Am 08.04.2021 um 09:46 schrieb Rainer M Krug:
Hi
I have a series of videos of moving particles (multiple particles per frame /
movie), and would like to add a circle around each particle and add a label. At
the moment I am using a script in R to plot, for each frame, these circles and
labels into a png with alpha channel, combine the pngs to a movie, and overlay
this movie to the original movie (see the thread 'Overlay images to frames in
video’ for the background).
Now I realised, that the actual plotting of the labels takes up nearly 40% of
the time of the R script and I would like to make this process faster.
I have found “draw_text” (https://ffmpeg.org/ffmpeg-filters.html#drawtext), but
I have no idea how I could do this. We can assume that I have a text file with
the following columns:
FRAME: the frame on which the labels and circle should be plotted
X: the x-coordinate
Y: the y=coordinate
LABEL: label for the point
Also possibly important, there are multiple (many) particles for which circles
and labels need to be plotted on each frame.
I also found the “sendcmd”
(https://ffmpeg.org/ffmpeg-filters.html#sendcmd_002c-asendcmd) but I do not get
my head around how I can combine these two.
It's possible to do this with FFmpeg if the number of particles is known
in advance and constant in all frames. Use several drawtext commands,
one for each particle. But with a changing number of particles I have no
idea how to do it. There are a few examples for sendcmd in chapter 2.86
of my book:
http://www.astro-electronic.de/FFmpeg_Book.pdf
Michael
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