Viewport transformation modules for use in a three-dimensional rendering system wherein vertices are received from an application in a strip. The viewport transformation modules include a fetch module configured to read from a vertex buffer: untransformed coordinate data for a vertex in a strip; information identifying a viewport associated with the vertex; and information identifying a viewport associated with one or more other vertices in the strip. The one or more other vertices in the strip are selected based on a provoking vertex of a primitive to be formed by the vertices in the strip and a number of vertices in the primitive. The viewport transformation modules also include a processing module that performs a viewport transformation on the untransformed coordinate data based on each of the identified viewports to generate transformed coordinate data for each identified viewport; and a write module that writes the transformed coordinate data for each identified viewport to the vertex buffer.