Line Follower using CV detecting angle

Follower_CV_3.py

from picamera.array import PiRGBArray
from picamera import PiCamera
import time
import cv2
import numpy as np
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BOARD)
GPIO.setup(40, GPIO.OUT)
GPIO.output(40, GPIO.HIGH)

camera = PiCamera()
camera.resolution = (640, 360)
camera.rotation = 180
rawCapture = PiRGBArray(camera, size=(640, 360))
time.sleep(0.1)


for frame in camera.capture_continuous(rawCapture, format="bgr", use_video_port=True):	
	image = frame.array	
	Blackline = cv2.inRange(image, (0,0,0), (60,60,60))	
	kernel = np.ones((3,3), np.uint8)
	Blackline = cv2.erode(Blackline, kernel, iterations=5)
	Blackline = cv2.dilate(Blackline, kernel, iterations=9)	
	img_blk,contours_blk, hierarchy_blk = cv2.findContours(Blackline.copy(),cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
	
	if len(contours_blk) > 0:	 
	 blackbox = cv2.minAreaRect(contours_blk[0])
	 (x_min, y_min), (w_min, h_min), ang = blackbox
	 if ang < -45 :
	  ang = 90 + ang
	 if w_min < h_min and ang > 0:	  
	  ang = (90-ang)*-1
	 if w_min > h_min and ang < 0:
	  ang = 90 + ang	  
	 setpoint = 320
	 error = int(x_min - setpoint) 
	 ang = int(ang)	 
	 box = cv2.boxPoints(blackbox)
	 box = np.int0(box)
	 cv2.drawContours(image,[box],0,(0,0,255),3)	 
	 cv2.putText(image,str(ang),(10, 40), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 0, 255), 2)
	 cv2.putText(image,str(error),(10, 320), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 0, 0), 2)
	 cv2.line(image, (int(x_min),200 ), (int(x_min),250 ), (255,0,0),3)
	 
	  	
	cv2.imshow("orginal with line", image)	
	rawCapture.truncate(0)	
	key = cv2.waitKey(1) & 0xFF	
	if key == ord("q"):
		break

GPIO.output(40, GPIO.LOW)

The error is used as the P input and the angle is used like the D input. I personally didn't uses a PID but rather used something similar to a PD controller. I used corrention = Kp * errror + Ka * angle. To understand how a PID works: P is a proportional correction, I is the integal or the accumulative error and used to correct in cases of steady state errors and D is the derivative or the trend the error is heading in and used to predict what will happen in the future to stop over shoot. So for line following you don't need the I part of a PID because u can't get steady state error in line following. I didn't use the D part of a PID because the angle tells you the same thing, the tend it is heading to in the future. So when I used the error and the angle it does the same thing as a PD controller but a little bit of a different way.

how did you calculate the speed of the motor? to make the forward movement and the turns : left and right