In various examples there is a computer-implemented method for forming an optimized three-dimensional model for a three-dimensional graphics image. The method comprises: obtaining the three-dimensional graphics image, the three-dimensional graphics image represented as at least one polygon mesh having a plurality of vertices connected by edges. The method selects one or more of the vertices to be removed by collapse decimation operation, the selection based on an amount of change in the polygon mesh introduced as a result of the decimation operation. The amount of change is computed using information about intersections where two different faces of the polygon mesh pass through one another.