No Hacksaw Required: Digital Amputation Using Blender

Written by William

August 1, 2023

In our upcoming short film, there is a scene in which an apocalyptic mailman (called the "British Creature") interacts with the main character through a doorbell video feed. Partway through, it is revealed that he is missing his arm. As the VFX guy, and also the actor that plays the creature, I decided it would be most practical to do this in post-production rather than relying on other methods.

We'd already spent quite a lot of time on the practical side of things. The makeup and prosthetics served as an excellent foundation for building the VFX. We also wrapped the arm we planned on amputating with tape and drew on rudimentary tracking dots with a marker.

The first step to creating this effect was to model a stump in Blender, that could then be motion tracked onto the arm. Since I'm not that great at modelling, especially organic shapes, I opted to 3D scan my own arm and use that as a starting point. The scan included much of the surrounding room, however we focused our data capture on the forearm.

I then extracted a small section of the geometry from the arm and cleaned up the geometry. The remesh modifier built into Blender worked very well in this instance, since I needed to fill in the open ends of the stump model. Instant Meshes can also be useful, plus it's free and open source.

Once the mesh was tidied, I used some free brushes from BlenderKit to sculpt the damaged end of the stump. I also cut the stump down in length since most of the arm was not going to be needed for the effect.

For texturing the skin, I merged a few sample photos of my arm in Photoshop. I also grabbed a meat/flesh texture for the wounded end of the stump. To blend between the two, I first created a mask texture that would describe where to paint wound and where to paint skin. I then merged it with a "cracked rock" texture that would create the splattered blood around the borders.

With a subtle use of subsurface scattering and some bump maps to create texture, I'm pretty happy with the final result.

For the final composition, I'll be motion tracking the stump into the scene and layering it with some additional effects such as the occasional blood splatter. Some blending will also need to be done, which should be aided by a heavy use of overlays.

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