1. A method of encoding video data, the method comprising: scanning a two-dimensional block of transform coefficients to produce a one-dimensional vector of the transform coefficients; determining values for significant coefficient flag syntax elements of the two-dimensional block indicative of whether the transform coefficients in the one-dimensional vector are significant; for a current significant coefficient flag syntax element of the significant coefficient flag syntax elements of the transform coefficients in the one-dimensional vector: determining a set of the significant coefficient flag syntax elements of the transform coefficients in the one-dimensional vector that were encoded prior to the current significant coefficient flag syntax element of the transform coefficients in the one-dimensional vector, the set having a size equal to a predetermined number; determining a percentage of the set of significant coefficient flag syntax elements of the transform coefficients in the one-dimensional vector that the corresponding values indicate are significant; selecting a context model for entropy encoding the value of the current significant coefficient flag based on the percentage of the set of significant coefficient flag syntax elements of the transform coefficients in the one-dimensional vector that are significant as indicated by the corresponding values; and entropy encoding the value of the current significant coefficient flag syntax element using the determined context model.