Breakdown of a visual effects shot in fire ants, a video and comprehensive article

Helicopter Chase Breakdown

Rather than just posting a breakdown video i thought it would be nicer to write about it a bit, the approach, problems, solutions and things-i-would-not-do-next-time. I hope it may help others and also i welcome any feedback or 'why-on-earth-would-you-do-it-that-way' comments…

This shot is part of a longer sequence, so it's out of context and fast but appears in the trailer so I can talk about it.

Our night micro crew, David our Drone DP, Kat, Rafi, Amelia and Eve assisting

We shot this run and gun with a very small footprint. We shot on a sigma fp, which was recording 12 bit UHD files to an SSD. It was handheld and focus being pulled off the lens, which was a 28mm Leica summicron. We chose to shoot the sigma rather than Red mostly because of it being so lightweight and can fit into a small bag but also because the full frame sensor is more sensitive than the Reds and so was able to use a higher ISO. In this case i shot higher than i intended to use because i wanted to protect shadow details and also have some detail to work with in post.

So we start by preparing a plate in Resolve, develop the DNG and get the image into a place to start - we export this as a RWG/LogG10 mxf file - which we take straight into Nuke. The reason for this is the roundtrip to Resolve is easier and the whole project is Red so the grade is much easier when the baseline for all the files are the same. MXF at 444 12bit to ensure we retain all the subtle data in a log format. I've done a lot of AB testing to make sure that the MXF does contain everything - on previous shows this includes roundtripping Red footage and i have the confidence that it's a good workflow with Resolve. The newer versions of Nuke have the updated SDK so for Red files would most likely bring them in direct but Nuke currently will not open Sigma FP DNGs.. yet...

With the shot in Nuke X now, i do a camera track. This proved pretty tricky. I have an extended version and often i will pre or post shoot slowly scan around a scene i know i will track, to help get some solid points and awareness of the scene in 3D. We were moving a bit too fast to do this properly though. Oops. Secondly i should have had a tape measure and surveyed some of the points. This issue with the track is that it is a fast pan, it's out of focus and the shot is full of rolling shutter, so it's skewed. It took a lot of attempts in Nuke to get a track and then actually i realised we have PF Track and ended up pulling a good track in that. However in Nuke the best approach was manual tracking of planar surfaces, on the building and importing user tracks along with the auto. I can't help feeling Nuke should have done a better job here. It's a bit of a bug bear as people can be quite dismissive of Nuke tracking but it can be really good.

However with a good enough track that's fine now.

We had Eve on set and the idea was to add practical lighting for the helicopter spot light which works really well in other scenes, especially reverse shots. We tried to orientate so she was hidden by Rafi but we moved where we wanted the helicopter so actually it's not ideal. It gave some great reference for how his hair would light but i was able to reconstruct that part of the shot at the end to remove the light and her by projecting a static frame onto a simple card set up and painting her out of that static frame. When the frame is being shot with the tracked camera then we get all the movement back. It works fine. We also roto'd out Rafi which we were going to need anyway because we need to comp stuff and have him on top.

So plate is ready and now we set up the scene in 3D. So unusually i like to do 3D in Nuke, we use VRay as the renderer for realistic stuff. From previous shots i have the helicopter asset textured and ready to go. So just work out the path where it comes into view over the building. Add menancing lights on it, light with an environmental HDR and also create a flood of orange light from the plaza so it comes from shadow into this light. The scene has cards for shadow blocking and masking where the building is. Once the animation path is good the result of that render is adjusted so it will comp. I tend to use a grade node set to hypot to bring up and match the blacks of the plate.

After that i set up different render scenes for the volumetric spotlight and the actual light on the ground. It's more manageable like that. The volume isn't that realistic because it wasn't foggy there but it looked better that way. Tracked the focus of the spotlight and added some randomness with expressions and it looks nice crawling up to Rafi.

Because the spot is really just hitting a flat projected surface with no texture i used 2D techniques to pull some highlights from the stones and colour from the grass to make it look a little better - and also drew a light texture in photoshop for the projection itself which helps a lot.

There have been two versions of this shot - the first was without any downdraft from the helicopter which was in the first shared version of the trailer - but i felt the helicopter wasn't totally in the scene plus i really wanted to mess around with Houdini and Pyro FX. So that's the real reason.

So i exported a simple version of the scene as a FBX file into houdini and set up two simulations and placed fans where the helicopter was going to be. I iterated over numerous versions - this was smoke based because i felt that represented the dust better - i did think of grains and particles but don't have a good way to render billions of them. I'd also considered Nuke particles but again the sheer number needed was going to be a tough call - although i suspect it could have worked.

The end result of the houdini simulations are exported VDB objects which i can take into Nuke and Vray, and finesses them in situ. This is a real advantage. I think it's way more flexible to tweak look in the comp like this, density and so on. So with the trails there, they are lit by lights in the scene based on the plate and also the light from the helicopter. I comped them in and achieved a balance between them being too much and too real. I had to track and roto some of the mid ground objects to help the distance smoke integrate, the bins are not totally obscured by it as are the plants.

A shot from the sequence but can't give too much away

Finally Rafi is comped and add some lens flares, of course. These were done via Video Copilot Optical Flares and had to resist making huge anamorphic flares, always need to see the helicopter to the end of the shot is at the end the image is almost flared out.

With all the elements it's a case of balancing luminance. The plate, being shot in RAW maintains it's photography integrity in Nuke, so i am able to light and adjust photographically and balance the sources to get that point where it's dark but not too dark things sit okay. I don't think it's perfectly realistic, but it tells a story and that's what's needed.

The file is rendered out and back into Resolve like the other footage RWG/LogG10 same as all the Red footage and then the same grade/look is applied.


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