Overdraw
Redrawing same pixels
Definition
Overdraw happens when the GPU shades the same pixel multiple times in one frame — once per overlapping transparent or alpha-tested surface in front of it. Particles, foliage, hair cards, and stacked decals are the main culprits. A scene with heavy overdraw can be CPU-cheap but GPU-fragment-bound: framerate tanks not because of triangles, but because each pixel is being painted 10 times over.
In production
UE5 visualises overdraw via "Shader Complexity" view mode (or the dedicated Quad Overdraw view), which colours pixels hotter as more shading passes stack on top of each other. The fix is usually to reduce particle count, simplify foliage alpha-tested geometry, or convert hair cards to opaque-with-cutout shaders where art direction allows, since alpha-blended surfaces can't be early-Z-rejected the way opaque geometry can. Unity's Frame Debugger and RenderDoc both let artists step through draw calls to spot exactly which layered transparent objects are causing the spike. A classic overdraw trap is a forest scene built from broad, overlapping alpha-tested tree-card billboards viewed at a grazing angle — the GPU ends up shading the same screen pixel five or six times per frame even though total triangle count is trivial, and the framerate drop can look identical to a CPU bottleneck unless you check the GPU profiler specifically.
