Opacity masks use the luminosity of the uppermost object to produce a transparent pattern on lower objects. When the masking object is filled with a solid color, the mask is uniform. If the luminosity is high (bright colors), the mask appears to have little or no effect. If the masking object is filled with a dark color, the artwork below appears more transparent. When the masking object is filled with a pattern or a gradient, the results can vary substantially. Figure 17.9 compares opacity masks created with various solid color, gradient, and pattern fills.