Harry Potter is one of the great publishing stories of our time and the Warner Brothers films have consistently produced Muggles gold. We provide our own Patronus to explain the true wizards behind Privet Lane, giants, and charms, as well as go inside the Ministry this week in our fxpodcast with Double Negative’s Paul Franklin. We also have notes from some other vfx shops involved in the movie.
Double Negative was awarded the vast majority of shots on the fifth Harry Potter film, making this the largest film Double Negative has done. The company has grown over recent years, and in this week’s podcast we talk to Paul Franklin from Double Negative about Harry Potter’s Order of the Phoenix, and the some 900 shots completed by Double Negative.
The film was directed by David Yates who is also directing the next film Harry Potter and the Half-Blood Prince. Yates actually had a lot of television experience earlier in his career including directing the critically acclaimed TV movie The Girl in the Cafe – a Richard Curtis penned script that was nominated for 2 Golden Globes.
While the film’s credits list 11 companies as having worked on the visual and special effects (see the end of the story for a listing), we catch up with Double Negative and a few of the other key effects houses that contributed to the film.
In late 2005, Tim Burke, overall VFX supervisor for Harry Potter and the Order of the Phoenix, approached Double Negative to come on board as the lead VFX vendor for the show. Burke was keen to bring Double Negative on board as early as possible in the visual effects development process so as to get the maximum level of creative integration. Work started in November 2005 with a small previsualisation and concept team being installed at Leavesden studios whilst Double Negative VFX supervisor, Paul Franklin, started laying down the groundwork for a pipeline that would touch almost every area of the finished film. As you can hear in our podcast this week, Paul Franklin found this early involvement critical to the film’s success.
Double Negativeâ€™s shot list was divided into four distinct geographical areas: Hogwarts, both inside and out; the Forbidden Forest, where Harry and his friends encounter Grawp, Hagridâ€™s teenage half brother; the Hall of Prophecy, a mysterious and cavernous storage space in the Ministry of Magic and the Veil Room, which lies at the very heart of the wizarding world.
As well as handling much of the exterior work for Hogwarts, Double Negative also created an extensive sequence set within the hidden Room of Requirements, a magical space that only reveals itself in time of need. For the scene in which Neville stumbles across the magical doorway to the Room, the production shot plates of the actor walking down a typical Hogwarts corridor with a blue screen filling the space where the door was to appear. The Double Negative team then developed a series of concept boards showing the door revealing itself as a carving in the raw stone of the wall. Once the design had been tweaked and approved, a hybrid 2D/3D approach was developed to create the scrolling patterns in the wall and the shot was completed with live action elements of falling sand streams.
Inside the Room, Harry and his friends quickly set about practicing their defensive magic. Along with various stunning and disarming spells, Harry shows the other members of the newly formed “Dumbledoreâ€™s Army” how to create a Patronus spell. Previously seen in Harry Potter and the Prisoner of Azkaban, the Patronus spell was completely redesigned by Double Negative. Each child casts a unique version of the Patronus with its own individual animal at its core. Double Negative creature animators used Maya to produce the veritable menagerie of otters, dogs, hares and horses which weave their ways between the children in spirited fashion. The 3D creatures were used to generate spiraling trails, which were then mapped with VFX shaders producing complex, coruscating patterns of light.
Led by 2D Supervisor, Jolene McCaffrey, 3D Supervisor, Justin Martin and sequence leads Tristan Myles and Victor Wade, the compositing team integrated the elements into the plates in Shake with the addition of a host of interactive lighting effects. The Room of Requirements also featured a large amount of invisible reconstruction work as the mirrored walls frequently threw back images of the shooting crew and lighting rigs, all of which had to be painstakingly
For the exterior shots of Hogwarts, Double Negative worked closely with the Harry Potter miniatures team to design sweeping camera moves over the castle. Multiple miniature passes were then combined together and carefully graded to give the correct levels of contrast and atmosphere to convey the huge size of the building. One especially complex shot features a continual traveling camera move that pushes past Hermione, out of the window into a torrential downpour. This in turn becomes a raging blizzard that contains a flash of Voldemort in the whirling snowflakes, eventually the snow clears to reveal the first light of dawn rising over the snow-bound village of Hogsmeade nestled amongst the white-clad mountains.
