samples: fix python samples compatibility with demo.py

samples/python/video_v4l2.py

@@ -17,51 +17,62 @@ Keys:
 # Python 2/3 compatibility
 from __future__ import print_function
 
+import numpy as np
 import cv2 as cv
 
-def decode_fourcc(v):
-    v = int(v)
-    return "".join([chr((v >> 8 * i) & 0xFF) for i in range(4)])
+def main():
 
-font = cv.FONT_HERSHEY_SIMPLEX
-color = (0, 255, 0)
+    def decode_fourcc(v):
+        v = int(v)
+        return "".join([chr((v >> 8 * i) & 0xFF) for i in range(4)])
 
-cap = cv.VideoCapture(0)
-cap.set(cv.CAP_PROP_AUTOFOCUS, False)  # Known bug: https://github.com/opencv/opencv/pull/5474
+    font = cv.FONT_HERSHEY_SIMPLEX
+    color = (0, 255, 0)
 
-cv.namedWindow("Video")
+    cap = cv.VideoCapture(0)
+    cap.set(cv.CAP_PROP_AUTOFOCUS, False)  # Known bug: https://github.com/opencv/opencv/pull/5474
 
-convert_rgb = True
-fps = int(cap.get(cv.CAP_PROP_FPS))
-focus = int(min(cap.get(cv.CAP_PROP_FOCUS) * 100, 2**31-1))  # ceil focus to C_LONG as Python3 int can go to +inf
+    cv.namedWindow("Video")
 
-cv.createTrackbar("FPS", "Video", fps, 30, lambda v: cap.set(cv.CAP_PROP_FPS, v))
-cv.createTrackbar("Focus", "Video", focus, 100, lambda v: cap.set(cv.CAP_PROP_FOCUS, v / 100))
+    convert_rgb = True
+    fps = int(cap.get(cv.CAP_PROP_FPS))
+    focus = int(min(cap.get(cv.CAP_PROP_FOCUS) * 100, 2**31-1))  # ceil focus to C_LONG as Python3 int can go to +inf
 
-while True:
-    status, img = cap.read()
+    cv.createTrackbar("FPS", "Video", fps, 30, lambda v: cap.set(cv.CAP_PROP_FPS, v))
+    cv.createTrackbar("Focus", "Video", focus, 100, lambda v: cap.set(cv.CAP_PROP_FOCUS, v / 100))
 
-    fourcc = decode_fourcc(cap.get(cv.CAP_PROP_FOURCC))
+    while True:
+        status, img = cap.read()
 
-    fps = cap.get(cv.CAP_PROP_FPS)
+        fourcc = decode_fourcc(cap.get(cv.CAP_PROP_FOURCC))
 
-    if not bool(cap.get(cv.CAP_PROP_CONVERT_RGB)):
-        if fourcc == "MJPG":
-            img = cv.imdecode(img, cv.IMREAD_GRAYSCALE)
-        elif fourcc == "YUYV":
-            img = cv.cvtColor(img, cv.COLOR_YUV2GRAY_YUYV)
-        else:
-            print("unsupported format")
+        fps = cap.get(cv.CAP_PROP_FPS)
+
+        if not bool(cap.get(cv.CAP_PROP_CONVERT_RGB)):
+            if fourcc == "MJPG":
+                img = cv.imdecode(img, cv.IMREAD_GRAYSCALE)
+            elif fourcc == "YUYV":
+                img = cv.cvtColor(img, cv.COLOR_YUV2GRAY_YUYV)
+            else:
+                print("unsupported format")
+                break
+
+        cv.putText(img, "Mode: {}".format(fourcc), (15, 40), font, 1.0, color)
+        cv.putText(img, "FPS: {}".format(fps), (15, 80), font, 1.0, color)
+        cv.imshow("Video", img)
+
+        k = cv.waitKey(1)
+
+        if k == 27:
             break
+        elif k == ord('g'):
+            convert_rgb = not convert_rgb
+            cap.set(cv.CAP_PROP_CONVERT_RGB, convert_rgb)
 
-    cv.putText(img, "Mode: {}".format(fourcc), (15, 40), font, 1.0, color)
-    cv.putText(img, "FPS: {}".format(fps), (15, 80), font, 1.0, color)
-    cv.imshow("Video", img)
+    print('Done')
 
-    k = cv.waitKey(1)
 
-    if k == 27:
-        break
-    elif k == ord('g'):
-        convert_rgb = not convert_rgb
-        cap.set(cv.CAP_PROP_CONVERT_RGB, convert_rgb)
+if __name__ == '__main__':
+    print(__doc__)
+    main()
+    cv.destroyAllWindows()