Update README.md

Signed-off-by: David Rotermund <54365609+davrot@users.noreply.github.com>

cv2/write_read_play/README.md

@@ -74,4 +74,116 @@ print(test_data.shape)  # -> (101, 101, 3, 101)
 write_video(frames=test_data, filename="test", fps=20.0)
 ```
 
+## Read the data from a mp4 file
 
+```python
+import numpy as np
+import cv2  # type: ignore
+
+
+def read_video(filename: str, display: bool = False) -> np.ndarray:
+    assert len(filename) > 0, "read_video: Filename is empty."
+
+    frames: np.ndarray = np.array([])
+
+    cap = cv2.VideoCapture(filename)
+
+    assert cap.isOpened() is True, "read_video: Error opening video stream or file!"
+
+    n: int = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
+    x: int = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
+    y: int = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
+
+    print(f"Reading {n} frames with {x} x {y} pixels.")
+
+    frames = np.zeros((y, x, 3, n)).astype(np.uint8)
+
+    i: int = 0
+
+    while cap.isOpened():
+        ret, frame = cap.read()
+
+        if ret is True:
+            frames[:, :, :, i] = frame
+            i += 1
+
+            if display is True:
+                cv2.imshow("Reading", frame)
+                cv2.waitKey(25)
+        else:
+            break
+
+    cap.release()
+
+    if display is True:
+        cv2.destroyWindow("Reading")
+
+    return frames
+
+
+movie = read_video("test.mp4", display=True)
+
+print(movie.shape) # -> (100, 100, 3, 101)
+```
+
+## Playback a video
+
+```python
+import numpy as np
+import cv2  # type: ignore
+import time
+
+
+def show_video(frames: np.ndarray, fps: float = 20.0) -> np.ndarray:
+    assert frames.size > 0, "The frame is empty."
+    assert frames.ndim == 4, "The frame has wrong dimensions."
+
+    n: int = frames.shape[3]
+    dt: float = 1 / fps
+
+    t: np.ndarray = np.zeros((n + 1))
+
+    t[0] = time.perf_counter()
+
+    for i in range(n):
+        frame = frames[:, :, :, i]
+        cv2.imshow("Display", frame)
+        t_wait: float = t[i] + dt - time.perf_counter()
+        retval = cv2.waitKey(int(max(1, 1000 * t_wait)))
+        t[i + 1] = time.perf_counter()
+        if retval != -1:
+            break
+    cv2.destroyWindow("Display")
+
+    return t
+
+
+# Create test data
+axis_x = np.arange(-50, 51)[:, np.newaxis, np.newaxis] / 50
+axis_y = np.arange(-50, 51)[np.newaxis, :, np.newaxis] / 50
+axis_z = np.arange(-50, 51)[np.newaxis, np.newaxis, :] / 50
+
+test_data = np.sqrt(axis_x**2 + axis_y**2 + axis_z**2) < 0.75
+
+
+# Adding an additional axis for the color channel
+test_data = test_data[:, :, :, np.newaxis]
+test_data = np.tile(test_data, (1, 1, 1, 3))
+test_data = test_data.astype(dtype=np.float32)
+
+# Put the time axis as last axis
+test_data = np.moveaxis(test_data, 0, -1)
+
+
+# Conversion to uint8
+test_data -= test_data.min()
+test_data /= test_data.max()
+test_data *= 255
+test_data = test_data.astype(dtype=np.uint8)
+
+
+print(test_data.shape)  # -> (101, 101, 3, 101)
+
+timings = show_video(frames=test_data, fps=20.0)
+print(timings.shape) # -> (102,)
+```