- pralhad on Colour Smear for Nuke (UPDATE v2.0):
i use this node education parpose
- Francois Leduc on FrameBlendMerge:
You could also use a TimeEcho node. There’s no Min mode, but to fix that, apply a Invert Node to your source, plug TimeEcho (in Max method) and apply another Invert node after. Go at the end of your timeline, change the “Frames to look at” to the number of frames or your shot (or less) and you should get your clean plate. Of course all of this is done on a stabilized shot.
- Richard Frazer on Colour Smear for Nuke (UPDATE v2.0):
Hi Josh. Thanks for the feedback. You have correctly discovered that this tool works best when you have a solid core for a matte with a feathered edge. Where it fails is if you have large areas that just have semi transparent alpha (such as your grimy window). I’d approach this by separating your actor with a rough roto and using the colour smear to deal with their edges. Then for the smudges maybe try extracting the green channel and using using it to drive a grade for your background, or...
- Josh Northeast on Colour Smear for Nuke (UPDATE v2.0):
Hey Richard! Absolutely love the tool. Saved my ass alot. I’m working with some greenscreen plates where there is a smudgy window behind the actor and a greenscreen behind that. We need to preserve the smudges on the window but that means it’s hard to use your tool to treat the edges because the alpha isn’t clean. Any tips? Cheers, Josh
- Matt on Keyframe Reduction script for Nuke:
Nice! Just used this on a projection/stabilization job and it worked great to simplify the original camera keyframes and smooth out the reprojected shot. Thanks!
- pralhad on Colour Smear for Nuke (UPDATE v2.0):
(sl)Edge-hammer to reduce light wrap
June 03, 2012
Here’s a quick tip that I learnt at MPC to help integrate composites where light wrap is causing a problem. It’s a fairly common trick, but I have Traian Constantinescu to thank for showing me. He calls it the “(sl)Edge Hammer”, because using a clamp node to perform a colour correct is a pretty brutal technique and “like using a sledge hammer to crack a nut”. It’s certainly a lot more crude than my Colour Smear technique, but is a quick a simple solution that will often get the result needed.
So a lot of the work that MPC did on Prometheus was modifying exterior landscapes (the movie has only just been released, so I’m still quite restricted in what I talk about with regards to VFX. I will post some of my shots later in the year once the footage has been officially released and cleared – until then keep an eye on MPCs website for VFX breakdowns). Many of the plates started with actors shot against a sky, but once the alien landscapes had been digitally added in, they ended up in front of a mountain or other dark background. This often meant that the rear lighting from the sky in the original plate left a halo that wrapped around the actors – a sure giveaway that the background had been altered. There are many ways of approaching how to fix this and this certainly won’t work in all cases, but this was a trick that I found useful on a number of shots.
Let’s start by comping something that roughly shows what I mean.
As you can see – the edges are very bright, but there is no lighting in the background that justifies the light wrap.
Now, where there are areas with a consistent colour, we can isolate the edges and treat them. The first thing to do is create an edge mask that matches the falloff of the light wrap. I’ve used a gizmo called FineEdgeDetect, as it allows for separate control of the inside and outside feathering of your edge detect. Here is the result.
The subjects jacket is an ideal area to treat, as its a fairly consistent tone of red. I add a Clamp node and for the “maximum” value I will use the colour picker to select a value that is close to the edge area of the jacket, but not so close that it is effected by the light wrap. You should spend some time adjusting this colour, as its a balancing act to get a value that isn’t to dark or light. You will also want to keyframe this value over time, as the colour of the jacket will change as the subject moves.
The result is pretty ugly looking, but we now matte that through our edge mask.
If I Merge this back over the comp it gives us a nice consistent tone of jacket colour right up to the edges. As the Clamp uses a single value, it won’t match the entire outline, so I also use a roto to restrict the areas where it is applied.
I then repeat this process several more times for the rest of her jacket, each time sampling a new colour to Clamp to, and using a roto to restrict the edge treatment area.
The end result shows a considerable reduction in the light wrap, and the foreground and background sit together a lot better. The obvious restrictions are that her face and hair not easily treated using the same technique (it works best on areas with little texture).
Using a Clamp is a very destructive technique, as all colour values lower than those sampled will be lost. Use this treatment sparingly! It is also a very quick way of destroying your grain, so be sure to re-grain your treated edges.
You can download the Nuke script and images here to see how I set up the nodes for this demo.
All images are taken from Flickr and used under the Creative Commons licence.