Dual conic

Similarly, the dual concept exists for lines. A conic envelope or dual conic is the locus of all lines ${\tt l}$ satisfying a homogeneous quadratic equation:

$${\tt l}^\top {\bf C}^* {\tt l} = 0 \, ,$$ (B11)

where ${\bf C}^*$ is a $3 \times 3$ symmetric matrix only defined up to scale. A dual conic thus also depends on five independent parameters.