A method of rasterizing a document using a plurality of threads interprets objects of the document by performing interpreting tasks associated with the objects. Objects associated with different pages are interpreted in parallel. A plurality of rasterizing tasks associated with the performed interpreting tasks are established, each performed interpreting task establishing a plurality of rasterizing tasks. The method estimates an amount of parallelisable work available to be performed using the plurality of threads. The amount of parallelisable work is estimated using the established rasterizing tasks and an expected number of interpreting tasks to be performed. The method selects, based on the estimated amount of parallelisable work, one of (i) an interpreting task to interpret objects of the document, and (ii) a rasterizing task from the established plurality of rasterizing tasks, and then executes the selected task using at least one thread to rasterize the document.