System and method for detecting reusable groups of drawing commands in a sequence of drawing commands

A system for detecting a group of drawing commands in an original sequence of drawing commands comprising a set of observation points defining locations at which an appearance of drawing commands is evaluated, wherein said group of drawing commands occurs multiple times in the original sequence of drawing commands, wherein the drawing commands in zero or more of the occurrences of said group are in a different order, wherein the drawing commands in zero or more of the occurrences of said group have intervening drawing commands not belonging to the occurrence, and wherein the system ensures that the sequence of drawing commands is reorderable such that, after reordering, the drawing commands of each occurrence of the group of repeated drawing commands have the same order and have no intervening drawing commands and the appearance of the reordered sequence of drawing commands at each observation point is the same as the appearance of the original sequence of drawing commands at the corresponding observation point, the system comprising:

• a processor;
• and
• memory, wherein the memory stores instructions that, when executed by the processor, cause the processor to:
• compute, for each drawing command in the original sequence of drawing commands, a checksum based on the drawing command and one or more appearance attributes associated with the drawing command;
• determine reused drawing commands in the original sequence of drawing commands by identifying drawing commands having the same checksum;
• and
• determine a sequence of such checksums identifying a group of reused drawing commands and a set of occurrences identifying where the drawing commands of the group of reused drawing commands occur in the original sequence of drawing commands,
• wherein each occurrence contains a set of locations of reused drawing commands with one drawing command associated with each of said checksums,
• wherein further for each checksum in the original sequence of checksums, the drawing commands associated with that checksum each draw at the same position relative to a reference point defined for the occurrence containing the respective drawing command, and
• wherein further the drawing commands of the group are reorderable such that, after reordering, the drawing commands contained in each occurrence of the group of repeated drawing commands have the same order and have no intervening drawing commands, and the appearance at each observation point of the reordered sequence is the same as the appearance at the corresponding observation point of the original sequence.