Simon Jones: green screen in HitFilm

Welcome to week 2. Last week, we talked about the core concepts of tracking and layered composting. And this week, we’re going to take it a step further by showing you how to cut holes in those layers using techniques called keying and masking. Here is the shot we’ll be making. This was shot using very common technique, which is to shoot the vehicle on a green screen. This has all kinds of benefits. For a start, you can concentrate on your filming and your acting without having to actually drive a car. So it’s a lot safer.

But it also means you can position your cameras and your lights in ways that would be either really difficult or perhaps even impossible to do on a moving car. There’s another project file this week for you to download. Once you’ve unzipped the files, double click the car projects to load it into film. Here we have to green screen clip loaded onto the timeline and ready to go. Head over to the Effects panel and open up the Keying folder. Keying is a technique whereby a specified part of the shot is automatically removed based upon certain criteria, for example, by using a green screen. In this case, we want to use the colour difference key.

This is a simple keyer for green screen and blue screen. Many compositors have more advanced keyers, including HitFilm 4 Pro. But this keyer in the free version of HitFilm 3 Express also does a really good job. So drag the Colour Difference key from the effects library onto the video layers. Make sure the layer is selected. And then head over to the Controls panel where we can tweak the settings. You’ll see that the effect has a Matte to View option. Selecting this shows a black and white version of the shot, which makes it easy to see the quality of the key. In this view, white represents visible, opaque areas. Black represents transparent areas, and anything grey is semi-transparent to some degree.

First, we’ll adjust the Gamma control until we’ve removed all of the green screen. The problem now is that we’ve also lost some of the foreground, that we want to keep. You can tell because it’s not entirely white. Adjusting the max setting brings back some of the foreground detail. Similarly, adjusting the minimum setting helps to remove any lingering green screen fuzz. This process depends heavily on the source footage. If you’ve shot your green screen footage carefully, it will key out nice and easily. If you’ve got your exposure, or your white balance wrong, or your subjects too close to the green screen and is casting shadows all over it, then you’re going to find it harder to get a clean key.

So do put that extra time into getting the shoot right. Turn off the View Matte option, and you can see that we now have our video layer with a big, black hole in it. We need to put in a background. And this can be anything you want. We’ve provided some sample backgrounds for you to try out. This time we need to make sure that we put our background underneath the video of the driver. So drag the day background video from the media panel onto the timeline, positioning it as the lowest layer. The tricky thing about filming out of the window of a car is that you’ll inevitably have some wobble.

Before we can use this clip effectively, it therefore needs to be stabilised. This can be done very easily using the same tracking features that we touched upon in week one. To stabilise the video, you normally need to do a double point track. By tracking two areas, HitFilm can then compare how they move relative to each other and figure out rotation data as well as position. We want to track the distant clouds, because they have very little horizontal movement. It will stabilise the shot while retaining the overall movement inherent in the video. So first, move the tracker over this cloud on the left. And once that’s done, change the track type to double points. And you’ll see the second tracker appears.

Position this one over the cloud on the other side of the shot. We can then track forwards through the clip. It should only take a few seconds. And once it’s done, change the purpose to stabilise then activate the rotation setting and hit Apply. Now, switch back to the viewer to see the results. Turn off the green screen layer using the little i icon so we can get a good look at what’s happened to the background. If you now move the playhead, it looks like the video layer is bouncing around like crazy, because that’s exactly what it’s doing. But check out the clouds. They are locked precisely in place in the viewer.

HitFilm is automatically adjusting the layer’s position to keep those clouds exactly in place. In fact, the whole shot is now completely stable with the layer’s erratic movement directly countering the movement that was in the video. We can now position that clip correctly so that is visible outside of the window. Don’t forget to turn the driver layer back on. You may need to increase the scale of the background and tweak its position a little so that it doesn’t reveal any gaps as it moves around in the shot. Something you’ll have already noticed is that this there is a visible green colouring around the edge of the keyed layer. This is called spill.

And it’s caused by the green screen colour reflecting off the foreground. You can see it particularly in his hair and also reflected off the car’s window frame. This can be easily countered using the spill removal effect. So in the effects panel, do a quick search for spill, and then add that to the driver’s layer. The green spill is automatically removed. OK, let’s increase the realism. Given the relative exposure of the interior, it’s likely that the exterior view would actually be a little bit blown out if we’d shot this for real. So I’m going to add a Crush Blacks and Whites effect and then ramp up the white value, which over-exposes the sky.

