A tessellation shader dynamically refines coarse geometry, transforming simple forms into intricate detail. Part of the Graphics Pipeline, it enables artists to create smoother surfaces and rich complexity with minimal initial data, enhancing visual realism on the GPU.
The core of tessellation involves defining tessellation factors that control the level of subdivision. These factors are often set in a specialized shader stage, such as the Hull Shader in DirectX or the Tessellation Control Shader in OpenGL:
// Example: Setting tessellation factors for a triangle patch
float tessFactorEdge0 = 4.0f; // Subdivision for the first edge
float tessFactorEdge1 = 4.0f; // Subdivision for the second edge
float tessFactorEdge2 = 4.0f; // Subdivision for the third edge
float tessFactorInside = 4.0f; // Subdivision for the interior of the triangle
After the tessellation factors are determined, a fixed-function Tessellator stage subdivides the geometry based on these factors, generating a higher density of vertices. These new vertices are then passed to the Domain Shader (DirectX) or Tessellation Evaluation Shader (OpenGL). This final programmable stage is where fine-grain details are applied, often by displacing the newly created vertices based on data like Height Maps or normal maps, creating realistic surfaces like terrain, character models, and organic shapes without increasing the base mesh complexity.
This process is crucial for implementing technologies like Adaptive Tessellation, where the level of detail changes dynamically based on the camera's distance to an object, optimizing performance by only applying high detail where it's most visible.