Double Negative CG artists matched the live action camera move on the Hogwarts set and then extended it out into the virtual night. Live action rain elements were combined with digital rain and snow and a complex VFX animation setup created the glimpse of the Dark Lordâ€™s head. The camera move then blended back into a motion control move on the Hogsmeade miniature which was integrated into a fully CG landscape derived from an aerial location shoot in Scotland. This shot was composited by Adam Paschke, who was the Senior Compositor and 2D Sequence Lead for the Establishing Shots; Dayne Cowan was the 3D Lead Artist. Lighting Artist, Viktor Rietveld, generated a combination of controller passes that allowed the lighting of Voldemort’s face to be adjusted in comp, as well as a series of choreographed particle simulations that moulded to proxy geometry.
A large number of iterations were produced to try and provide as much scope for the right timing, shape of formation and dissipation for this effect. The landscape was extensively changed with matte painting to bulk up the rather sparse snowfall in the Highlands and numerous enhancements such as smoke plumes, lamps in windows and pedestrians were added to the village. Finally three digital doubles of Harry, Hermione and Ron were placed walking up the central street.
The Forbidden Forest
To explain the reason behind his mysterious absences and furtive behaviour, Hagrid leads the children into the depths of the Forbidden Forest where he reveals Grawp, his younger half brother â€“ a teenage, full size giant. Grawp was a major challenge on several different levels. His sheer size and his specific look meant that shooting an actor using the same techniques to make Hagrid look big would be impossible, so Grawp was realised in a completely digital manner. The character would also need to be capable of delivering an emotionally complex performance that was made even more challenging by the fact that he stays mute throughout both his scenes. Finally, Grawp would need to be able to interact physically with the real actors, lifting Hermione and later Umbridge off their feet and into the air.
Development on Grawp started as soon as Double Negative joined the project in the winter of 2005. A small Double Negative previsualisation team worked closely with director David Yates to create an extensive animatic of Grawpâ€™s two scenes. The preliminary animation was then loaded into a custom-built virtual set system, which was built by Double Negative R&D in collaboration with JDC who supplied the cameras used in the making of the film. The virtual set system allowed the director and crew to be able to see Grawp on set composited into the live video feed from the main camera and thus produce tightly choreographed camera moves and performances that responded accurately to the giantâ€™s notional position on set.
Double Negative then set about creating a highly detailed digital model from the creature departmentâ€™s designs, adding a sophisticated animation rig and a fully simulated muscle system beneath Grawp’s digital “skin”. A reference performance from the actor Tony Maudsley was used as a starting point by the animators. Of particular note was the advanced facial animation rig that allowed animators to work with data derived from any source â€“ motion capture, rotoscoping, procedural animation â€“ and combine it with keyframe action. Eventually, as Grawpâ€™s character continued its development, all source data was completely replaced with keyframe action making Grawp into a completely original piece of animation. A team of nine animators worked on the character animation, making continuity and consistency a high priority.
To enable Grawp to pick up the actors, Double Negative supplied camera and character animation data to the production unit. Plates of Emma Watson and Imelda Staunton were then shot with custom motion control rigs. A highly detailed skin simulation shader was created to create Grawpâ€™s skin and a wide range of character details were added including nicks and scratches from undergrowth and mud splatters that gave authenticity to Grawpâ€™s youthful and playful personality. All the CG elements were then carefully graded and integrated with the live action plates and a combination of 3D and matte painting was used to extend the impressively large set into the soaring reaches of the cathedral like Forest. Great time and effort was taken to make sure that the lighting in the forest set extensions compliment Grawp, including an non-key pass which was mixed with beauty pass to break up the light over Grawp and give the impression of dappled light through the foliage. The Grawp team was lead by Head of Animation, Eamonn Butler, CG Supervisor and 3D leads Richard Clarke, Ged Wright and Andrew Whitehurst, 2D Supervisor and 2D lead, Jelena Stojanovic and Tom Rolfe and Lead Animator, Colin McEvoy.
The Hall of Prophecy
In his desperation to rescue Sirius Black (Gary Oldman) from Voldemort, Harry leads his friends into the seemingly infinite space of the Hall of Prophecy, a vast storeroom of magical predictions in the depths of the Ministry of Magic. The productionâ€™s art department had designed a regular series of regimented shelves stretching in all directions; each shelf would bear a multitude of prophecy spheres, all of differing sizes. Everything was to be made of glass and each prophecy sphere would hold a swirling mass of magical fluids and vapours. Early in preproduction a practical set build was vetoed in favour of a totally virtual approach. The actors were filmed on a nearly featureless green screen set which then required very specific match-moving of the predominantly hand-held camera moves.
