Sink Hole Animation

I made a little animation which fakes a sink hole in the platform outside the Umeå Östra train station. This relies on camera tracking which takes a real scene and incorporates it in a simulated 3D environment.

Prerequisities 

A camera, (in my case a Canon 550D), some bright post-its, Blender (free and open source), Gimp or other image processing software and a few hours of modelling time and a nights sleep for rendering the scene.

Pick a scene

Figure out a scene. To make the camera tracking easiser to automate, pick a bright scene so that you will have short shutter times. Also, (which I failed to follow) try to keep your camera steady and make pans slow and steady to avoid blurry and unsharp images in your film. Record in highest possible resolution of your camera.

Object tracking

You must find some high contrast objects in the scene which you will use to track during the movement of the real camera in order for Blender to figure out the relative position of these objects. Preferrably use objects which will have parallax shifts when you move around in the scene. Also use a few objects that lie in your ground plane (the train platform) so that you can create a correct coordinate system.

In this scene, I want good resolution around the sink hole, but there are few high contrast objects, so I scattered some yellow pieces of post it notes around the area. 

Record your movie, import it into Blender and use the new Movie Clip Editor, which has come in one of the latest releases, to tag tracking objects. In the best of worlds, the software can follow the movement of the high contrast objects through the whole movie clip. Didn't work like a charm for me, had to manually help the tracker when the camera panned fast, so that's a note to self for future trackings. When you solve the system, Blender will figure out how the camera has moved during the photo shoot and create a camera animation for that for the virtual camera used later when rendering.

Modelling and rendering

I made a real simple hole model in Blender with some gravel textures overlaying eachother. Then by using the composition features of Blender you can merge each frame in the original movie clip with the rendered model with the correct camera position. In this example, the large disk in the image representing the ground is not rendered in the final composition, but it can receive renderable shadows, so that you can have your own 3D objects cast shadows onto the ground in the movie.

For reference, the original movie looks like

This is based in an excellent explanation of the Blender camera tracking functionality by Andrew Price on BlenderGuru.