It really does not really matter what 3D program you use for a 3D integration VFX. You can use 3dsMax, Cinema4D or any other program you like but
In 3dsMax I first created simple geometric shapes for the ground and placed a vRay camera in the scene with the same position, angle, focal length, aperture, shutter speed and ISO settings as the camera I used to shoot the scene.
I made sure that the geometry, once rendered, would fit nicely over the geometry (the pier) in the shot when being composited in After Effects. Test this first! It sucks if you find out when you try to finish your VFX that things don't fit as expected!
Here comes (the first) tricky part.
I want crows to come out of Celina's shape as she spins. I decided against having them shoot out of her entire body as I wanted to utilise the spin and have the crows emerge from her legs first, then her body, then her head just as she completes the motion.
This means I need to spawn the particles over time, from bottom to top of Celina's shape as she performs the spin. For this, I needed to create an animated texture that defines the areas where the particles spawn to be used in 3dsMax.
First, I added a Colorama effect to my rotoscoped layer of Celina and rendered it out in black and white:
Next, I animated a mask to move from bottom to top over the layer and rendered out the animation. It looks like a horizontal window is moving up the shot as Celina's white outline does a spin
My brotherNils once created a dissolve to crows effect for one of his short films and he was nice enough to let me use his awesome animated crow model for my particles
In 3dsMax I created a standard pFlow. If you do not know what that is, 'pFlow' is a particle system in 3dsMax. I set up a plane exactly where Celina was standing at the pier and assigned the animated birth texture to it.
I replaced the birth event in my particle flow with a Birth Texture operator and set up the particle to be emitted from the white areas of my animated birth texture. Next I added a Shape event and assigned the crow model to my particles. You will have to set up a few other things like animations, facing, randomness, etc to have the crows spawn (fairly) realistically out of the animated texture.
Here comes the (second) tricky part!
We want to render out the crows. We also want to render out the shadows they cast, however we do NOT want to render the ground. Since I used the vRay renderer inside 3dsMax, I created a material that would not be visible but catch and render only the shadows of any objects in the scene.
In vRay, you can create a vRayWrapperMtl and then assign a standard material as the base material.
Note that while you can see the ground in this example, in the final render using my vRayWrapperMtl, the ground will be invisible, but the crows' shadows would be rendered with their correct alpha value.
One more thing I rendered out (and I like to because it usally proves useful for creating 3D integration VFX) is a zDepth pass. This is a pass that renders a greyscale image coloured by how far objects are from your camera.
The further away from the camera something is, the darker it will be. This layer can be useful for a number of things when you composite your VFX in After Effects. You can use it to add and blend stock footage elements into your scene using zDepth compositing or you can use it for depth of field or other special effects
I used Render Elements provided by vRay to render this pass out separately.
Now it's time to return to Adobe After Effects to composite our original footage with the rendered layers to create the final dissolve into crows effect!