1. A method of specifying time-dependent computer graphics data, the method comprising: receiving a first layer data structure including first computer graphics data specified using a first time reference; receiving a second layer data structure including second computer graphics data specified using a second time reference; receiving a first layer relationship specifying a time relationship between the first and second layer data structures; determining a time transformation specifying a relationship between the first time reference and the second time reference; transforming the first computer graphics data using the time transformation to create transformed first computer graphics data specified using the second time reference; compositing the second computer graphics data and the transformed first computer graphics data to create composite computer graphics data, wherein the composite computer graphics data includes the second computer graphics data and at least a portion of the transformed first computer graphics data, receiving a third layer data structure including third computer graphics data specified using a third time reference; receiving a second layer relationship specifying a time relationship between the first and third layer data structures; determining a second time transformation specifying a relationship between the first time reference and the third time reference; transforming the first computer graphics data using the second time transformation to create additional transformed first computer graphics data specified using the third time reference; and compositing the third computer graphics data and the additional transformed first computer graphics data to create additional composite computer graphics data, wherein the additional composite computer graphics data includes the third computer graphics data and at least a second portion of the additional transformed first computer graphics data, wherein a first portion of the first computer graphics data corresponding with the portion of the transformed first computer graphics data included in the composite computer graphics data is different from a second portion of the first computer graphics data corresponding with the second portion of the additional transformed first computer graphics data included in the additional composite computer graphics data.