The Hall of Prophecy team was lead by CG Supervisor, Justin Martin, 2D Supervisor, Jolene McCaffrey, Sequence Supervisor, David Vickery and R&D Lead, Trina Roy. The complexity of the environment produced very long 3D render times for the shots with some frames taking many hours to process so a custom lighting pipeline was built which allowed the 3D and 2D artists to “re-light” the rendered images interactively within Shake, thus keeping the need to fully recalculate frames to an absolute minimum. The overhead on the computers was further reduced though clever use of Rendermanâ€™s new “brickmap” 3D texture feature, which enabled background detail to be added in a highly efficient manner. Sequence Supervisor, David Vickery notes that “without the brickmaps we would never have been able to render the shelves off to infinity…the render farm would still be going in a few years time!!!”
The Trelawny prophecy was created with a custom version of the shader used in the rest of the prophecy spheres. Extra layers of detail and movement were added and elements of a digital Trelawny head were built from a full 3D recreation of Emma Thompsonâ€™s performance. The prophecy sphere was carefully tracked into Daniel Radcliffeâ€™s hand and the Trelawny face was revealed within the prophecy murk in time to the soundtrack.
When all appears lost and the children are about to be caught by the Death Eaters, Ginny unleashes a powerful Reducto spell, which causes the shelves to topple in a spectacular domino destruction effect. Ginnyâ€™s wand blast was created with a combination of particle dynamics and fluid effects, the resultant explosion was generated with more dynamics and animating 3D holdout mattes that expand through the rows of shelves. The toppling shelves and smashing spheres made extensive use of dnDynamite, Double Negative’s in-house dynamics toolset, which can handle far higher levels of complexity than any commercially available package.
Dynamite was used to drive the collapse and break-up of the shelves, along with the animation of the spheres and stands that crash to the floor in a myriad of splintered shards. A library of simulations was built, including, “collapse”, “crumble”, “wobble” and “topple” which gave a basis with which the team could choreograph the shots. The rendered break-up was then composited together with filmed elements of falling debris and multiple CG additions including shattering glass and billowing plumes of heavy smoke from broken prophecies to create a convincing picture of unleashed chaos a world away from an empty green screen studio just outside of London.
The Veil Room
After having barely escaped the destruction of the Hall of Prophecy the children find themselves plummeting through space in wild freefall. Stopping short of disaster at the last moment Harry and his friends dust themselves down and survey the huge amphitheater-like environment of the Veil Room that surrounds the mysterious Arch and Veil.
The basis of the Veil Room was a partial set built on a stage at Leavesden Studios, consisting of the Arch-bearing Island and the centre of a drum shaped arena, which was surrounded by a circular ceramic wall. Height restrictions on the stage meant that the wall and arch topped off at approximately twenty feet, so all up angles on the set required extensive digital cleanup work to remove the lighting rigs and the addition of a digital set extension to give the sense of a vast space reaching up into the gloomy heights. The Veil itself was created through a highly detailed digital cloth simulation, which was itself based upon reference elements of live action cloth supplied by the production. The transparency and highlight quality of the Veil was carefully controlled in compositing and tweaked on a shot by shot basis to give just enough density to make it readable in every situation, but not so much that it became opaque and milky. In most situations a digital Veil was inserted into a practical in-camera Arch, which was digitally extended when necessary, but a fully digital Arch, Veil and Arena was used in the opening two establishing shots of the Veil Room.
Within moments of arriving in the Veil Room Harry and the others are set upon by the Death Eaters who swoop down in coils of black smoke, enveloping everything in a dark maelstrom. When the mist clears Harry finds himself alone on the island and looks around to see that his companions are now the prisoners of the Death Eaters. Faced with the possible death of his friends Harry is compelled to hand over the precious prophecy sphere to Malfoy.