Although you’d normally try to avoid this while shooting for real, adding in a few, subtle, deliberate errors can actually help a composite look more real. If everything looks too perfect, people can tell that it’s a visual effects shot. Now that our background is quite bright, it should be interacting more with the foreground. Just like the green of the green screen spilled into his hair in the window frame, so I should add a new background. To do this, find the Light Wrap effect and drag it onto the driver layer. In the controls panel, open up the Source Layer menu and select the background. This automatically allows the light from the background to spill over onto the foreground.

I tend to switch the blend mode to lighten, because it looks more realistic. And for this shot, I’m going to boost the radius up to about 45. It’s a subtle difference, but it really helps. I’m also going to add a colour temperature effect to the driver and use it to give him a cooler, bluer appearance so that he matches the background. You could, of course, grade the background in the opposite direction if you wanted, depending on the specific look you’re actually going for. To complete the shot, we’re going to use a greyed layer. This is done for the new layer menu just like the point layer in week 1.

So go to that New Layer menu and select a Greyed Layer. These are special layers which affect every other layer underneath them. It’s a very useful way to quickly grade an entire shot in one go. On to this greyed layer, I’ll add a Glow effect. I’m going to use this to add more light blue from outside the window. I’m going to ramp the radius all the way up to 512 so that it’s super-diffuse. The intensity can then be adjusted to get the look you want. Take it down to 0.5 or below for a subtle look. Or if you want it to like the sun is right outside the window, you can ramp it up to 1.2 or above.

But generally, subtler is often better with this kind of thing. I’m also going to add a Levels histogram. This can be used to apply a specific look, which depends on your own tastes and the story that you’re trying to tell. For this shot, I’m going to tweak the red channel raising the input black and the gamma. And then I’ll switch to the green channel and boost the gamma up just a touch. Finally, I’m going to find the Vignette effect and add that to darken the edges. The crucial thing here is that the greyed layer is affecting the entire shot so that the foreground and the background are both being adjusted equally.

By applying this grade to the entire shot, we’re binding those layers together and giving them a unified look. A lot of what we’ve done here in the second half this tutorial is optional. But by adding in that extra detail, you increase the realism of shot. Now, occasionally, you’re going to have the dreaded situation of an actor wearing a green costume in front of a green screen. The simple solution is just to swap that green screen out for a blue screen. Seriously, it is worth the effort and the investment of having a backup blue screen for those exact situations. But if you do end up with a situation like this, here is how you handle it.

To start with, I’ll use the same initial approach, adding Colour Difference key and adjusting the settings to remove the green screen from the window. I’ll ignore for now that I’m also removing his green shirt. I’ll add the Spill Removal effect to get rid of those green fringes. So now, what we can do is duplicate the driver layer. So I’m going to choose Duplicate from that layer’s menu. And on the lower copy, I’m going to turn off the key and the spill removal. This is where the masks come in. In an ideal world, you’d always use a key, because it does everything automatically and instantly.

But practically, masks are there for when you need to manually specify exactly what is being retained or removed. Up in the viewer, the Pen tool can be used to draw a mask shape. So make sure the Pen tool is selected and make sure that the lower driver layer is selected. And now draw a quick loose shape around his shirt. If I turn off the top layer, you can see where this is done. All that remains of that lower layer is the shirt. But when combined with top layer, then they’ll work together to create the final composite with the top one doing the green screen removal and the bottom one filling back in that green shirt.

In the project file, we’ve also included a handheld version of the driver shot and a nighttime version. It’s now your turn to experiment. Can you figure out how to composite the background convincingly into the handheld versions? You will, of course, need to do a bit more tracking. You can see in this example that by having a handheld foreground the whole shot feels even more realistic. So this tutorial is all about their car sequence on a green screen. But the same techniques work for pretty much anything else. You can put your actor into a plane or a spaceship even.

Or if you want, you can change the view out the window of your set or location, make it look like you’re on the top floor of the Empire State Building looking out over Manhattan. So whatever you can think of, green screen makes it easy to pull off. So next we’re going to be talking about to change a small number of extras into a much larger crowd scene. Well done for getting this far, and many thanks for watching. I’ll see you next week.