Are you trying to occasionally create a single jpeg from the mp4 buffer output of node-red-contrib-mp4frag?
And then you want to base64 encode the jpeg buffer before sending it via mqtt?
If yes to the first question, then you can trigger the mp4 buffer output by sending a command msg to node-red-contrib-mp4frag and then receive that in a node-red-contrib-ffmpeg-spawn node which will process the mp4 buffer and transcode a jpeg. see this post and check out the shared flow showing ffmpeg-spawn --> mp4frag --> ffmpeg-spawn --> pipe2jpeg.
If yes to the second question, I am not sure how you convert buffer to base64 string. There is probably a node for that.
