View Full Version : Relighting a live action fire scene in shake?

04 April 2009, 07:47 AM
So I have a scene in a student film I am working on that revolves around a campfire... the camp we were filming in didn't allow large fires on the grounds. because of this, we're forced to come up with another solution. so i had them shoot all the live action using 1k keno lights to give the characters all a diffuse light to start with, now i want to layer some type of color correction to add the lighting and flickering from the fire.

my idea so far is to multiply a ramp by a random and mask it out with a roto, then lay it over the original footage... i was thinking of maybe locking a camera off looking at a flat grid with procedural flames on it, both color and alpha, then layer that over the original? mind you, im a houdini artist, so shake is all greek to me...

my real question is how would you go about doing this? am i on the right track... at all? is there an easier more efficient? and where can i learn some shake script? thanks in advance guys!

04 April 2009, 06:19 AM
I'd say, first, shoot some (or find some) reference material! My first thoughts are that you could probably get away with something quite simple to just give the overall effect of the lighting shifts, rather than getting bogged down and making it too complex.

Gestalt is key in matte painting and compositing - the unified look of all the elements gives a greater effect than the individual elements of effects themselves.

As for the intensity, most compositing apps will let you do a pixel analysis of footage. So for example, you could get some footage of real flames, blur them quite heavily (so the pixel analysis doesn't have to work so hard) and analyse the luminance. You can then plug those figures into your colour correction/brightness to get the flickering effect

04 April 2009, 11:55 AM
So the lighting on the characters is more or less flat?

It is really dificult to re-light a scene in a 2d compositing package. Depending on how much your characters are moving probably the best way would be to make 3d dummies of them, track the original plate, make a camp fire sim in Houdini just to get the interactive lighting right over the characters, then import your camera and render some passes of the light produced by the fire as an alpha, probably breaking it down by character and also by the diferent materials the ligt is interacting with. then bring those alphas into your comp application and do a color correction using them, i guess you won't really have to animate any parameters here since the alphas are gonna give you the flickering already. Then you need to efine your fire element and comp it where it was suppose dto be originally

It makes sense in my head but probably is gonna be a lot of work,



04 April 2009, 11:59 PM
No no, you would still have to have some kind of roto on the characters to define where the lighting would go, but my gut feeling is that you could be reasonably loose and it would give the effect or impression you are after. Like I said, get some reference and see how the shadows play and how you make be able to fake it with lo-res roto.

Rigging and animating stand in characters and doing houdini sims seems a little over the top. You might do that for a feature with the budget and the expertise, but in my experience, for a student film I feel you will get bogged down in the technical and details.

I guess my point is, you would be surprised what you can get away with. As a student you tend to go the "easist" route (eg sims) which can lead to days of R and D and faffing about when you could have quickly solved the problem using a more lateral approach.

I might do a couple of tests this weekend and see what I come up with.

04 April 2009, 08:05 AM
Rigging and animating stand in characters and doing houdini sims seems a little over the top. You might do that for a feature with the budget and the expertise, but in my experience, for a student film I feel you will get bogged down in the technical and details.


He said he was a houdini airist didn't he?

I never said he shouldn't use roto at all. I just gave an outline of the steps I would follow to re - light a scene that has very diferent characteristics to the one he's after. loose roto will give you big blobby areas of diferent lighting, whereas camp fire tend to give ver contrasty areas and definde shadows. You would need a fair amount of roto to reproduce that effect if you don't have some of it in the original plate

As I stated in my post, I think the soultion I am giving is not the easiest, but still valid. and your "no, no" seems quite patronising and pretentious, as if your answer was the best solution for him, how do you know without seeing the origonal footage? we are speculating here and presenting solutions, this is not a popularity contest

04 April 2009, 10:50 AM
Settle down there matey. I meant "no no, I wasn't making myself clear", not "no no, that is the wrong way to do it"

If the op is a competent houdini artist, then rigging, tracking and animating convincing body doubles, then doing fire and lighting sims might be a viable option.

Personally, I would be inclined to try the quick and dirty approach first before committing to that, but that's just me.

Take a chill pill man, and re-read my posts in the spirit of trying to help out a fellow artist.

05 May 2009, 10:47 AM
For relighting for something like this, I would generally go with something like DoubleSupercool suggested... As long as it was shot with some kind of light coming from the same place that the fire light would be coming from, it should be doable. I've done this on quite a few shows, and, while it's a pain, it's not impossible.

To grade them through the light, you could either pull some kind of rough soft lumakey to get the lit vs shadow areas, or (and this has worked surprisingly well, as long as you're not pushing it too far), use a GammaPivot operator, pivoted around the shadow density (if this doesn't exist for you, you divide your image by the pivot point, then Gamma, the multiply it back by the pivot point.

Then you can use your flame footage and run it through a pixelanalyze to get the colour and intensity of the light on the people - that way when your fire flickers, the light on your people will flicker along with it.

CGTalk Moderation
05 May 2009, 10:47 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.