For the purpose of computer recognition, it is possible to split valve morphology into component groups of characters (= features). The outline of the valve is an obvious source of morphological characters; for example, size is an important identifier (but see the life-cycle related changes in size, below). Shape is another suite of characters that are very important for identification. Symmetry is one of its components: the greatest proportion of diatom genera are bilaterally symmetrical about two orthogonal axes (the apical and transapical axes, Fig. 3, Fig. 4A); most are also symmetrical about the third orthogonal axis as well (the pervalvar axis), but this is of less importance since automatic identification will be concerned with diatoms in valve view. A much smaller number of genera are bilaterally symmetrical about only one axis; this can be either the transapical (Fig. 4B) or the apical (Fig. 4C) axis, although some of the species included are very nearly symmetrical in both planes; some bipolar genera have no planes of symmetry, but rotational symmetry instead (Fig. 4D). A large group of genera are more or less circular (Fig. 4E). Other components of shape can be analysed mathematically: a simple measure of rectangularity can be informative in some genera, while fitting the outline to a polynomial expansion may be more appropriate in others, using each polynomial element as a taxonomic character. 
The pattern of ornamentation (Fig. 5) is the other major source of identifiers in diatoms. Usually the valve is covered with a more or less regular pattern of lines of skeletal pores (the striae), the periodicity and angle of which have great significance. Another important character is the presence or absence of a raphe (an elongated fissure through the valve wall, involved in cell motility). Very small morphological details such as the shape of the raphe endings are very important as sources of taxonomic (i.e. classificatory) evidence, but in many cases they are not essential for identification. However, the structure of the striae (the main pattern-forming lines on the diatom) is important for both identification and classification; whether they appear as more or less simple lines, or composed of single or double rows of pores. Another very useful character (for both processes) is how the pattern of striae changes over the valve face and the shape of the axial and central area.