# Kochanek–Bartels spline

In mathematics, a Kochanek–Bartels spline or Kochanek–Bartels curve is a cubic Hermite spline with tension, bias, and continuity parameters defined to change the behavior of the tangents.

Given n + 1 knots,

p0, ..., pn,

to be interpolated with n cubic Hermite curve segments, for each curve we have a starting point pi and an ending point pi+1 with starting tangent di and ending tangent di+1 defined by

${\displaystyle \mathbf {d} _{i}={\frac {(1-t)(1+b)(1+c)}{2}}(\mathbf {p} _{i}-\mathbf {p} _{i-1})+{\frac {(1-t)(1-b)(1-c)}{2}}(\mathbf {p} _{i+1}-\mathbf {p} _{i})}$
${\displaystyle \mathbf {d} _{i+1}={\frac {(1-t)(1+b)(1-c)}{2}}(\mathbf {p} _{i+1}-\mathbf {p} _{i})+{\frac {(1-t)(1-b)(1+c)}{2}}(\mathbf {p} _{i+2}-\mathbf {p} _{i+1})}$

where...

 t tension Changes the length of the tangent vector b bias Primarily changes the direction of the tangent vector c continuity Changes the sharpness in change between tangents

Setting each parameter to zero would give a Catmull–Rom spline.

The source code found here of Steve Noskowicz in 1996 actually describes the impact that each of these values has on the drawn curve:

 Tension T = +1→ Tight T = −1→ Round Bias B = +1→ Post Shoot B = −1→ Pre shoot Continuity C = +1→ Inverted corners C = −1→ Box corners

The code includes matrix summary needed to generate these splines in a BASIC dialect.