update doc

Signed-off-by: Takayuki Murooka <[email protected]>

perception/multi_object_tracker/models.md

@@ -65,6 +65,18 @@ $$
 \end{array}\right]
 $$
 
+#### remarks on the output twist
+
+Remarks that the velocity $v_{k}$ is the norm of velocity of vehicle, not the longitudinal velocity.
+So the output twist in the object coordinate $(x,y)$ is calculated as follows.
+
+$$
+\begin{aligned}
+v_{x} &= v_{k} \cos \left(\beta_{k}\right) \\
+v_{y} &= v_{k} \sin \left(\beta_{k}\right)
+\end{aligned}
+$$
+
 ## Anchor point based estimation
 
 To separate the estimation of the position and the shape, we use anchor point based position estimation.