Taichi: [misc] Video generation tutorial

[yuanming-hu]

Concisely describe the proposed feature
A lot of people are having trouble making a video to demonstrate their Taichi programs.

Now we actually already have GUI.show(filename) to save the frames, ti video (frames to mp4) and ti gif (mp4 to GIF) command. We just need to add a tutorial on how to use these handy functions.

Additional comments
We may also need to briefly doc how to install ffmpeg on Windows/OS X. I guess most Linux releases come with ffmpeg.

[Rullec]

Great!
My plan is to add an export_result.rst doc that exaplains the usage of GUI.show, ti.video and ti.gif in our docs/ directory. Do you think it's ok?

[archibate]

Yes, please add a brief usage description for ti video, ti video_xxx and ti gif to export_result.rst, in the Miscellaneous section, thanks!

[archibate]

Also remember to mention ti.imread/imwrite/imshow :smile:

[archibate]

这里已经有一位热心的小朋友做好了：https://forum.taichi.graphics/t/homework-0-new-shader-ffmpeg-gif/469

[Rullec]

速度真的够快，，，那你感觉我们还需要加一个rst到仓库里面去吗？如果需要的话我晚上提一个PR

[yuanming-hu]

速度真的够快，，，那你感觉我们还需要加一个rst到仓库里面去吗？如果需要的话我晚上提一个PR

Yes, please. Providing ti video and ti gif is still very helpful. They are much easier to use than raw ffmpeg.