1. A method of rendering a computer graphic image modelling a surface of a virtual object, comprising: generating, by a processor, a plurality of elementary geometries by subdividing an input geometry based on a tessellation level, a plurality of vertices being associated with each elementary geometry, each vertex being defined with coordinates, wherein the tessellation level corresponds to a quality of the computer graphic image, assigning, by the processor, an index value to each elementary geometry of at least a part of the elementary geometries, the index value being computed from the coordinates defining at least one vertex of the elementary geometry, the elementary geometries correspond to triangles, the triangles being obtained from quadrilaterals subdivided into two triangles, each quadrilateral comprising a lower triangle and an upper triangle, the assigning of the index value comprising, implemented for each elementary geometry of the at least a part of the elementary geometries: determining minimal coordinates (x min , y min ) of the elementary geometry, x min and y min being normalized between 0 and 1, determining if the elementary geometry is an upper triangle or a lower triangle, determining an index value to be assigned to the elementary geometry, the index value being equal to: i.x min +2.i.j.y min , if the elementary geometry is a lower triangle, or i ⁢ ⁢ x min + 2 ⁢ ij ⁡ ( y min + 1 2 ⁢ j ) , if the elementary geometry is an upper triangle, wherein i×j corresponds to a number of quadrilaterals; storing the assigned index value assigned to the each elementary geometry in a random access memory; and rendering providing, by the processor the computer graphic image to a display for rendering by using the indexed elementary geometries.