add python sample to test/ showcase new cap_v4l2 features

samples/python2/video_v4l2.py

+#!/usr/bin/env python
+
+'''
+VideoCapture sample showcasing  some features of the Video4Linux2 backend
+
+Sample shows how VideoCapture class can be used to control parameters
+of a webcam such as focus or framerate.
+Also the sample provides an example how to access raw images delivered
+by the hardware to get a grayscale image in a very efficient fashion.
+
+Keys:
+    ESC    - exit
+    g      - toggle optimized grayscale conversion
+
+'''
+
+import cv2
+
+def decode_fourcc(v):
+    v = int(v)
+    return "".join([chr((v >> 8 * i) & 0xFF) for i in range(4)])
+
+font = cv2.FONT_HERSHEY_SIMPLEX
+color = (0, 255, 0)
+
+cap = cv2.VideoCapture(0)
+cap.set(cv2.CAP_PROP_AUTOFOCUS, False)
+
+cv2.namedWindow("Video")
+
+convert_rgb = True
+fps = int(cap.get(cv2.CAP_PROP_FPS))
+focus = int(cap.get(cv2.CAP_PROP_FOCUS)) * 100
+
+cv2.createTrackbar("FPS", "Video", fps, 30, lambda v: cap.set(cv2.CAP_PROP_FPS, v))
+cv2.createTrackbar("Focus", "Video", focus, 100, lambda v: cap.set(cv2.CAP_PROP_FOCUS, v / 100))
+
+while True:
+    status, img = cap.read()
+
+    fourcc = decode_fourcc(cap.get(cv2.CAP_PROP_FOURCC))
+
+    fps = cap.get(cv2.CAP_PROP_FPS)
+
+    if not bool(cap.get(cv2.CAP_PROP_CONVERT_RGB)):
+        if fourcc == "MJPG":
+            img = cv2.imdecode(img, cv2.IMREAD_GRAYSCALE)
+        elif fourcc == "YUYV":
+            img = cv2.cvtColor(img, cv2.COLOR_YUV2GRAY_YUYV)
+        else:
+            print("unsupported format")
+            break
+
+    cv2.putText(img, "Mode: {}".format(fourcc), (15, 40), font, 1.0, color)
+    cv2.putText(img, "FPS: {}".format(fps), (15, 80), font, 1.0, color)
+    cv2.imshow("Video", img)
+
+    k = cv2.waitKey(1)
+
+    if k == 27:
+        break
+    elif k == ord("g"):
+        convert_rgb = not convert_rgb
+        cap.set(cv2.CAP_PROP_CONVERT_RGB, convert_rgb)