Methods and computer graphics systems are provided for compressing vertex parameter data. The vertex parameter data comprises a data block comprising data of vertices relating to at least one parameter. The data of each of the vertices includes multiple data segments at respective positions. The data in the data block is analyzed to determine a compression grouping scheme according to which data segments of the vertices are grouped together into segment blocks for compression. The analysis of the data determines a compression grouping scheme which is suited to the distribution of the data in the data block, to thereby improve the compression ratio which can be achieved when compressing the data block.