A tangent plane describes directions that remain locally on a level surface, so the function value does not change to first order. For a level contour, the corresponding tangent direction follows the contour; movement perpendicular to it gives the greatest change. The gradient is normal to the tangent surface. section 3.3.2
Use the tangent–normal relationship to interpret loss landscapes:
- Moving along a tangent direction stays on the same level contour or surface. section 3.3.2
- Moving along the normal, which is the gradient direction, crosses level surfaces with maximum change. section 3.3.2
- In model training, these directions describe how parameter movement changes or preserves the loss. section 3.3.2
Circular level contours
For L(w0, w1) = √(w0² + w1²), a level contour is a circle. Moving around its circumference follows the tangent and leaves L unchanged; moving radially follows the normal and changes L most rapidly. section 3.3.2