Having been briefly seen in the Hall of Prophecy, the Death Eater apparate effect came into its own in this scene. Elaborating on the design developed by Double Negative for Harry Potter and the Goblet of Fire, the Death Eaters were built up from swirling streams of thick black smoke and whipping ribbons and tendrils of tattered cloth, enhancing their swooping movements through the air. All smoke elements were created with Mayaâ€™s fluid and particle dynamics and then rendered with Double Negativeâ€™s proprietary DNB volumetric toolset. Cloth dynamics were carried out in Houdini and then lit in Double Negativeâ€™s Rex system before rendering in Renderman. The Trelawny prophecy sphere from the Hall of Prophecy was adjusted to match the grade and look of the Veil Room and the interior gaseous fluids were animated to give the sense of a loss of energy and life when held by the evil Malfoy, the elements were carefully integrated into the live action with particular attention being paid to the environment reflections and shadows in the glass surface.
The destruction of the prophecy was achieved entirely in CG through a combination of rigid body and fluid dynamics. The elements were composited with a flickering light effect taken from the plate of Jason Isaacs, which tied the whole shot together. In a moment of magical ying and yang the wizards of the Order apparate in swirling vortices of shining vapour and gossamer strands, the brilliant white light balancing the choking black of the Death Eaters. Numerous pyrotechnic hits, smoke and dust elements were added to the action shots to increase the sense of energy and jeopardy in the battle. Wand effects were given extra emphasis with the addition of subtle amounts of turbulent displacement to sell the idea of powerful forces being unleashed by the spells. A big centerpiece shot following the flying wizards in swooping aerial combat involved a complex, precisely choreographed camera move through the practical set which was then extended and enhanced with additional lighting effects, reflections and pyro hits.
Bellatrixâ€™s wand attack on Sirius used fluid dynamics and displacement effects to create a uniquely aggressive version of the Avada Kedavara spell with its characteristic green cast. To emphasise the dreadful effect on Sirius, Gary Oldmanâ€™s skin colour was progressively drained over several shots leaving him pale and ashen, clouding was added to his eyes imparting a blank, dead look. The cloth system was custom animated for the shots to produce the sweeping Veil layers that wrapped around Sirius like a funereal winding sheet. As the Veil surrounded him, Gary Oldmanâ€™s element was softened, graded and progressively mixed away to give the feeling of dissolution into the spirit world. The work on the Veil Room was lead by CG Supervisor and 3D Lead, Justin Martin and Phil Johnson and 2D Supervisor, Jolene McCaffrey.
Over a period of 19 months Double Negative fielded a team of well over 250 artists, developers and technicians to deliver approximately 950 finished VFX shots for the film, a record for any one vendor in the history of the Harry Potter film series. “This was a unique opportunity for us to get involved at a very early stage on one of the largest visual effects projects in the world,” comments Paul Franklin. “It was a privilege to work so closely with the Harry Potter production team and I believe that the outstanding work produced by our artists will help raise our profile to the highest possible level.”
While the majority of the shots were done by Double Negative, several other companies were also involved. MPC had previously done the nose removal on Voldemort and once again the production worked in favour of giving shots of any particular type to the company that had perviously done this exact type of effect. This also held true with Double Negative, having donea all the Hogwart’s train window replacement shots in several of the early movies.
The majority of MPCâ€™s work utilized their Maya and Shake pipeline. The fireworks seen tormenting Dolores Umbridge were enhanced using a proprietary shader which replicates real firework colour temperatures and uses each particle as an individual light source. And in much the same way MPC revisited the lack of Valdemort’s noise, the wand duel effect from the previous movie was further developed for the atrium based battle between Harry and his nemesis, Voldemort.
Bespoke particle systems were exploited for the â€˜shockwaveâ€™ and â€˜sand tornadoâ€™ sequences with the PAPI rigid body dynamics system allowing fast and accurate simulations of shattering glass seen in the explosion shots. The Atrium battle sequence was one of their biggest challenges and required a wide range of effects work. Huge scale set extensions complete the atrium environment, and hydro simulations used Flowline to create realistic fluid effects for the fantastical fire serpent and water prison effects.
