Sky Replacement with Adobe After Effects Tutorial

You have a perfect composition, a picturesque setting, beautiful actors and brilliant lighting, however the exterior of your shot does not thrill, thanks to horrible weather. This scenario is pretty common when it comes to the troubles of on-location filming. Thankfully, there are ways to remedy such a situation and not only salvage, but enhance the visual effect of your image. In this tutorial, we will take a look at replacing a sky and background to add new life to an underwhelming image. A basic knowledge of lighting and Adobe After Effects is encouraged for this tutorial.

1. Shoot the main action

The first element in this composite is the main action, or your source footage. This example shows a couple on the balcony, enjoying a glass of wine. The view outside of this penthouse on this particular day leaves a lot to be desired, thus in order to make the scene more engaging, we want to replace the sky and background with a visually dynamic backdrop. Fortunately, the sky was pretty overcast and washed out, therefore I exposed for the interior and let the sky blow out to try and capture it at an even exposure. Knowing I want to use the sky as a luma matte in After Effects, blowing it out in camera will allow me to select a majority of it with ease, not having to fight with a gradient of tones or hues. Once the action is captured, we take this footage and drop it into our After Effects sequence, creating a new comp to the settings of the source footage.

2. Key out the background elements

With the new comp created and the footage in place, the next step is to get rid of the sky so our new background can be placed into the scene. There are a number of ways to achieve this, but I chose to use the Color Range tool in the effects panel. By selecting the dropper and targeting the sky, the washed out color tones are masked, leaving behind everything else. I kept the color space drop-down turned to the Lab setting, and turned the fuzziness to 20 percent to help perfect the edges of the selection. With a majority of the sky selected, the next step is to perfect the matte.

3. Perfect the Matte

For this step, I dropped in a Key Cleaner, keeping the strength to 100 percent and the edge radius at 20. This tool helps to polish up any fringe elements of the mask. After this, I was still left with a subtle green outline around the subjects, and the scene as a whole gave off a general green hue. To remedy this, I use the Advanced Spill Suppressor to get rid of both. Additionally, color grading the source footage can be done to eliminate any remaining green in the midtones and highlights within source footage by simply pulling that hue down with a Curves adjustment. With the matte at where I want it, the following step will see the addition of the background plate.

4. Add the background plate

For this example, I have a nice, high resolution image of the Chicago skyline. I hid the main footage, and added this background element to the relative position I want for the final composite. Be sure to make this object 3D in order to adjust its depth and final positioning in space. Knowing what your background will be prior to filming the main source footage is critical in understanding how to properly film and execute the capture of the footage. The next stage details how to perfect the background plate.

5. Expand the plate and color grade

Knowing I have two windows that will be seen in the final image, I went ahead and duplicated this background image and spaced it out to align it with where the second window will be. I repeat the steps of 3D aligning to get the scale and positioning perfect. In order to match the exposure of my main footage, I go ahead and add a Brightness and Contrast effect, turning the brightness to 80. It is important to note that I knew what the ballpark, general exposure of the exterior should be, based upon how I lit and shot the main action footage. At this stage, it is just a matter of fine-tuning the image to match visually. I finalized the color of the image by adding a Hue/Saturation effect, dropping the saturation ever slightly to mimic the hazy atmosphere of city buildings in the distance, and polished it off with the addition of a Gaussian Blur to take away a bit of the sharpness.

6. Unhide main action footage

At this stage, the composite is almost complete. All that is left is unhiding the main action footage, and making sure all elements are in-line. Any tweaking can be done by repeating steps four and five, adjusting the distance, spacing, and color of the background plate relative to the main footage.

Final Notes

As mentioned previously, there is more than one way to get the final result pictured. Ultimately, the conditions of the shoot will help determine which method to use, whether it be installing a green screen, or simply using the washed out sky as a luma matte like this scenario showcased. At the end of the day it comes down to efficiency, and the tools at your disposal. For a look at how each step flows, the following is an eight-second overview of the steps listed above:

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