton, Vine's head of 3D. "It's all about the bounced lighting. It's not like old-fashioned ambient or diffuse occlusion. [Because it's a physically accurate raytrace renderer] you can put a single light into an environment and get all your indirect lighting for free.