We define a new shading tool called a Bidirectional Appearance Distribution Function (BADF) tailored to the direct control of stylized appearance. A BADF can be thought of as defining the appearance of a sphere from all possible illumination directions. Our BADF formulation generalizes and improves upon previous stylized shading techniques by enabling the direct control of shading profiles in screen space, exaggerating surface features in a flexible manner, and letting users control stylized appearance from multiple lighting or viewing directions. This allows users to start from a simple shading behavior, and refine from there towards greater stylization. Our GPU implementation works in real-time, which benefits both editing, and rendering in interactive systems. These features make BADFs an efficient tool for many applications in artistic and scientific illustration domains.