VFX – Sky Replacement

Gary leapt into his first real lesson today with something that seems very useful, Sky replacement. This involves removing the sky from an existing image or footage and replacing it with another. Gary managed to fool us with some of his own photos, asking which was the real sky. Trick question, none of them were! Proving how effective the technique is.

We would go over two main techniques this lesson, linear colour keying and colorama. The first would be used in instances where we had a clear blue sky and the second where skies were white/grey. If we completed both these exercises on still images there was footage provided to apply the same techniques to video using motion tracking. All photos for this exercise were provided from Gary’s own photos.

Here is what I decided to start with:


As the time of day is fast approaching night but the sky was still blue, I figured it would be easy to swap it out for a starry sky. The rest I’ll try and summarise in a few bullet points, the process isn’t complex or long but it doesn’t hurt to give myself a few notes for the future!

  • Import the photo and use it to make a new composition
  • Drag the photo onto the timeline and duplicate it
  • Import the photo of the sky you wish to use and place it on the timeline between the two copies of the photo. Position as required
  • On the top layer, Effects > Linear Colour Key > Pick out the colour of your sky using the eye dropper
  • Edit tolerance and softness values to get desired effect
  • Add a Matte Choker effect to blend the edges of the keying


On top of this I used a Levels & Curves effect to colour correct the original image to fake lighting conditions closer to true night.

Onto the second method. Here is the starting photo for the colorama method, starting with a grey sky:


This time the original image gets duplicated three times and a colorama effect gets added to the top most layer. Colorama gets changed from RGB to Ramp Grey and the white/black levels get edited by hand to give you a mask.

Now the second duplicate down in the timeline has the Luma Inverted Track Matte turned on and the rest of the process is the same as above. Position sky, blend edges and colour correction.

Here is the end result:


Since there was time after these two exercises I decided to attempt making the same kind of edits on footage. This did prove troublesome. The ideal settings for colorama to get a mask differed throughout the duration of the footage, leaving the need for some garbage masking. However for some reason the normal method for achieving this didn’t work and even eluded Gary. Could be an issue with After Effects or simply the order in which we’re doing things. We’ll all look into this for next week as the problem needs to be solved.

While the end result isn’t perfect due to holes in the mask, here is the work in progress:



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