Dissolve Into Crows 3D VFX – Part 2

In the first part of this VFX tutorial I showed you how to set up a particle system in 3dsMax using Particle Flow to spawn a murder of crows. In this part we will be taking the rendered crows as well as the z-depth pass we generated and composite them with our base footage to complete this awesome visual effect!

Preparing Your After Effects Composition

What you need for this tutorial is:

• Your clean plate (the shot without the actor in it)
• Footage with your actor in it. You need to rotoscope the actor out for the duration of the effect since we will dissolve themin front of the clean plate
• The rendered crows from part 1 of this tutorial
• The z-depth pass for the rendered crows (this is optional)

In Adobe After Effects you need to create a new composition with your clean plate at the very bottom and the rotoscoped actor on top. Then import the rendered crows. Ensure that you right click imported crows footage in the project window and go to ‘Interpret Footage’. Double check that the frame rate matches the frame rate of your footage.

Now place the rendered crows on top of your 2 layers. Since we timed them to match our scene when we set up everything up in 3dsMax, the crows should be spawned with the correct timing.

The first issue you may encounter is the crows not fitting into the shot very well in terms of their colour. This is not usually that big a deal although it can save you some time to make sure the colour matches when exporting from 3dsMax. Since my crows came out rather blue, I applied a simple Hue/Saturation effect to the crows layer and drained a lot of the saturation and lightness to make the crows look nice and black.

Since in 3dsMax I only set up a deflector for the ground plane of the pier and not for the sides, I noticed a few crows penetrate the sides of my geometry. To fix this, I applied a number of animated subtractive masks to cover up the crows for those moments.

There might be other cleanups you have to do depending on your footage and the effect you are going for. Most of them you should be able to solve with simple techniques in After Effects.

I cut off the rotoscoped layer of Celina spinning at the moment when all crows have spawned. This causes her to disappear when the crows emerge - but she disappears rather suddenly.

Since we don’t want our actor to simply vanish from one frame to the next, we now need to add the ‘dissolving’ effect to our layer. For this we will use a very simple tool in Adobe After Effects: track mattes.

Dissolving Your Actor

Duplicate the crows layer and place it directly above your rotoscoped actor layer. Then set the track matte option of the actor’s layer to Alpha Inverted. The layer will now become invisible wherever the crows layer is visible.

Disable the crows layer temporarily and scrub through your footage. Your actor should now seem to crumble apart in the pattern of the crows spawning. This is not perfect since it’s not a real dissolve effect, but for what we are trying to achieve it will suffice.

To complete the dissolve effect, add an animated subtractive mask to sweep along your actors body and make them vanish in time with the crumbling dissolve effect. Scrub through your footage and you should now see your actor crumble apart and then vanish.

If you re-enable the crows layer on top of the actor dissolving you will notice that the crows perfectly cover up the crumbling effect. That is because the crows and the track matte layer are perfectly in sync. To fix this, select the track matte layer and pull it forward a little in the timeline to offset it against the crows.

It’s a subtle change, but you can now see your actor fraying out just before the crows emerge. A pretty cool effect!

While the crows all cast shadows in my composited shot as expected, I also wanted a big dark shadow for the entire murder or crows to add a bit of darkness to the effect.

You can do this by adding a simple black solid to your scene and masking out the area of your scene that should receive the shadow. Feather the mask out by a lot and then animate the opacity so the shadow appears as the crows are spawning and then disappears as the crows fly away.

Let’s look at using z-depth compositing to integrate our murder of crows with stock footage elements in Adobe After Effects!

Z-Depth Compositing

One thing I recommend when rendering anything from your 3D program for compositing in After Effects is to also export a z-depth pass. The z-depth pass contains depth information for you 3D elements (in relation to the camera) and it can be used very easily to realistically composite stock footage elements together with your virtual objects.

For example, let’s assume we wanted to add a big cloud of smoke to the scene, just where your actor is standing. You could simply grab some smoke stock footage and slap it onto your scene. You can find free stock footage elements for your own use either on Detonation Films free stock footage page or on Footagecreate’s free effects page.

In my example, the smoke sits on top of the crows, but even if you placed the layer below the crows, you would notice that the two layers don’t interact - the crows would sit either fully on top or fully behind the smoke.

We can use the depth information from the z-depth pass to more realistically integrate these two layers. This technique is referred to as Z-Depth Compositiong.

Import your z-depth pass (making sure the frame rate matches your footage) and drop it on top of your crows layer. The two layers should be perfectly in sync. When working with z-depth passes, I recommend switching your project over to use full 32bit rather than the default 8 to give you finer control. You can do this by alt-clicking twice on the ‘8 bcc’ label just below your project window.

The z-depth pass is a greyscale image that visualises the distance of the crows from the camera. By using this layer as a matte onto any stock footage elements we place in the scene, we can fake an interaction between the 3D rendered elements and our flat 2D stock footage.

The great thing about the z-depth pass is that you can change it on the fly during compositing. For this, simply apply an Exposure effect to the layer. You can change the Exposure, Offset and Gamma Correction properties to change how strongly the depth information of your layer is presented, how much depth it has (e.g. expand or contract the grey scale range) and how harsh the edges of your objects will appear.

I have tweaked mine so only the crows that are closer to the camera than where Celina is standing, where I will place the smoke stock footage element, are visible in my z-depth layer.

Finally, set the track matte option for the smoke layer to ‘Luma Inverted’. The smoke will only be visible where the crows are further away from the camera than where Celina is standing. This conveys the impression that the 3D crows are obscuring the smoke layer when they are in front of it, but are being obscured when they fly behind it.

To show you the true power of the z-depth pass, try changing your Exposure effect. You will notice how it pushes the smoke layer further to the front or the back of the crows. This gives you full power over where in your scene you want to position your stock footage elemenets in relation to the rendered 3D elements.

I just wanted to give you a small example of how to do z-depth compositing. Feel free to delete the smoke layer again if you don’t like it.

There is one more detail I want to add to our dissolve into crows effect and that is a small dark shadow that rushes over Celina’s body just ahead of her fraying out and dissolving into crows. This will add a bit more detail to the VFX and make it look more interesting.

Adding A Detailed Crow Shadow

To achieve this, duplicate the rotoscoped layer of your actor as well as the crows layer. Select these two layers and then precompose them. In my example, I called this new composition ‘Dark Shadow’.

Jump into this new composition. You should see the crows being spawned on top of your actor. Set the track matte option for the layer with your actor, the bottom one, to ‘Alpha Inverted’. This should hide your crows layer and you should get a clip of your actor dissolving.

Offset the crows matte just a little bit to the left so the pattern of crows is a few frames ahead of the crows layer in the parent composition. Otherwise she shadow will be perfectly covered up by the crows.

Return to the parent composition, select the Dark Shadow layer and apply a hue and saturation effect to it. Bring down the saturation and the lightness considerably to make this overlay layer nice and dark. If you now scrub through your composition, you should see a dark shadow precede the dissolve effect.

You can tweak the timing of the dissolve effect by either offsetting the Dark Shadow layer or by offsetting the crows matte we used inside the Dark Shadow composition. I reduced the offset to only a couple of frames to make the dark shadow a little less obvious and I also lowered its opacity to about 50%. Feel free to tweak this to your liking!

And that is how you can dissolve a person into a murder of crows. Creating 3D integration VFX can become quite complicated and time consuming. If you are looking for a more detailed introduction to creating these types of effects, check out my 6 part 3D Integration VFX series.

Feel free to leave any questions you may have in the comment section below.