23. A method of compressing a stream of data representing a requested dose pattern for a lithographic apparatus, comprising: analyzing the stream of data to identify sequences of data that occur more than once; selecting a group from the identified repeating sequences of data to encode for later decompression by a dictionary decoder; and forming at least a portion of a compressed representation of the data stream by replacing at least one of the repeating sequences in the group by a reference to a location in a dictionary memory from which an uncompressed version of the repeating sequence will be accessible during decompression by the dictionary decoder.
23. A method of compressing a stream of data representing a requested dose pattern for a lithographic apparatus, comprising: analyzing the stream of data to identify sequences of data that occur more than once; selecting a group from the identified repeating sequences of data to encode for later decompression by a dictionary decoder; and forming at least a portion of a compressed representation of the data stream by replacing at least one of the repeating sequences in the group by a reference to a location in a dictionary memory from which an uncompressed version of the repeating sequence will be accessible during decompression by the dictionary decoder. 24. The method of claim 23 , further comprising: selecting a first group from the identified repeating sequences of data to encode for later decompression by a first dictionary decoder; selecting a second group from the identified repeating sequences of data to encode for later decompression by a second dictionary decoder; forming at least a portion of a compressed representation of the data stream by replacing at least one of the repeating sequences in the first group by a reference to a location in an external dictionary memory from which an uncompressed version of the repeating sequence will be accessible during decompression by the first dictionary decoder, wherein the external dictionary memory is external to the first dictionary decoder; and forming at least a portion of the compressed representation of the data stream by replacing at least one of the repeating sequences in the second group by a reference to a location in an internal dictionary memory from which an uncompressed version of the repeating sequence will be accessible during decompression by the second dictionary decoder, wherein the internal dictionary memory is internal to the second dictionary decoder.