8. Computer apparatus for interpreting a program for the creation of an image, wherein the image is made up of multiple graphical elements and said program is defined in a graphical programming language, said computer apparatus comprising: means for constructing at least one expression tree describing operations, required to create the image; means for automatically altering at least one portion of said expression tree so as to reduce a corresponding execution time of a series of instructions derived from said expression tree without substantially altering an image created by the execution of said series of instructions, and means for converting said altered expression tree to a corresponding series of instructions for creating the image, wherein said altering means comprises: means for determining an alternative form of expression of portions of said at least one expression tree, means for estimating a cost of execution of a corresponding series of instructions of said alternative form of expression, and means for altering said portion to said alternative form of expression when the cost of execution of said alternative form of said portion is lower than the cost of execution of a present form of said portion, wherein said expression tree includes leaf nodes and said estimation means includes means for determining the size of descendant graphical elements corresponding to the descendants of the current node, and means for determining the cost of execution from at least the size of said descendant graphical elements.
8. Computer apparatus for interpreting a program for the creation of an image, wherein the image is made up of multiple graphical elements and said program is defined in a graphical programming language, said computer apparatus comprising: means for constructing at least one expression tree describing operations, required to create the image; means for automatically altering at least one portion of said expression tree so as to reduce a corresponding execution time of a series of instructions derived from said expression tree without substantially altering an image created by the execution of said series of instructions, and means for converting said altered expression tree to a corresponding series of instructions for creating the image, wherein said altering means comprises: means for determining an alternative form of expression of portions of said at least one expression tree, means for estimating a cost of execution of a corresponding series of instructions of said alternative form of expression, and means for altering said portion to said alternative form of expression when the cost of execution of said alternative form of said portion is lower than the cost of execution of a present form of said portion, wherein said expression tree includes leaf nodes and said estimation means includes means for determining the size of descendant graphical elements corresponding to the descendants of the current node, and means for determining the cost of execution from at least the size of said descendant graphical elements. 9. Computer apparatus as claimed in claim 8, further comprising: means for executing said series of instructions as to create said image.