You may have heard of the term “sky replacement”. The term is quite self-explanatory, it is the process of eliminating the sky from a shot so to replace it with a different sky image.
The reasons for this can be different, you may have a dull overcast sky, or maybe some parts could be clipped, or you could go for a completely different setup and create a sci-fi environment or a more catching background for that romantic kiss. Whatever be the reason, here we have five nifty techniques that can help you achieve the desired result, thanks to Jordy Vandeput and his team at Cinecom.net.
The first one to be fair it’s not a real replacement, it’s more of a sky enhancement, but it’s the simplest one of the group, so we’ll start with that. Get a nice shot, possibly taken with a tripod, or even handheld if you prefer. Just mind, you’ll need a little bit ok tracking later on, where you have an abundant portion of the shot that is taken up by the sky.
We could give it a little make up and add some astral element to make it a little bit more interesting. Search for a picture of the moon or some planet that could suit your needs, and get both the picture and the shot in Premiere Pro. Obviously, be sure to have the rights to use the image before going on.
Once you’ve dragged both the images in the timeline you can mask out the space around the planet using the mask in the opacity section of the Effects Control.
Feather a little bit the selection so to not have a rough edge, and then proceed to change the blending mode of the planet to screen. You’ll probably need to lower a tad the opacity to blend in a little more the images. Position the planet as needed and there you are!
If you did shoot handheld you just need a quick round trip in After Effects, where you can track an element on the back, like a tree and then parent the level of the planet image to the tracking info you got, easy peasy.
Our second option is a good one of you have a shot where there isn’t much detail or rough edges on the background that lean on the sky. A good example, as you can see in the picture, is a building, or some kind of construction with straight and clear edges. Once you have the correct framing on a tripod (this time it will be necessary to stay planted there) you have to take to shots, one in the daytime and one during the night, or at least at dusk.
Put your tracks on the timeline, having your dark shot on top. Again in the opacity tab, you can mask out following the horizon, or like, in this case, the contour of the building.
Furthermore, you can feather a little bit the edges of the mask so to avoid a steep transition. If you wish to achieve the opposite effect you can just switch the clips in the timeline putting on top the daytime one or simply inverting the mask.
This technique, unlike the others, relays on having instead a nice blue sky. It is the one that can get the best replacement, and it uses the Ultra Key in Premiere. Drag your shot in the timeline and apply the effect on the clip using the eyedropper to pick up the sky. You’ll see that it will turn black and reveal the clips underneath. Using the options in the keyer you can clean up the mask and light spillings in the frame if present.
To get a better look at what you’re keying, you can switch the view of the output inside the effect options from composite to alpha channel and you’ll get a black and white image where white represents the area of the image you will see, and black will be keyed out. It’s also recommended to adjust the image in the Lumetri panel you’ve shot so to blend nicely background and foreground, maybe faking a nighttime shoot.
This one is a slight variation of the previous one. If you don’t have a chance to shoot on a blue sky you can get the same result using the Luma key on a bright overcast sky, using as an advantage the dull grey clouds.
It is important to have a good tonal separation between the grey/white overexposed sky and your talent and the foreground. Using the curves we can clip a little bit the brighter parts of the image to increase the separation. We can then apply the Luma Key onto the clip.
You’ll probably have a little bit of contamination on the key if there are small details like hairs or leaves in the shot, and getting rid of them with only the Luma key will be close to impossible. What can you do to correct it? You can add the Ultra Key, but instead of using it as before you can use its options to clean the mask out and get a nicer image, specifically with the choke and soften parameters in the Clean Matte tab of Ultra.
This time we’ll go with a green screen. As always when working with a green screen it is important, to make easy the keying, that the cloth must be evenly lit, and you should avoid any kind of wrinkle or imperfection on the surface. The key here (pun intended!) is to match the lighting condition of the background you’ll apply to the light on the talent.
In this example, you see what seems a daring composition. The idea is to be the person with a shot of the northern lights, adding a slight moving green light on the face of the talent.
You would think that using a green tint on the face would make for a hard time going to key the green screen, but the reality here is that there is a sufficient difference between the luminance and saturation of the screen and the face so it’ not at all as hard as you could imagine.
Granted, this technique can be successful only if you know in advance what will be the background you’ll composite on, so to match in the best way possible the lighting scheme. So, there you have it, these are five ways to improve the energy of your shot with little to no fuss in Premiere Pro. Give it a try!
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