Overdraw
Drawing the same pixel many times.
Every transparent sprite, particle and UI panel covering a pixel makes the GPU shade that pixel again. Ten layers of smoke over the screen can cost more than the whole level. Overdraw view shows the hot spots; trimming transparent quads to the visible shape and using fewer, smaller particles fixes them.
Try this: Hold Cast to fill the room with smoke. Big see-through quads stack up and the frame time climbs. Turn on the heatmap to see pixels drawn 40 times, then try Trimmed puffs.
Big transparent quads stack up: every pixel gets shaded again and again.
Prompt bit
Works for 2D or 3D in any engine (Unity, Unreal, Godot, web). Paste the short one for a quick fix, or the detailed one when you want it done properly.
Cut overdraw: check the overdraw view, trim transparent sprites to tight meshes instead of full quads, cap screen-filling particles to a few large ones, draw opaque objects front to back, and drop effects that fade out over 2 s or longer at low quality.
Reduce overdraw (2D or 3D; Unity Scene view Overdraw mode, Unreal Shader Complexity and Quad Overdraw views, browser dev tools for canvas). Look at the overdraw view in the busiest scene. Fixes: use tight sprite meshes (Unity "Tight" mesh type) instead of full rectangles; prefer fewer larger particles over many small transparent ones, and lower the max particle count at low quality; avoid full-screen transparent UI panels that stay hidden behind others (disable them); render opaque geometry front to back so depth testing rejects hidden pixels; use alpha cutout instead of blending where it looks fine; render expensive effects at half resolution. Re-measure GPU time after each change.
Where it's used
- Mobile games: fill rate is the usual bottleneck
- Particle-heavy games: smoke, fire and magic
- 2D games: big transparent sprites and layers
- UI: stacked panels and backgrounds
- VR: high resolution makes overdraw expensive
Common mistakes
Full-screen transparent quads for small effects, hundreds of tiny overlapping particles, hidden UI canvases that still render, sorting opaque objects back to front, and optimizing without looking at the overdraw view.
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