Animating a sky in Nuke based on luminance

Written by Llyr Williams on .

This is a little trick which I’ve been using for a few years. The aim is to quickly add a sense of depth and motion to a sky matte-painting or photograph. When I first developed this technique I was using 3d Studio Max and its volume select modifier, for this example I’ll be using geo displacement in Nuke. 

This is an example using this technique with some extra lighting effects added

http://vimeo.com/33310960

Step 1: Choose the sky plate. This technique works best on a cloudy sky where there’s a bit of contrast between the sky and clouds.

Sky still

 

Next we need to animate the sky based on the inverted luminance values of the image.

Step 2: In Nuke import the sky image and connect it to a Card. Create a DisplaceGeo node and connect the card to it. We now need to connect an inverted version of the sky to the DisplaceGeo node using an Invert node.  If you view your card now you should see some displacement with the darker areas being pulled out. You’ll need to subdivide your card quite a bit to pick up the cloud detail, in this example I set rows/columns to 64 and 112.

Node tree

 

This Z displacement is a useful guide to check that we’re getting a nice even consistent separation, if not then we can always add a Grade node to get the desired result. In this example I’ve also set the  scale to 1 on my DisplaceGeo node.

Card displacement

 

Step 3: We’re only interested in animating along the X axis (left to right) so we need to set weight Z to 0 and weight X to 1. We can now adjust the offset X value to animate the image based on its luminance, so the darker clouds will move more than the lighter sky. There is of course a point at which the geometry will start to overlap on itself, we just need to find the point at which this becomes noticeable and keep within those boundaries when animating. Connect the Card to a Scene, create and position your Camera and attach to a ScanlineRender node.

DisplaceGeo Properties

 

To slow the clouds in the distance simply add a ramp to the displacement image.

Displacement map

 

Displacement map

Summary:

There are obvious limitations to this technique but I’ve found it to be a great way to quickly add a bit more depth to skies, especially being able to do it all within Nuke.

You could also cut out the 3D aspect of this process and simply use an IDistort to to animate the image.

llyrwilliams.wordpress.com

 

 

 

 

Comments   

 
# Jay Sambuev 2011-12-07 22:00
I very like your tutorial, can you show how did you add flashlights to the skies?
 
 
# Guillaume Charron 2011-12-09 07:34
Pretty pretty pretty !! Thanks for the tip.
 
 
# srihari babu 2011-12-09 10:04
good way theri how did you add in flash light
 
 
# Llyr Williams 2011-12-09 10:39
Thanks for your comments. The lightning behind the clouds is done based on luminance as well, I'll upload a still of the node tree.
 
 
# kapil more 2011-12-10 06:20
I like the tuts.the cloud looking like water waves.what should i do to the footage that it's looks like original cloud ?
 
 
# dario fosco 2012-01-04 05:57
nice tutorial thanks, I'm new in nuke, I followed everything but I'm still getting problem rendering out the final video. My previous works was mainly about rotopainting, Any tip?
 
 
# Isi Bruce-Clayton 2012-01-31 02:34
Hey guys,

Im new to Nuke and really want to specialise in it, this particular tutorial caught my eye this is quite a silly question to ask, but how do I import an image into the grid? i've tried to do it but the image doesn't show up on any of the folders?
 
 
# dario fosco 2012-01-31 09:34
Isi Bruce-Clayton: You need to switch from 2d to 3d in the drop-down menu on the top right of the viewer.
 
 
# fed x 2013-02-18 21:57
really nice! great way
 
 
# Junaum De Luca 2013-05-02 08:13
Nice, really nice! thanks!
 
 
# Llyr Williams 2014-11-12 21:53
The lightning effect is simply a ColorCorrect node added at the end. Gain is adjusted in the highlights only and in the Ranges tab the curves are adjusted to control the falloff (how much light is visible through the clouds).
 
 
# Adam Eddy 2015-06-23 15:38
Excellent work! - many thanks
 

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