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Sky replacement is a technique where visual effects artists can take a boring looking, blown out, overcast sky and turn it into something spectacular—changing the time of day, the weather... you can even change the location by changing the sky.
If you have you ever tried to shoot at night, you probably know how difficult it is to properly expose your actors AND capture a beautiful starry night and a full moon. What you often end up with is a moon that looks like a speck of light and stars that aren’t even visible. That can be remedied by shooting during the day and replacing the sky with a night sky. You could use stock footage, digitally rendered content, a matte painting or a combination of things. Then color grade your shot.
First, I’m going to show you how I took this bland, cloudy sky behind the Firth of Forth Bridge outside of Edinburgh, Scotland, and replaced it with a beautiful blue sky. Then, we’ll get a bit more involved and make it a lovely sunset scene. Lastly, I’m also going to show you an example of how to turned the day scene into night.
Sky Replace Using CoreMelt Sky Replace
CoreMelt Sky Replace is a cool little plug-in that makes sky replacement easy. It’s available in the CoreMelt Complete v2 Plug-in Set (MSRP: $199.00, Toolfarm price: $189.05). Apply CoreMelt Sky Replace directly to the footage layer. Depending on the color of the sky, the default settings may already give you some nice improvements.
The Sky Top and Sky Bottom parameters are for adjusting the position of the sky and the horizon. Hold down the shift key so that the gradient remains horizontal. To change the color of the sky, use the color picker in New Color. To see the matte, just click Show Key. I’ve normally want to make my matte as black and white as possible, but in this case, it’s nice to keep some of the grayscale levels to keep some of the natural clouds. To modify the key, adjust the shade selected under Old Color and blur the key to soften the edges a bit.
This plug-in is ideal for shots that have a lot of contrast against the sky, like this one. If you have a lot of elements in the shot that are the same color as the sky, it won’t work as well. CoreMelt Sky Replace is a powerful little plug-in and really does make simple sky improvements a breeze.
Using Colorama to Create a Luma Matte
The next method I’m going to show you involves using Colorama, included in Adobe After Effects (MSRP: $239.88, Toolfarm price $239.00 for 12 month subscription). This is a technique that I learned quite a while ago from a Video Copilot tutorial. Thanks, Andrew!
To use this method, first duplicate that footage with the sky that you want to remove. The top layer will end up being a track matte. Apply Colorama to the top layer. Under Output Cycle, set it to Ramp Gray. Set a couple more points on the cycle: black at about 5 o’clock and white at about 7 o’clock. Slide the colors around the circle until a nice black and white matte is achieved. Ideally, all of the clouds will be removed so that you have the most options. In my scene I made sure I have a nice clean matte for the bridge.
In my shot, notice that some of the white can be seen because of reflections in the water and because of the white boat. The shot doesn’t move much, so I added a black Solid Layer to hide it. I made the solid bit wider than my shot, named it Water and moved it into position. I drew a mask on the layer but overshoot it a bit so when I track it, the edges of the solid can’t be seen.
I could track the scene in After Effects, but I took it into Imagineer mocha Pro (MSRP: $1,495.00, Toolfarm Price: $1,420.25). I tracked the land in the background, and used the data to keep the mask that I just created in place. I also tracked the bridge and the water for other purposes. I selected the land track and exported the tracking data to my clipboard. Then, in After Effects, I created a Null Object, which I named Water Position. Making sure I’m at the first frame, I pasted the tracking data onto the Water Position Null. Hot tip: tap the U key to see all of the keyframes that have been applied to a layer. For the Water layer, I selected the Water Position Null as the Parent. It completely covers the white problem areas that I needed to cover.
I pre-composed these three layers: Bridge, Water and Water Position (Layer > Precompose). I’ll called my precomp Bridge Luma Matte.
Remember the bottom layer, with the duplicate of the original Bridge movie? That is below Bridge Luma Matte. I set the Track Matte on the Bridge movie layer to Luma Inverted Matte Bridge Luma Matte.
