Tessellation and Displacement: Build Detailed Surfaces Without Cracks

Separate subdivision from surface definition

Tessellation creates additional samples of a patch; it does not automatically invent useful detail. A surface evaluator determines where those samples belong, and displacement can move them using a height function. The resulting triangles can change the visible silhouette. Start with a clear surface definition and height scale before increasing subdivision to hide artifacts.

Direct3D's tessellation overview separates patch control, fixed-function domain subdivision and evaluation of generated vertex positions. Other APIs use different stage names, so map responsibilities before porting shader code. [1]

Make neighboring patches agree

Cracks appear when adjoining patches disagree about where their common boundary lies. Shared edges need compatible subdivision factors and spacing, plus an evaluator that produces matching positions. Vulkan's invariance rules describe which edge samples are guaranteed to agree under matching conditions. They do not make inconsistent displacement functions agree. [2]

Use the same inputs when computing a shared edge's factor. A factor based independently on each patch center can differ across that edge. Check boundary height samples, texture coordinates and displacement directions as well: matching sample counts cannot repair a height discontinuity.

Worked example: count a defined grid

Consider an illustrative square divided into a regular grid of 4 segments per side. It contains 16 small quads, or 32 triangles if each quad is split once. Increasing to 8 segments per side produces 64 quads and 128 triangles.

The triangle count increased by a factor of 4 when linear subdivision doubled. This is arithmetic for the stated grid, not a promise about every hardware tessellation pattern. It shows why a visually small factor change can create much more geometry. If both sides of a seam generate matching samples but one evaluates a boundary height of 0.02 metres and the other 0 metres, a gap still remains.

Validate a seam before an entire landscape

Build a test with two adjacent patches and a height feature crossing their boundary. Inspect wireframe, silhouette and shading while moving the camera. Change subdivision gradually and verify that the seam remains closed. Then test texture seams and nonuniform object scaling.

Choose a texture sampling level deliberately in the surface evaluator and keep the height definition consistent at shared edges. Update displaced bounds, and ensure normals describe the displaced surface well enough for the intended lighting. Test the same displacement in every pass that needs matching geometry.

Limit generated work deliberately

More triangles do not guarantee more visible detail. Tiny primitives, expensive displacement evaluations and transition instability can dominate the result. Measure geometry generation and downstream shading together. Clamp subdivision to supported limits and provide an appropriate path on unsupported devices. Rendering displacement also does not automatically update collision geometry; gameplay interaction needs a separately defined representation.

Sources

• [1] Tessellation Stages — Microsoft, Direct3D documentation.
• [2] Tessellation Invariance — Khronos Group, Vulkan specification.