Drag your explosion element into your composition to create a new layer.
Turn the explosion layer into a 3D layer. Using the top view, move it to the position indicated by the Motorcycle Marker null object - just like we did with the clean plate layer.
Return to your active camera view and scale up the explosion to fit in nicely with your scene. Position it so it sit just in front of all the chunks we created for the motorcycle.
I am actually going to add a second explosion element to make the detonation look more powerful. A simple way to do this is to duplicate the explosion layer we already added, select it, then drag your new stock footage onto it while holding down the ALT key. This will replace the contents of the layer without changing its position in the scene or any of its properties.
Feel free to position this layer any way you want to sit nicely in your shot. I've decided to change the blend mode of this layer to Add to blend it nicely with the layer underneath and give some more intensity to the explosion.
Next, let's create the light that is being cast by the bike disintegrating in a ball of fire.What we will do is create some layers to be used as <em>mattes</em> for our lighting.We will create some white matte layers first to define the areas where the final lighting will be visible.
For this, select your tracked footage and the 3D Camera Tracker effect assigned to it to reveal all the 3D track points. Move your cursor over the scene until you see a bulls-eye that matches the ground the motorcycle stands on. Right click and create a new solid.
Open the settings for the solid by selecting it and pressing CTRL/SHIFT + Y, change its colour to white and call it 'Light Floor'. Next, scale it up to cover most of the ground in the scene.
Add a rough mask over the area that would receive most of the light from the explosion and feather it out. Scrub through your shot to ensure the layer and mask cover all the areas you want to receive light in the final effect.
Now, animate the opacity of the layer to fade in when the explosion strikes and then fade out very slowly as the fireball disappears.
I am going to add a small mask just on the curb because the area would remain in shadow and it will make the light look a little more realistic. Set the mask mode to subtract to remove the curb part from the Light Floor layer and add a tiny bit of feathering.
Now repeat the process and create a light layer for the back of the scene. Again, select a flat area on the back wall in the tracked 3D scene and create a new solid.
Scale the solid up to cover the entire back area of your shot.
Again, animate the opacity of the back lighting matte to be in time with the motorcycle explosion
Playing back the scene, the ground the bike is standing on as well as the background of the scene should turn nice white and then the light layers will fade away.
Now let's create the actual lighting layers. We want to use the shot of the scene without the motorcycle in it for the lighting layer.This means that we need to pre-compose our base footage, the camera and the clean plate layer we added to hide the motorcycle.
Because we have other 3D elements in this composition and they depend on the camera, you first need to duplicate the camera in the composition.
Once you've duplicated the camera, select the clean plate layer, the original camera and the original footage layer and pre-compose them by pressing CTRL/SHIFT + C
I will call this composition 'Motorcycle Vanishing'. Duplicate the new composition and place the copy directly below the Light Floor layer. Set the blend mode of this copy to Add and set the track matte option to 'Alpha'.
Duplicate the Motorcycle Vanishing composition again, move this copy below your Light Back layer and set the blend mode to Add and the track matte option to Alpha. Both the Light Back and the Light Floor layers should now be set to invisible as they are being used as track mattes onto the two copies of the Motorcycle Vanishing layer.
The new lighting layers (Motorcycle Vanishing layer with Additive blend mode) will be visible wherever the layer directly above them is opaque so the white of our Light Floor and Light Back layers now controls how strongly we see the additive light.
That looks quite cool, but I think the lighting looks a little bit too cold. Let's add a bit of warmth to the light to match our explosion. For this, apply a Tint effect to your lighting layers. Change the Map White To colour to yellow or orange to match your fireball.
This looks much more like the lighting you'd expect from a fiery explosion. Adjust this to your liking.
It's a pretty cool effect already, but let's make it even cooler by adding some finer shrapnel particles to the explosion!