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Reactive Lighting

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Spencer TycksenReactive Lighting
by on Feb 13, 2012 at 5:10:30 pm

Hey Everyone,

I've got a truck scene where I need to add some reactive lighting. We created the shot with the subject sitting on the back of what is supposed to be a moving truck. The truck was stationary while we shot. To help create effect that the truck is in motion, I would like to add some reactive lighting (i.e. passing overhead street lights, etc.). I figured I would experiments with simply adding light layers and adding motion to them to see if that would work. But before I did that I thought I would ask on here for suggestions. Thanks in advance!

Spencer


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Tudor "Ted" JelescuRe: Reactive Lighting
by on Feb 13, 2012 at 6:01:47 pm

There is no easy way of doing this. If you want to play with AE Lights, I would try to roto the character. Then create a depth map with small details for the back of the truck and using the mask from the roto, a basic depth map for the character. When creating the depth maps make sure that the levels of grey correspond to the position in z space of the items in your scene (for example, the lightest gray on the back of the truck should not be lighter than the darkest gray on the character.
You can use then FreeForm Pro to displace the image and 3d AE Lights animated towards the camera to create highlights and shadows.
This tutorial may help if you're not familiar with FreeForm Pro and displacement (it uses Free Form- the stock version but the principle is the same):
http://library.creativecow.net/jelescu_tudor/3D-from-2D-image_AE/1

Tudor "Ted" Jelescu
Senior VFX Artist


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Tudor "Ted" JelescuRe: Reactive Lighting
by on Feb 14, 2012 at 9:33:40 am

Can you post a still?

Tudor "Ted" Jelescu
Senior VFX Artist


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Michael SzalapskiRe: Reactive Lighting
by on Feb 13, 2012 at 7:02:49 pm

In the future, this is much easier to do on set. Just take some lights and move them around.

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.


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Andrew SomersRe: Reactive Lighting
by on Feb 13, 2012 at 10:10:57 pm

I'm going to echo what everyone else is saying - use some ellipsoids on set, with a grip swinging it across the subject from time to time to give the motion.

If you don't have an ellipsoid you can also use an old slide projector. The idea is to use a lamp with a highly focused beam, so that it can be moved over the subject, and has little to no spill.

If you REALLY want to make it work, get some video projectors, and run footage through them of driving, and bounce them off the windshield.


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Conrad OlsonRe: Reactive Lighting
by on Feb 13, 2012 at 10:25:39 pm

Using 3D lights in After Effects is probably over kill. It's not like you have a 3D model of the tuck or the actor that you can use to cast shadows.

You can probably achieve a lot with some 2D effects.

Find a piece of footage of a view from a car window at night with lights going past. Grade it to crush out all of the darker details and leave you with the lights, then blur it and add it over the shot of the actor. Mess around with the blending modes (try add first) and the opacity. This will give you some moving lights over the actor.

To give it more of a directional look pull some luma keys of the actor or create some roto shapes on one side of their face and apply them to the lighting layer before you add it to the original layer.

---

conradolson.com


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Andrew SomersRe: Reactive Lighting
by on Feb 13, 2012 at 10:29:28 pm

HOWEVER: If you want to add this in post, and make it look credible, you can:

1) Do a complete 3D track of the shot.

2) Track the objects in the shot too.

3) Import the track and the tracked objects into Cinema 4D/Maya/3DSmax or another 3D application.

4) Build the objects so that they match the real objects "fairly" close - but high detail is not necessary.

5) Color the objects a simple neutral grey only. Adjust the reflectivity/specularity etc. so that it is similar to the real objects.

6) Animate some lights over the objects to emulate street lights, etc.

7) Render out a pass with NO ambient lighting, only your animated lights. what you should get is a mostly black image, with white where the lights hit only.


Now the trick:

8) In AE, create an adjustment layer and put EXPOSURE on it.

9) Take the render pass from the 3D app above, and place it immediately above the adjustment layer.

10) Set the adjustment layer to LUMA key, so that the "light" pass keys the adjustment layer.

11) Adjust the exposure plug in on the adjustment layer to substantially brighten where the lights are hitting.




ALTERNATE METHOD:

Step 1: Reshoot with real lights on set.


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Conrad OlsonRe: Reactive Lighting
by on Feb 13, 2012 at 11:13:22 pm

This is overkill. Some 2D roto shapes and some luma keying will give you a decent enough result.

---

conradolson.com


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Andrew SomersRe: Reactive Lighting
by on Feb 14, 2012 at 12:17:43 am

Conrad stated: "This is overkill. Some 2D roto shapes and some luma keying will give you a decent enough result.
"

Is it? Maybe - maybe not - I haven't seen the shot. Without seeing it, and without knowing the context that it will play in, you can't say if it is an approach that is useful. 2D roto shapes may or may not work well, and you don't know what "decent enough" will be for the shot/client/film etc.

The point with my post is that this kind of effect is *easiest* to create as a practical on set.


Nevertheless, 3D light catcher/shadow catchers are not difficult to make - and while creating catchers may consume some time, getting roto shapes to work correctly and look convincing also takes time.

Relatively flat light/shadow catchers can be made directly in After Effects (walls, ground, even car hoods, can be cheated this way, etc).

It depends on the shot and the desired effect if the catcher method is the way to go, vs. rotos or other 2D methods.


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Conrad OlsonRe: Reactive Lighting
by on Feb 14, 2012 at 12:27:15 am

[Andrew Somers] "Without seeing it, and without knowing the context that it will play in, you can't say if it is an approach that is useful. 2D roto shapes may or may not work well, and you don't know what "decent enough" will be for the shot/client/film etc."

That's fair enough but you haven't seen the shot either and are suggesting the most complicated solution possible. Why not go with the simplest solution first and then work up from there if that doesn't work?

