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Compositing: Giving Depth to Stock Footage
Added on: Sun Aug 27 2000
Page: 2 3 

Lets say you wanted to make an explosion, blowing an object up and making debris fly at the screen, of course rather than using CG fire which a lot of the time looks, well... CG. You might want to use stock footage, video footage of fire which you can overlay into your 3D scene making the explosion look a lot more realistic.

Of course the first problem of doing this is the fact that the footage is 2D!
It's a flat image, rather than being 3D and having depth, so throwing it into your scene will look cool, but if you want it to interact with the geometry and have debris fly through it it's going to be quite difficult, because it's either behind the explosion or in front, it won't be inbetween since it's just a flat 2D sprite, so that's going to look quite fake.

So that's what this tutorial will cover, how to make debris and objects shoot out of an explosion and actually look like they shoot out, and also look cool and easy to set up. After this tutorial I might get onto writing a really neat technique for doing the same
thing, but at a more advanced level. Check out this video clip here, explosion.avi, which is just a test using this technique, but gives you an idea of what you can do.
Anyway the other technique uses a plugin from Cebas called Real Lense Flare although whereas the technique in this tutorial just uses max 3's standard tools.

To start off we're using this standard 3dsmax scene which I've created for you just for the sake of focusing more on what this tutorial is meant to cover.

Load up depth.max after you download it here:

This file is rather simple, a house which explodes, what we want to do is set up the depth of it, so that we can create a lumience map for the debris so that it controls when the debris is visible and when it's behind the firey explosion.

This is a pretty simple process but go to the effects panel and click on file output. A pop up screen similiar to the one below should apear.

Go down the parameters and adjust the channel to depth(greyscale) and if effect source bitmap isn't selected enable it, and deactivate the fit entire scene button so that you can adjust the parameters, click on the none button and select camera 1.

Now adjust the near Z value to -7.381 and far Z to -337.921. Click on the interactive button to see what this does, essentially what it does is create a 256 grey frame based on the depth of the objects to the camera and the values control what is and isn't visible from the distance of the camera so you can tell the debris that is within a certain distance to be visible and the rest not to be, and use this as a lumience map.

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