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+# OpenCV2: Play, write, read a video
+{:.no_toc}
+
+<nav markdown="1" class="toc-class">
+* TOC
+{:toc}
+</nav>
+
+## The goal
+
+How to playback a video? Howto save it into a mp4 file? Howto load from a mp4 file?
+
+Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
+
+## Write the video into a mp4 file
+
+```python
+
+import numpy as np
+import cv2  # type: ignore
+import tqdm  # type: ignore
+
+
+def write_video(frames: np.ndarray, filename: str = "test", fps: float = 20.0) -> None:
+    assert len(filename) > 0, "write_video: Filename is empty."
+    assert frames.size > 0, "write_video: The frame is empty."
+    assert frames.ndim == 4, "write_video: The frame has wrong dimensions."
+
+    x: int = frames.shape[1]
+    y: int = frames.shape[0]
+    n: int = frames.shape[3]
+    fullname: str = filename + ".mp4"
+
+    fourcc = cv2.VideoWriter_fourcc("m", "p", "4", "v")
+    out = cv2.VideoWriter(fullname, fourcc, fps, (x, y))
+    assert (
+        out.isOpened() is True
+    ), "write_video: Unknown error occurred during writing, closing file!"
+
+    print(f"Writing frames into {fullname} at {fps:.1f} FPS.")
+
+    for i in tqdm.trange(n):
+        out.write(frames[..., i])
+
+    out.release()
+
+
+# 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)
+
+write_video(frames=test_data, filename="test", fps=20.0)
+```
+
+