Important elements for modelling were studied with reference to lighting in pictorial portraits. In addition to the three known elements (the direction of lighting, the area ratio of light and shade, and the contrast ratio of light and shade), two new elements (the shading properties and the gradation index) were proposed. The shading is produced on the lighted surface by Lambert cosine law. The gradation index (G) represents the gradation degree of the boundary between light and shade.
The gradation index is expressed by the following formula:_??_where E' is the individual direct lighting illumination vector, γ is the including angle between E' and the composite illumination vector (E_c). Fig. 15 illustrates the correlation between G value and actual appearance by uniformly bright circular surface source. Fig. 20 made in accordance E'/E_c sin γ and E'/E_c cos γ represents construction for E_c and G value.

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