WRF.md, creation of animations

WRF.md

@@ -681,5 +681,58 @@ For 3D visualization of WRF output, it is recommended to use either [Paraview](h
 
 Not tested yet.
 
+##### Creating a video
+
+Whether done with Paraview or with Mayavi, the visualization will result in a collection of png files, e.g., `InnValley.%04d.png`. There are several tools to convert invidual frames into movies. Among them, `ffmpeg` and `apngasm`.
+
+The basic method to create an `mp4` movie is:
+```sh
+ffmpeg -i InnValley.%04d.png -c:v libx264 -r 12 -pix_fmt yuv420p InnValley.mp4
+```
+
+The method above might return an error if frames have an odd number of pixels in one dimension:
+```sh
+[libx264 @ 0x5651e5f02980] height not divisible by 2 (1066x1083)
+```
+
+The fix is as follows:
+```sh
+ffmpeg -i InnValley.%04d.png -c:v libx264 -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2" -r 12 -pix_fmt yuv420p InnValley.mp4
+```
+
+It is possible to add movie repetitions (similar to a loop). In this case, 3 additional loops are appended after the first one:
+```sh
+ffmpeg -stream_loop 3 -framerate 12 -i InnValley.%04d.png -c:v libx264 -vf "pad=ceil(iw/2)*2:ceil(ih/2)*2" -pix_fmt yuv420p InnValley.mp4
+```
+
+It also possible to generate movies in other formats, better suited for the web:
+
+* webp (most efficient compression for loops):
+    ```sh
+    ffmpeg -framerate 12 -i InnValley.%04d.png InnValley.webp
+    ```
+
+* animated png (bigger in size):
+    ```sh
+    apngasm InnValley.png InnValley.0*png
+    ```
+
+* gif (much bigger in size):
+    ```sh
+    ffmpeg -framerate 12 -i InnValley.%04d.png InnValley.gif
+    ```
+
+* To get a feeling of the file sizes:
+    ```sh
+    (mypy39) stefano@stefano-XPS-13-9370:~/Desktop/Paraview_animation/anim$ du -hcs InnValley.0*png
+    59M	total
+
+    (mypy39) stefano@stefano-XPS-13-9370:~/Desktop/Paraview_animation/anim$ du -hcs InnValley.[pgmw]*
+    70M	InnValley.gif
+    14M	InnValley.mp4
+    51M	InnValley.png
+    4,5M	InnValley.webp
+    ```
+
 ## Useful tools