Also returning was London powerhouse, Framestore CFC. Heading the team for Framestore CFC was veteran VFX Supervisor, Craig Lyn. “We have made a name for ourselves producing creatures with real character for the Potter films, creatures such as the Cornish Pixies and Buckbeak the Hippogriff” he says. This time, Framestore CFC provided the centaurs and Kreacher. The centaurs described in the book are not the cute, pretty creatures, but rather fierce, mysterious beast-men, in whom the human half leans towards the caveman in appearance. They feature chiefly in one key sequence in Harry Potter and the Order of the Phoenix: an encounter with Harry, Ron, Hermione and the appalling Professor Umbridge in the Forbidden Forest. During a tense face-off, one of the centaurs is hurt by Professor Umbridge, who subsequently finds herself being carried off into the woods by the enraged half-men.
“Purely from a technical point of view, there were a number of aspects to the centaurs that Framestore CFC hadn’t tackled before,” says Lyn, “Skin moving over muscle, the muscle dynamics and the dynamic hair simulations, for example. Not only that, but there were up to twelve of them onscreen at once.” The team strove to create a whole animal, rather than just a man with a CG horses hindquarters added – the approach taken in some other films. “We wanted to make it feel like a man was thinking and making the horse part move,” says Solomon, “One of the trickiest shots is where Bane – one of the centaurs – is caught by a rope, rears up and falls to the ground. We had some rearing horses but the mechanics of a horse rearing is different – what they do is throw their heads down low in anticipation, and then throw their heads up. So we had to make it feel like a man jumping, keeping the back very straight, and yet like a horse as it lay thrashing on the ground.”
CG Supervisor Ben White concurs. “The centaurs presented a double challenge,” he says, “Trying to combine believable human and horse torsos was a great deal harder than doing either one separately. When the centaurs move around they have to move in a convincing horse like manner, and yet still have the characteristics of a human – which is very tricky stuff to do , both in terms of animation and the movement of muscle and skin. To really push the realism of the characters we developed a new method for doing sliding skin over underlying volume. We prototyped the skin and muscle behaviour in Houdini, then our R&D department then developed this into an in-house Maya plug in, which we used to actually run the shots.
Kreacher, whom Harry encounters briefly at Sirius Black’s house, is an elderly, somewhat sinister house-elf. Craig Lyn is particularly proud of the work that went in to winning the job of creating him. “The number of shots Kreacher is in isn’t huge, but at the same time it’s a tricky bit of work because he’s a humanoid character who acts, delivers lines and fills the whole frame. Three of the guys on our side decided that they really, really wanted a shot at him. On their own initiative they built, modeled, textured and did a turn-table of his head and sent that off to the client. And it was that work which won us the gig.”
Although he delivers just a few lines, Kreacher’s words are loaded with meaning and his body language is similarly freighted. You tend, as an animator, to animate – to put in the traditional, slightly heightened stuff, with the creature hitting its marks perfectly and so forth,” says Max Solomon, “but we soon realised that that was not what was wanted here. So we worked against our instincts, and it was quite strange. We kept thinking he’s not really doing much, he’s not moving, he’s not really acting – but actually that does feel more real. A little hunchbacked old man doesn’t actually move or gesticulate a lot, so doing almost nothing was the right choice.”
Technically, says Ben White, “we took an entirely muscle based approach to Kreacher’s facial animation system, extending the functionality of the tools we designed for the centaurs and taking them even further, to give him skin that’s appropriately soft and stretchy for such an elderly character. We filmed actor Timothy Bateson as he sat in a chair and did his vocal takes, using his facial expressions and mannerisms as reference (although no motion capture was used), and these were incredibly useful for the facial performance. But the actual combination of body language, stance, how Kreacher moves around and how he reacts to Harry – that was entirely the creation of our animation team. This subtlety of animation, combined with the sophisticated skin shading and really believable eye lighting gave us something I’m really proud of. It’s a beautiful, understated little performance: you feel that you can really see his mind in his eye, like he’s a living being.”
Rising Sun Pictures
One other company that returned also was Rising Sun Pictures (RSP). We caught up with RSP’s Kat Szuminska, to find out more.
fxg: How many Harry Potter films have RSP done ? This is your second?
Szuminska: It’s Rising Sun’s second, Harry Potter and the Goblet of fire was our first.
fxg: How many shots did you do? And were these similar in complexity to last time?
Szuminska: We completed 82 shots for Order of the Phoenix, which is double the size of our HP4 shot count, but still a tenth the size of the largest vendor, Double Negative in the UK. Its incredible to represent Australia in this international VFX-fest of a story, alongside the world’s leading creators of VFX. In our last outing with Potter we created bespoke magical fire effects, the goblet of fire itself, the triwizard cup and age line as well as the firebird for the durmstrang students’ big entrance. The sparklers in this show were our only firey foray and Carsten Kolve had a lot of fun making procedural sparklers in houdini for the twins to juggle.
