Shading is the technique of adding depth, dimension, and form to objects by varying color and brightness based on how light interacts with surfaces. It's what makes 2D drawings and 3D renders appear three-dimensional.

Shading techniques:

- Flat shading: Single color per face (low poly look)

- Gouraud shading: Smooth color interpolation

- Phong shading: Smooth lighting calculation

- Cel shading: Cartoon-style flat colors with hard edges

- PBR shading: Physically-based realistic materials

Shading creates:

- Highlights: Brightest areas where light hits directly

- Mid-tones: Medium brightness areas

- Shadows: Darkest areas where light doesn't reach

- Reflections: Light bouncing off surfaces

- Ambient occlusion: Soft shadows in crevices

Shading is important for:

- Creating depth perception

- Defining object form

- Making objects look realistic

- Adding visual interest

- Guiding viewer attention

In 3D icon design, shading helps icons appear dimensional and professional, making them stand out from flat designs.