VFX – Sky Replacement

We covered a very interesting topic in our VFX class today: having covered keying in previous lessons, this time we were going to be learning how to do sky replacements. While it may not sound exceptional, the impact something like this could have on footage was astounding in some of the example Gary showed us – particularly when he fooled us with his own edited photos!

There were two types of sky replacements we were going to cover. One was going to be more useful for a blue sky (linear color keying) and the other would be better for a white or grey sky (colorama). We would be learning the process on still photos first, but there would be the option to try out the replacement using simple video footage. This would mean doing the same 2-point tracking we’ve already learned and then parenting up the sky to the null object so it tracks with the footage.

I took one of the blue-sky photos and added it into a new After Effects project, then duplicated it so I could sandwich my sky stock photo between them. From there, I could add my linear wipe to the sky layer, adjust the effect settings, and make sure any transition changes aren’t keyed in the timeline. Following that, I could add the linear color key effect to the second layer of footage.

linearcolorkey

I could pick out the colour of the sky and adjust the matching tolerance and softness until I could convincingly blend my new sky with the original photo.

skyreplace1.gif

The GIF isn’t exactly showing the best quality of the sky, but I wanted to include something that showed the changes I made to the original photo – I did some colour correction to the environment so that the entire thing looked like it was closer to the dusk/dawn the sky was showing.

Moving on, I followed the steps for the colorama method used for situations where the sky is more grey/white. This time, I needed three copies of the original photograph and then could add a colorama effect to the top-most layer. Instead of using the default RGB options, we would use the Ramp Grey option; this way, we could choose the blacks and greys to attempt to contrasts the colours of the sky versus the scenery. We used the second copy of the photo to make a luma inverted track matte.

Skipping through a few more steps of setting up the sky, using a matte choker on a pre composed layer, and then playing with some settings to get a more natural edge to the skyline, I ended up with the following look:

skyreplace2

I wish I’d chosen a better quality photograph for the sky, if only because it doesn’t match the higher resolution of the photograph. I wanted to go for a more sci-fi look to the sky (obviously) and I think the colour correction is subtle enough to work, but the sky would need to be better.

In the final time of the class, I also got to try out these new skills on footage rather than still photos. There were a few issues in manipulating the skyline between the houses and the sky using the colorama method, which we’ll have to look into further. For some reason, the sky’s white/black ratio would change half way through the footage. Perhaps it’s something in After Effects itself or some other setting needs to be altered.

The original footage is below:

skyreplace3original

I think I managed to make it look passable enough if not looked at too closely! I look forward to trying it again on other footage – perhaps even some I’ve shot myself.

skyreplace3

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Posted in VFX

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