7. EFFECT OF INCLINATION TO LIGHT

 

A plane directly facing a light source receives the full flux of light; as the plane rotates away, the amount of light striking a unit area, and hence the amount of light energy reflected, diminishes in proportion to the cosine of the angle of rotation. Again, it is the linear light energy (radiance) that is affected by this factor; you need to convert from this if you want to get an accurate fall-off of nonlinear brightness (Table 10.2). For practical painting purposes this sort of calculation may not be necessary unless particular accuracy is needed (all of the sphere illustrations on this site, for example, were modelled purely by "eyeballing").

Table 10.2. Relative fall-off of radiance and brightness of reflected light with angle of inclination to direction of light.

This fall off of brightness with inclination to the light source is of course the basis of our efforts to model form in drawing and painting using tone. The interesting point to note in the table is how slowly apparent brightness diminishes at low to moderate inclinations away from the light, which is why such a large area can be treated, at least as a first approximation, as a simple "full-light" zone.

In digital painting, the problem of synchronizing the change in brightness of the different components of a multicoloured surface can generally be solved very simply if the way the colouration behaves is analyzed and understood. In Figure 10.14, an imaginary strip of apple skin was conceived as having an underlying uniform green colouration, modified in patches by varying concentrations of red pigmentation. The green component was therefore modelled (using the principle of uniform saturation) in one layer (Figure 10.14C), and the red component was painted in an irregular pattern of overlapping brushstrokes on an overlying layer in multiply mode (Figure 10.14B).

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