This time we delved right into core photorealistic effects. There was really a nice variety of work for us from cute animated fireworks through to mainstay invisible effects, set extensions, CG projected environments, full CG Grimmald Place and CG owl. The challenges are really different in this kind of work. There’s a small sequence where the children run down the hill to see Hagrid. This was shot green screen in Leavesden. We used stills of the Scottish location to build up a projected environment which we enhanced with a sprinkling of 3D grass to give some added dimension to the hill. Luke Emrose created simple proxy geometry & lovingly grew Maya Paint Effects grass over the top. We had terrific feedback from Tim (Burke) that no one even guessed that the children hadn’t been to Scotland. That’s the best compliment you can get really.
fxg: Can you explain how you did the reveal or building of Sirius Black’s house? What was the pipeline?
Szuminska: The brief for this sequence required us to ‘magic’ No.12 Grimmauld Place out from in between a terrace of muggle houses. For the wide shots, some careful lighting sits the CG into the plate, butted up against the practical houses. The ‘real’ houses (on a set in Leavesden) are static, but the camera is constantly on the move, so these elements were shot using motion control so we could match them up easily. Moody and the others watch from across the street on a greenscreen on a separate motion control pass. The reveal involves the left hand side of the rest of the street pushed out by number 12.
Dan Bethell, RSP’s CG supervisor explains “we built six houses in total (including No.12) from a collection of reference photos and a lidar scan of the set. Nick Pitt-Owen and Daniel Thompson provided detailed modeling and texturing of brickwork, built individual window fixtures, doors, balconies, and curtains. There’s even a pot plant which wobbles along for dramatic effect. The two close up shots of number 12 are entirely CG, the main components of No.12 were hand animated to emphasise the force & weight of the house pushing out into the muggle world. Dynamics bought hero curtains and a plant to life and multiple layers of particle effects, combined with 2D elements, generated dust and debris to augment the shots.”
For rendering we used the 3Delight renderer, together with custom shaders & our render-time geometry injector (Venom). Our crack team of Kyle Goodsell and Keith Herft composted the shots together in Shake.
fxg: Can you explain the integration of miniatures on the Hogwarts owl shot down to Hagrids Hut please, was this motion control – what files did you get?
Szuminska: This was another sequence we previzzed. The first element was shot from a helicopter looking down on location with Harry walking on the path in a field, empty except for a skeleton structure indicating the location of Hagrid’s Hut. We’d build that in 3d later! We designed a camera move using a matching lens, that would extend back to the head of this shot and flow nicely to the beginning at Hogwarts. At that stage we just used an accurate previz quality CG model of Hogwarts. Once Tim Burke was happy with the move we passed it on to the motion control unit who recreated the CG camera move out on the moco rig.
We got terrific textures and reference material from production for Hagrid’s Hut. With Hedwig, originally the plan was to use a real owl flying in a studio. We reverse engineered a camera move around the bird, rather than trying to train a bird to follow a CG designed camera move. The theory held up well, but the bird’s performance just didn’t look quite right on the day. In its place we then created a CG Hedwig. We started by building a feather system in Houdini, the feathers cast onto a reference owl which is then driven by animation exported from maya.Tim (Burke) provided lots of great reference of owls shot in previous Potter films,. With that footage, along with “the Life of Birds”, we had a great time exploring the intricacies of the snowy owl’s silent flight style. There’s hardly any ruffling in a real snowy owl’s feathers, that’s how they fly so quietly. For us it meant that only really subtle dynamics in the feathers, were needed to bring Hedwig to life.
fxg: RSP did some negative repair work didn’t you ? What software did you use and what approaches did the compositors take? Did you use any special plugins or software?
Szuminska: We used the very special combination of Furnace’s wire removal and rig removal software from their delectable suite of tools available for shake. You would have to have a lot of spare compositors sitting around to do it any other way!
Full visual effects credits:(source IMDB)
Gentle Giant Studios
Industrial Light & Magic (ILM)
Moving Picture Company (MPC)
Rising Sun Pictures
Visual Effects Company, For motion control