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Advanced Decal Systems for Bloody Footprints

· 2 min read
Thang Le
Thang Le
Senior Lead Engineer

Decals

The Trail of Terror​

Bloody footprints are a staple of horror storytelling. They guide the player, build tension, and tell a silent story of a struggle. However, in a game where both the player and the monsters can leave dynamic trails, managing thousands of decals can quickly become a performance nightmare.

The Problem with Standard Decals​

Unity's standard Decal Projector is powerful but can be expensive when used in high quantities. Each projector is essentially a specialized camera pass. If you have 500 bloody footprints in a hallway, you're looking at a significant hit to your draw calls and GPU fill rate.

The Solution: GPU-Driven Decal Atlasing​

For Lil Sis, we developed a custom decal system that bypasses the standard projectors for small, repetitive details like footprints.

  1. The Atlas: We store all our blood variations in a single high-res texture atlas.
  2. The Mesh Batcher: Instead of a "Projector," we use a single mesh that is generated on the fly. When a footprint is created, we add its vertices to a Dynamic Mesh. This allows us to render hundreds of footprints in a Single Draw Call.
  3. Projector-Less Projection: We use a custom shader that performs the projection logic in the vertex shader. By passing the ground's normal and position data, we can "shrink-wrap" the footprint mesh to the surface.

Fade-out and Memory Management​

To prevent memory leaks, we use a Circular Buffer for our decals. When the buffer is full, the oldest footprint simply fades out and its vertices are repurposed for the newest one.

// In the decal shader
float alpha = smoothstep(_MaxLifetime, _MinLifetime, v.age);
col.a *= alpha;

This system allows us to have rooms covered in blood and footprints without the player ever noticing a dip in performance. It's a perfect example of how "technical tricks" can be used to support "narrative atmosphere."