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Interactive Lighting is something that many people seem to forget about or don't want to bother with while they are compositing and putting together a shot. Interactive Lighting will make some of your composites 100% better as it really helps to blend everything together and sell the shot as real.
For example, if someone has a flame in their hand, it'll look like a composite. There is no life to it. You take the same shot, and have that fire cast a glow on the hand and on the face of the person, as well as faintly illuminating the close surrounding background and BAM! Suddenly that flame is looking more believable.
I've assembled a small tutorial based on a force lightning shot I did for one of my compositing lessons in school. I heavily used Interactive Lighting so I thought it would be a great way to show how it can be done really easy with just a few mattes, color correction and blurs.
Technique 1: Edge Highlights
Here is an example of how I got the edges of my Emperor character to be
illuminated by the lightning he's throwing. This is a useful technique that I've used a lot.
Step 1: Isolate your Actor
If it's a greenscreen element, then that's great, because you'll just have to pull a key. If your footage is shot on location, your going to have to go in and rotoscope. Either way, you'll end up with an isolated actor.
Step 2: Colour Correct
Now, Colour Correct your character to the shade that you want the glow to be. My actor will be throwing blue lightning, so I corrected him to a blue shade.
Step 3: Create a Matte
Leaving our colour corrected actor from Step 2, go back to the footage from Step 1, and make a matte of him, so he's white against black.
Step 4: Emboss
Take your matte and use a standard Emboss filter. Adjust it so the side your actor will be highlighted on will be covered in a thick white line.
Step 5: Colour Correct
Colour Correct your embossed footage of your actor so you keep the thick white line and you reduce the rest to black.
Step 6: Blur
Add a Gaussian Blur to the corrected footage, so you have a nice soft blur on your edging. Please note that the blur overlaps the edge of your actor and would spill into the background, this is no good and will be fixed in the next step.
Step 7: Multiply Layers
Take the matte you made of your footage in Step 3, and set your transfer mode to Multiply against your blurred footage in Step 6. The result is that your gaussian blur will cut off once it reaches the edge of your actor, so you'll get no spill into the background. Just a nice soft highlight on your edges.
Step 8: Matte
Now using your result from Step 7, use this as a matte against your glow corrected footage from Step 2. The result will be an edge highlight that's the correct glow colour.
Step 9: Put It Together
You now have a matte of your edge glow on it. Place your footage from Step 1 in behind your result from Step 8 and there you go, you have a character with highlights on them. Using a mask on the footage result of Step 8 allows you to adjust how much of the highlight is going to be seen, like in my lightning shot, where the amount is always changing to give a flickering effect.
Technique 2: Lighting with Footage
This is a really easy technique to get interactive lighting on your background using a piece of footage. If it's a flame in the palm of your hand, you'd use the isolated flame as your footage piece.
Step 1: Get Your Background
Use a clip of your isolated background without any actors on it. Mine was a screenshot of a set piece from another Star Wars fanfilm, which I photo edited so i could add windows to space and holes for lights.
Step 2: Luma Key
Now make a Luma Key of your background footage. Like all keys, luma keys operate on a greyscale, with black being non-transparent, white being completly transparent, and the grey scale in between. Adjust your Luma so the areas you want to highlight are pure white or almost pure white and most of the background is black. This was my result:
Step 3: Isolate Your Footage
Leaving your Luma Key for now, decide what footage your going to use to illuminate the scene with. Since this was a force lightning test, I used a rendered clip of my lightning against a black background.
Step 4: Colour Correct (Optional)
This step is optional, and is only needed if your footage is too dim and you want a brighter result. I colour corrected my footage so my lightning was a bright blue instead of the faint light blue that it is originally. You can always go back and adjust this if your result isn't to your liking.
Step 5: Blur
Now blur your footage using a Gaussian Blur so you just get shades of your intended glow colour.
Step 6: Scale
Now scale your footage so it takes up all of your screen. It doesn't really matter if it's stretched or not, just that you have a nice wash of varying shades of colour.
Step 7: Matte
Using the luma key from Step 2, use this as a matte against your scaled footage from Step 6. The result will be highlighted areas of your background taking on the colours of your glow.
Step 8: Mask (Optional)
This step is optional as well. If you have areas of your highlighted background from Step 7 that you don't want your glow to cast upon, just mask out those areas from the result of Step 7.
Step 9: Put It Together
Place your highlighted background overtop your original background from Step 1. Set the transfer mode of your highlighted background to Add. The result will be your room with highlights on it that flicker and change as your blurred footage plays.
Technique 3: Lighting with Footage, Optional Technique
In my sequence I have an actor thats struck by the force lightning, so he needs to have a glow cast upon him as the lightning wraps around his body. This is a variation of Technique 2.
Step 1: Isolate Your Actor
As before, you need to have your actor isolated. This is a DV Greenscreen shot that was a huge trouble to key, so his edges aren't the greatest.
Step 2: Luma Key
Just like in the last technique, you need to pull a luma key from your actor.
Step 3: Matte
Now, take your luma key and use it as a matte against the blurred footage from Step 5 of the last technique. If you haven't done that, follow Steps 3 to 5 under Technique 2. The result of this matte will be your actor covered in a glow.
Step 4: Put It Together
Now, take your matted glow from Step 3 and set its transfer layer to Add over top of the footage of your actor from Step 1. The result will be your actor being covered by a glow.
Note: I like to use Add because the glow colours come out more intense. Feel free to experiment with any other transfer mode such as Screen or Overlay.
These are the techniques. They don't take too long to do, and once you've done them a few times it will take you mere seconds to get all the glows and mattes. The following is a result of all the Interactive Lighting put together:
I hope this helps out everyone with getting some interactive lighting into the scene. I tried to leave the terms as general as possible, so it could be accomplished in any compositing program. Good Luck! I'll leave you all with the final shot with all the elements composited together: