check usb cams

# https://www.codingforentrepreneurs.com/blog/opencv-python-web-camera-quick-test/
# https://stackoverflow.com/questions/41406547/how-to-detect-a-full-black-color-image-in-opencv-python
# Getting a USB error, 
import numpy as np
import cv2

font = cv2.FONT_HERSHEY_SIMPLEX


NUM_CAMS = 2
cap=[]

for n in range(NUM_CAMS):
    try:
        # [ WARN:0] global C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-95hbg2jt\opencv\modules\videoio\src\cap_msmf.cpp (435) `anonymous-namespace'::SourceReaderCB::~SourceReaderCB terminating async callback
        #  cv2.CAP_DSHOW // This get's rid of the WARN:0 error
        cap.append(cv2.VideoCapture(n))
    except Exception as ex:
        print(ex)
print(f'detected cameras: {len(cap)}')
for i in range(len(cap)):
    print(f'cap[{i}]: {cap[i].isOpened()}')
     
try:
    for i in range(len(cap)):
        ret, frame = cap[i].read()
        
        if frame is None:
            print("Image is empty")
            frame = np.zeros((512,512,3), np.uint8)
            cv2.putText(frame,f'ret: {ret},Empty Image',(10,50), font, 1,(255,255,255),2)

        elif np.mean(frame) == 0:
            print ("Image is black")
        else:
            print ("Colored image")
        # Display the resulting frame
        cv2.imshow(f'Cam: {i}', frame)

except Exception as ex:
    print(ex)

# input("Press Enter to continue...")

x = 0
y = 0
w = 500
h = 100
while(True):
    try:
        for i in range(len(cap)):
            ret, frame = cap[i].read()
            if (ret):
                gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
                count = cv2.countNonZero(gray)
                cv2.rectangle(frame, (x, x), (x + w, y + h), (0,0,0), -1)
                cv2.putText(frame,f'countNonZero {count}',(10,50), font, 1,(255,255,255),2)
                cv2.imshow(f'Cam: {i}', frame)
    
    except Exception as ex:
        print(ex)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

for n in range(NUM_CAMS):
    cap[n].release()
cv2.destroyAllWindows()