I’m not an expert in Video Editing but what I want to understand the logic of Whatsapp video processing.
First of all I have noticed that whatever the file is, Whatsapp sets the limit of Uploaded videos to 16MB, after which whatsapp crops the video to not exceed the limit. is this a convention or it’s a personal choice ?
Secondly, When a video is recorded using the Camera it’s not compressed by default, so whatsapp compresses it using
FFMPEG I guess, and it takes no time. (tried for a video of 1min 1920×1080 with 125MB of size, becomes 640×360 with 5MB of size in no time, and the upload starts automatically).. how may they do this ? and why the choice of 640×360, It seems to me very fast for 2 asynchronous tasks : Compression + Upload.
When I run the compression command
ffmpeg -y -i in.mp4 -codec:v libx264 -crf 23 -preset medium -codec:a libfdk_aac -vbr 4 -vf scale=-1:640,format=yuv420p out.mp4 it takes approximatively 1 min and the video is being rotated !! 😀
Finally, when we download a video from Youtube it’s already compressed (I guess) and whatsapp doesn’t even try to compress it. So I think that it automatically detects thats the video is compressed. How can we detect this ?
Here are possible answers to your questions:
Quest. 1: Its a personal choice. The whatsapp team is trying to offer the best User Experience (UX) they could to users of their app, that is why they have kept a limit of 16MB for video file. Imagine how long it would take to upload a file of about 125MB. Hence, the app compresses the file for quicker upload and seamless experience.
Quest. 2: I guess you already answered this question yourself – Asynchronous programming. The large video file you feed it, gets encoded into a compressed format according to the algorithm they have written for the app. As Devs., we all know about algorithms and we all know there are things you can do to speed up execution. I guess they implemented their very own algorithm using Asynchronous programming that speeds up the process. The ffmpeg library you mentioned i guess was coded in C which i think doesn’t support async call (not so sure though). After this, upload takes over.
Quest. 3&Finally: Codecs are standards. If you encode a video file to MPEG4, then try to re-encode it again to MPEG4 even using another program, you will get the same result as far as both programs are using same encoding standards, i.e. they didn’t implement a specific algorithm for their programs(this takes years of work). So, when your Whatsapp tries to encode the file, it gives the same result.
Hope I have been able to answer your questions.