Written by Julian Pössnicker on April 2022
A few scenes of the music video “Let me dream” include a glass terrarium that gets shattered. Using a real shattered glass was not really an option, because the crack in the glass should expand over time and in a certain direction. Would the glass be prepared for breakage onset, the speed and the direction in which the crack would expand cannot be fully controlled. It could also be a safety issue for the actress and the crew. So, this effect had to be done with Visual Effects.
Onset we prepared the scene for this effect. Tracking markers were placed on top of the glass as a guideline for the actress to look und of course for the motion tracking. Without the tracking markers applied the glass would be impossible to track. Because glass is normally transmissive.
After the motion tracking and marker removal the glass shattering effect can be added to the clip. For a convincing effect the following elements must be added:
The glass has a thickness and therefore the crack has a thickness. The crack must be in 3d because the camera doesn’t look straight onto the glass. The parallax of the crack has to be visible.
Additionally, the cracked glass elements would distort the visible things behind it. Because the elements would move in directions and the refracted parts would change.
The edges of the cracked glass elements would reflect different parts of the scene.
For the look development, an Action Essentials shattered glass texture is composited onto the clean plate. The following picture shows the result:
The composited 2d cracks were not convincing. The parallax and the inner reflections are not present.
To get a convincing effect the crack has to be done in 3D. To get realistic lighting and reflections, multiple pictures are taken on set and then combined into a 360-degree HDR environment map. The glass plate is modeled and fractured with the cell fracture plug-in in Blender. The shader is fully transparent. Along the crack edges, a glass shader is applied.
For the following compositing step a mat-cap version is rendered out:
The mat-cap is normally used as a masking channel. All objects get a different color assigned. If for example one color is keyed in the compositor, the black and white mask can be used to isolate the object. For this effect, it is used differently.
The first step in compositing was distortion. The mat-cap is used as an input for the Displacement Map effect in After Effects. This effect transforms the pixels in certain directions guided by the input color of the displacement map. For example, the red parts of the mat-cap move the pixel of the clip to the left by 12 pixels. But each color has a different direction. In the end, each broken glass element distorts the image differently.
After that, the rendering of the shattered glass is composited over the displacement. With a few curves and glow effects, and additional reflections on the left-hand side for the left glass, the shattered glass is a lot more convincing compared to the simple broken glass texture. The result can be seen in the following video:3D Academy Color Encoding System ACES After Effects Blender Cycles-X glass HDR let me dream Mocha motion tracking shattered VFX visual effects