[Andrew Somers] "The point with my post is that this kind of effect is *easiest* to create as a practical on set."

True. But it's already shot. It's not always practical to do things on set or to re-shoot things. As much as we hate 'fix it in post' it happens all the time. Spencer's original question was about how to fix this in After Effect, not how should he have shot this in the first place.

---

conradolson.com


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Andrew SomersRe: Reactive Lighting
by on Feb 14, 2012 at 4:37:37 am

Conrad: "and are suggesting the most complicated solution possible. Why not go with the simplest solution first and then work up from there if that doesn't work?
"

It's not the "most complicated" possible - Having a ton of manual paint/roto is complicated too, not to mention labor intensive.

Manual paint/roto can work well, and it generally the easiest solution for really fast light changes, like camera flash pops, muzzle flashes, etc.

But here we are talking about things like overhead street lights coming in from a distance, then passing,, then leaving for a distance. If the track is traveling at a constant speed, the speed of the light passes will be "variable" in that they start slow, speed up to the pass, then slow down post-pass, as the of the angle of incidence changes non-linearly.

The lights create highlights, and also cast shadows. The shadows in particular move a lot relative to the passing light(s), getting longer, shorter, deforming on the walls and across objects and corners.

Making a bunch of rotos to emulate light and shadow, and animating them to create a convincing effect is complicated and time consuming.

On the other hand, creating a 3D track in something like PFTrack is relatively trivial.

Using the track data to locate shadow catchers is also fairly trivial. The most primitive method - using flat 3D layers in AE with masks to shape the layer to the set geometry is also straight forward.

Animating the AE lights in this situation is trivial. This level is simple, and works surprisingly well. flat layers work find as shadow catchers even on curved object like car hoods. And if the object is static in the scene, these catchers don't even have to be animated.

In the case of a person in a truck bed, you could have catchers for the sides and floor, and then just deal with the body of the actor with a couple mask-shaped layers that bisect and are at 90 degrees to each other (i.e. the paper cutout man trick).


To improve the results, the next step up is to use 3D objects instead of flat layers - primitives, or something more complex like a body - detail is not critical for shadow catchers (since these are to be used to control an exposure adjustment layer), and so library mannequins can typcially be dropped in. An app like C4D is better suited to this more advanced method than AE, as AE's object support is poor - but using a 3D app like C4D that does not mean the method is overly complicated.

Both the AE flat-3D-layer and the C4D 3D object catcher method allows for things that are hard to mimic as a 2D paint job. Specularity on shiny objects, shadow,s and highlights that all move together in response to the lighting conditions, against the objects and set, and without concern for camera motion.

Let's assume his camera is handheld an/or being rocked to mimic truck motion. Or it has a lateral move. Now not only are you trying to animate masks to emulate light over the course of the truck passing under a light, but also keep those masks oriented against the objects they need to affect while the camera is moving.

So, you are going to have to track the shot so you can have a layer with masks that move relative to various objects/set elements. Well, if you have to track it for the roto/paint method, then why not just use a good 3D track and use catchers?


Here are some advantages of using 3D catchers:

1) You can add, change, or remove lights at a later time without having to re-do a ton of animated rotos - all that is needed is a new render pass after the lights are changed. SInce you (in most cases) only need a single channel, and not ray traced, this is a quick render.

2) You can add atmosphere/smoke to the lights as a separate channel to increase scene integration.

3) You can have realistic reactions on specular, reflective, transparent, etc. objects.

4) Since you have the environment at this point, it's trivial to add in reflection maps

5) You can use multiple channels for different lights, and then use the different channels to have color variations between the light sources, as well as using separate channels for the left and right side of the truck, to allow for greater control in the final comp.




Now in contract, the roto method may seem simpler because it does not need any 3D build, and sure, it may be a fine cheat on very short light passes, like a car whipping by in the opposite direction - but how is it going to look on extended light passes that take a few seconds to complete? - these slower, natural light passes are going to be harder to make convincing than a quick flash of light.


Here's an example of real night driving:







This is just some youtube video of a guy driving at night, under street lights. Note the complexity of the interplay of light and shadow. Note that a typical light pass is two to three seconds long over the small space of the dash board, but this would probably play longer over the entire space of the truck bed.

To emulate this look with a bunch of masks is quite complicated - I will not agree that masks are some how "simpler" just because they are 2D. You still have to track, animate, adjust, and spend time on this.


---


Having said all of this, I use 2D cheat techniques whenever appropriate, or a combo of 2D and 3D, etc. It all depends on what kinds of compromises you need or want to make. And certainly it depends on your staff and their capabilities. And it depends on the shot.



Conrad: " you haven't seen the shot either and are suggesting the most complicated solution possible. Why not go with the simplest solution first and then work up from there if that doesn't work?"

The method I outlined in my previous post details a method that will give good results regardless of what the shot is. Now, if that shot is setup is such a way that a 2D cheat will work, sure - use the cheat. But the OP asked for how to make lights "reactive" after the fact - shadow & light catchers are the easiest way to reliably create an element to do this. You may not have tried or used the workflow, but I do and in fact I am doing a series of shots right now using catchers. 2D masks will not work in most of these shots, while the catchers are nearly effortless to setup and provide excellent results.


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Roland R. KahlenbergRe: Reactive Lighting
by on Feb 14, 2012 at 5:19:28 am

You may want to look at using a gobo effect using AE's Lights. Here's a write-up -> http://provideocoalition.com/index.php/cmg_keyframes/story/gobos_and_gels/ that should help.

Cheers
RoRK

Intensive AE & Mocha Training in Singapore and Malaysia
Adobe ACE/ACI (version 7) & Imagineer Systems Inc Approved Mocha Trainer


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