A method involves calculating 302 a vertex tessellation factor for each corner vertex in one or more input patches. Tessellation 304 is then performed on the plurality of input patches using the vertex tessellation factors. The tessellation operation involves adding 305 one or more new vertices and calculating 306 a displacement factor for each newly added vertex. A world space parameter for each vertex is subsequently determined 308 by calculating 310 a target world space parameter for each vertex and then modifying 312 the target world space parameter for a vertex using the displacement factor for that vertex. The target world space parameter may be calculated by interpolating the world space parameters of the parent vertices and may be modified to lie between the target world space parameter and a starting world state parameter based on the value of the displacement factor. The displacement factor may be a function of the vertex tessellation factors of the parent vertices. The world space parameter may be
