25. Computer apparatus according to claim 24 wherein the expression tree includes leaf nodes having associated graphical elements and internal nodes having associated codes for combining graphical elements, and wherein the portions include internal nodes and said means for estimating for the cost of execution of the alternative forms comprises: first means for determining a size of descendant graphical elements corresponding to descendants of a current node, and means for determining a cost from the size of the descendant graphical elements.
24. Computer apparatus according to claim 23, wherein said means for transforming further comprises: means for determining alternative forms of expression of portions of the at least one expression tree; means for estimating a cost of execution of a corresponding series of instructions of the alternative form of expression; and means for altering the portion to one of the alternative forms of expression hen an expected execution time of the alternative form of the portion is lower than an expected execution time of a present form of the portion.