Re: [Plplot-general] JPG file driver

[Torquil M. <to...@gm...>]

On Tuesday 22 January 2008, Oliver Bandel wrote:
> Zitat von Torquil Macdonald S=F8rensen <to...@gm...>:
> > Hello, sorry to ask so many questions... I am trying to get plplot to
> > rewrite
> > the plot to the same file several times (part of my quest to write to
> > a pipe
> > into ffmpeg). In between plots (plbop().....pleop()) I delete the
> > file
>
> [...]
>
>
> When you give the outputfile the name "-"
> plplot-lib will write to stdout!!!
>
> You can make your executable that writes
> a stream to stdout and then redirect the output of
> your file into the fifo!
>
> $ my-stream-generator > my_fifo
>
>
> But at the other end of the fifo,
> then you maybe have to cut the stream into seperated files...
> ...depends on your reading process / program, if it can read
> plplot-lib's output directly.

Hello Oliver, thanks for your suggestion and explanation about how plbop()=
=20
works with the JPG-driver. I had already tried using the STDOUT instead of =
an=20
ordinary file in plplot, but it didn't work to use the data that plplot wro=
te=20
to stdout as input to ffmpeg. From what I could understand it was not=20
equivalent to "cat *.jpg | ffmpeg_command". I ended ut with only one frame =
in=20
the ffmpeg-generated video.

But I have managed to find a workaround for now (it works but is a bit=20
inefficient). I start and stop plplot between each plot page with plinit()=
=20
and plend(). Between each plot I then send the newly created "file.jpg" int=
o=20
an ofstream that is really a FIFO-file "fifo.mjpeg" (which is equivalent to=
=20
STDOUT, but nicer, I think). Then plplot overwrites "file.jpg" when I start=
=20
it up again with plinit() and changes "file.jpg" to the next plot page, and=
=20
the process goes on and on sending each newly created "file.jpg" into the=20
pipe going to ffmpeg. I think the main point here is that the pipe is alway=
s=20
open until the program has ended, if I understand correctly.
At the same time I have a ffmpeg skript reading from the FIFO-file. That wa=
y I=20
am sure that the data going into my FIFO is equivalent to doing cat *.jpg >=
=20
fifo.mjpeg. I have already gotten my ffmpeg-script to work with such input.=
=20
The fifo.mjpeg must be created first, of course, using "mkfifo fifo.mjpeg".=
 I=20
will experiment some more trying do get the STDOUT from plplot to work=20
directly with the STDIN on ffmpeg, with out using an additional ofstream in=
=20
the c++ code.

But at least I have now reached my goal to create a video on the fly, witho=
ut=20
temporary image files, I I'm really happy about that :-)

I will post my example programs here later, in case somebody is interested =
in=20
doing something similar.

Thanks to everone on the list that helped me with all of this!

Best regards,
Torquil S=F8rensen