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Dynamically subdividing polygons into smaller triangles to add geometric detail exactly where and when it is needed.
Tessellation takes existing polygons and splits them into smaller triangles on the GPU in real time, then displaces those new vertices using a height map to add actual geometric detail. Unlike normal mapping, which only fakes surface detail, tessellation creates real geometry that has correct silhouettes, casts proper shadows, and interacts with lighting physically. A flat plane with tessellation and a displacement map can become a detailed rocky terrain with actual bumps and crevices. The GPU's tessellation stage controls how much subdivision happens based on factors like camera distance, allowing surfaces to become more detailed as you approach them.
Example
Crysis 2 and 3 were early showcases for hardware tessellation, with concrete barriers and walls gaining actual geometric detail up close. The Witcher 3 uses tessellation on terrain and water surfaces to add physical depth to the ground Geralt walks on. Batman: Arkham Knight applied tessellation to Batman's cape and suit, giving the fabric real folds and creases that moved with the character rather than being painted-on texture details.
Why it matters
Tessellation bridges the gap between the visual fidelity of normal mapping and the geometric truth of high-poly modeling. It adds real detail where it matters most without requiring impossibly dense base meshes. While Nanite and other mesh streaming systems are changing the geometry landscape, tessellation remains a valuable tool for adding detail to surfaces that need to deform or interact with physics.
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