I created some clouds with the BCC Clouds plug-in in Boris Continuum Complete (Price: $995). I’ll create a new solid Layer (Layer > New Solid). I called it Clouds and made it comp size. I placed it behind my bridge layer and applied BCC Clouds to that layer. There are a lot of presets, and I selected a preset named Blue Sky Cloud 2.
Color Grading the Scene
A very easy way to color grade it to add an Adjustment Layer and apply a plug-in like Red Giant Magic Bullet Looks (MSRP: $399.00, Toolfarm price: $379.05). I like to click through the presets and I’m always impressed how it brings the foreground and background into the same color family.
Changing the Time of Day
To create the sunset look behind the bridge, there are a few things that I added to the mix. Instead of going through it step by step, I’m just going to break down the process a bit, so you can try it on your own.
I used the same Bridge Matte that I created in previous steps. I used BCC Clouds again and changed it to give the sky a warmer look. I also added a gradient above that to give it a nice blue at the top. To create the sun, I used BCC Lens Flare 3D on an Adjustment Layer.
You could easily use another tool such as Red Giant Knoll Light Factory (MSRP: $199.00, Toolfarm Price: $189.05), Video Copilot Optical Flares (MSRP: $124.95, Toolfarm Price: $118.70). There are some great flares in the GenArts Sapphire (Price: $499 for a one year subscription) package as well.
Any lens flare with an obscuration feature, which allows the sun to go behind the little X’s on the bridge, and that will give you a nice flicker effect. I thought it was a bit overpowering, so I duplicated my Sun effect. I have no obscuration in the background but the front instance of the sun has the flicker. It really helps to tone down the effect.
Positioning the Sun With Mocha and a Null Object
What I ended up doing was creating a Null Object called Sun Position. I pasted the data from mocha into the Sun Position. On each of the flare layers, I linked the Position X,Y, a parameter in BCC Lens Flare, directly to the position parameter on the Sun Position Null. That will cause the sun to follow the land with parallax and it will look more realistic.
Since the bridge is now backlight by the sun, I applied levels to the Bridge Alpha pre-comp layer and darkened it. I also wanted to reflect some of the color of the sky onto the water. I duplicated that Bridge Alpha Precomp layer, renamed it, and then masked off everything but the water. I applied a few color filters to the layer to match the colors in the sky but also keep the water texture. I used are Ramp, Calculations and Photo Filter.
I also added a bridge shadow which is basically a duplicate of the Bridge Alpha Pre-Comp layer and it is scaled down and placed into position. It was also masked so that I don’t see the water on the layer. I have a little reflection matte that I added to it. It’s pretty light and you won’t see it much, but it’s there.
I have an Adjustment Layer for color grading and I applied Magic Bullet Looks to the layer and set up a custom look.
A Day for Night Example
If you’ve tried to shoot a foggy night scene like this, you’d need to time it for the full moon and a night with rolling fog, and it would be very difficult to expose properly. I was going for this surreal cartoony style with the oversized moon, and of course, Nessie. Nessie was created by some friends of mine for the first ArtPrize Competition in Grand Rapids, MI and it floated in the Grand River. It’s now at the zoo. Incidentally, they are the same friends who shot the footage of this bridge, The Firth of Forth bridge in Scotland.
The moon was created with Moonlight, a plug-in in SUGARfx Light Pack (MSRP: $99.00, Toolfarm Price: $94.05). I also highly recommend a plug-in in GenArts Monsters (Price: $499) called M_Luna to create the moon. It has some features that will allow you to accurately place the moon in the right position in the sky for specific cities and on a select date for accuracy.
The rolling fog was created with Boris Continuum Complete Particle Emitter 3D. I started with a preset called White Cloud Fly Through and then I modified it.
Color Grading was once again done with Red Giant Magic Bullet Looks.
So there you have it. The same scene with sky replacement and color grading, giving it a completely different mood and time.
All of the plug-ins that I’ve mentioned are available at Toolfarm.com.
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