1. A method of rasterising a document using a plurality of threads, the method comprising: interpreting objects of the document by performing interpreting tasks associated with said objects, wherein objects associated with different pages are interpreted in parallel; establishing a plurality of rasterising tasks associated with the performed interpreting tasks, each performed interpreting task establishing a plurality of rasterising tasks; estimating an amount of parallelisable work available to be performed using the plurality of threads, the amount of parallelisable work being estimated using the established plurality of rasterising tasks and an expected number of interpreting tasks to be performed; selecting, based on the estimated amount of parallelisable work, one of: (i) an interpreting task to interpret objects of the document, and (ii) a rasterising task from the established plurality of rasterising tasks; and executing the selected task using at least one thread to rasterize the document.