test_cameras.py: add a python script for simple camera devices test

The script will show the camera video in a window. It supports multiple
cameras. Also it find available camera devices automatically.

Usage:
  Press 'q' to quit.
  press 'n' to change to next camera device.
  Press 's' to save current picture in current folder.

Signed-off-by: JoshWu <rainyfeeling@outlook.com>

test_cameras.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+
+import cv2
+import time
+
+# get the installed camera list for initialization.
+def get_cam_lst(cam_lst=range(0, 24)):
+    arr = []
+    for iCam in cam_lst:
+        cap = cv2.VideoCapture(iCam)
+        if not cap.read()[0]:
+            continue
+        else:
+            arr.append(iCam)
+
+        cap.release()
+    return arr
+
+def show_cam_img(caps, cam_list):
+    print("INFO: Press 'q' to quit! Press 's' to save a picture, 'n' to change to next camera device!")
+    idx = 0
+    while True:
+        cap_device = caps[idx]
+        ret, frame = cap_device.read()
+        if ret:
+            cv2.imshow('video', frame)
+        else:
+            print("ERROR: failed read frame!")
+
+        # quit the test
+        c = cv2.waitKey(1)
+        if c == ord('q'):
+            break
+
+        # change to next camera device
+        if c == ord('n'):
+            idx += 1
+            if idx >= len(caps):
+                idx = 0
+            continue
+
+        # save the picture
+        if c == ord('s'):
+            if ret:
+                name = 'video{0}_{1}.png'.format(cam_list[idx],
+                            time.strftime("%Y-%m-%d_%H:%M:%S", time.localtime()))
+                cv2.imwrite(name, frame)
+                print("saved file: %s!" %name)
+
+    cv2.destroyAllWindows()
+
+def init_caps(cam_list, resolution=(1280,720)):
+    caps = []
+    for iCam in cam_list:
+        cap = cv2.VideoCapture(iCam)
+        cap.set(3, resolution[0])
+        cap.set(4, resolution[1])
+        caps.append(cap)
+
+    return caps
+
+def deinit_caps(cap_list):
+    for cap in cap_list:
+        cap.release()
+
+def show_cameras(video_list=None):
+    if video_list == None:
+        print("Start to search all available camera devices, please wait... ")
+        cam_list = get_cam_lst()
+        err_msg = "cannot find any video device!"
+    else:
+        cam_list = get_cam_lst(video_list)
+        err_msg = "cannot find available video device in list: {0}!".format(video_list) +\
+                    "\nPlease check the video devices in /dev/v4l/by-path/ folder!"
+
+    if len(cam_list) < 1:
+        print("ERROR: " + err_msg)
+        return
+
+    print("Available video device list is {}".format(cam_list))
+    caps = init_caps(cam_list)
+    show_cam_img(caps, cam_list)
+    deinit_caps(caps)
+
+if __name__ == "__main__":
+    # User can specify the video list here.
+    #show_cameras([2, 6, 10, 14])
+
+    # Or search all available video devices automatically.
+    show_cameras()