# Donkey-Car formular

Hello.
Can someone briefly explain how you come up with the following formula at donkey-car (def figure_out_my_steering(line, img)). I would be very grateful for a short derivation.Thanks very much.
Link: openmv-projects/line_follower_main.py at master · openmv/openmv-projects · GitHub

def figure_out_my_steering(line, img):
global old_cx_normal

``````# Rho is computed using the inverse of this code below in the actual OpenMV Cam code.
# This formula comes from the Hough line detection formula (see the wikipedia page for more).
# Anyway, the output of this calculations below are a point centered vertically in the middle
# of the image and to the left or right such that the line goes through it (cx may be off the image).
cy = img.height() / 2
cx = (line.rho() - (cy * math.sin(math.radians(line.theta())))) / math.cos(math.radians(line.theta()))

# "cx_middle" is now the distance from the center of the line. This is our error method to stay
# on the line. "cx_normal" normalizes the error to something like -1/+1 (it will go over this).
cx_middle = cx - (img.width() / 2)
cx_normal = cx_middle / (img.width() / 2)
# Note that "cx_normal" may be larger than -1/+1. When the value is between -1/+1 this means the
# robot is driving basically straight and needs to only turn lightly left or right. When the value
# is outside -1/+1 it means you need to turn VERY hard to the left or right to get back on the
# line. This maps to the case of the robot driving into a horizontal line. "cx_normal" will
# then approach -inf/+inf depending on how horizontal the line is. What's nice is that this
# is exactly the behavior we want and it gets up back on the line!

if old_cx_normal != None: old_cx_normal = (cx_normal * MIXING_RATE) + (old_cx_normal * (1.0 - MIXING_RATE))
else: old_cx_normal = cx_normal
return old_cx_normal``````

was the Hough transform used here? But I don’t see where.

The explanation is in the code… I tried to explain it as well as I could. See this webpage and read it: Hough transform - Wikipedia In particular the theory section for the math.

was the Hough transform used here? But I don’t see where.

No, it’s not, but, the way lines are represented is the same.

All clear. Now I understand. thank you very much!
But one small question. What exactly does NATIVE_MOTOR_CONTROLLER do?
Is this there if you don’t want to use the servo shield and want to generate the PWM signals directly with the OPEN MV CAM? I don’t understand what that thing is used for?
A heartfelt thank you in advance

Yes, so you can control motors using the motor shield.