readme.lavpipe

Motion JPEG编解码器源代码

This README describes the lavpipe tools: how to use them,
and how they work (partially).
The current implementation still is not tested very well, so be
warned (and please report any errors you encounter).

At the moment, there are only two filters for lavpipe,
transist.flt and matteblend.flt, which means that you can
use it only to make simple blending transistions between two
movies or to blend one video over another using a predefined
matte (yuv image or lav movie). But it is very easy to code
new filters or extend existing ones. Read on.

Contents:
1. What are the tools?
2. What can we do with it?
3. What else can we do with it?
4. That's all?! How can we make them do more?
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1. What are the tools?                                                       /
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The tools that are involved so far are:
 - lav2yuv, which decompresses the given input files (or any portions
            of them) and gives us raw yuv frame streams that can be
            piped through the several tools.
 - lavpipe, which reads out a "LAV Pipe List" (e.g. *.pli) file, a
            "recipe" which tells it how to combine several input
            movies using several yuv stream filters.
 - transist.flt and matteblend.flt, all the filters that already exist.
 - yuv2lav or mpeg2enc, which compress the resulting yuv stream
                        into an mjpeg or mpeg file.

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2. So hat can we do with it?                                                 /
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Example one: Make a transistion from one movie to another one.

Let's assume that we have got two 352x288 PAL files:
intro.avi (1040 frames) and epilogue.qt (3920 frames). Now we want
to make a transistion between them that lasts for two seconds
(2 sec = 50 frames, as they are PAL movies).
We also have to take care that both share the same dimensions - if
they are of different sizes, we can use lavscale once it is finished.

Our task now is to write a "recipe" for lavpipe and thus tell it how
to do the work. If we store our work as trans.pli, the final call will
simply be: "lavpipe trans.pli | yuv2lav -o result.avi" (lavpipe
writes a yuv stream to stdout as lav2yuv does).

The first line of trans.pli must be "LAV Pipe List".

The second line contains one single number, the number of input
streams that we will use. (In our case: "2")

Now for each of the two input streams a line containing the
command that produces the stream is added. First for intro.avi:
"lav2yuv -o $o -f $n -n 1 intro.avi"
The -o $o and -f $n parameters are necessary as lavpipe somehow
has to inform lav2yuv which frames it should output. $o will
be replaced by the offset and $n will be replaced by the number
of frames that lavpipe wants lav2yuv to output. The -n 1
parameter is of course optional, and any other parameters to
lav2yuv could be added. The second line for epilogue.qt might
look like this: "lav2yuv -o $o -f $n epilogue.qt"

Now follow all the sequences of the Pipe List, each of which
consists of a listing of the input streams used and a command
line to the filter program.

The first sequence will simply reproduce all but the last 50
frames of intro.avi (that are 1040 - 50 = 990 frames). Its
first line only contains "990", the number of frames. The
second line is "1", the number of streams used.
The next line contains the index of the stream to use and the
offset (how many frames to skip in the beginning).
In our case both index and offset are 0, so the line would be:
"0 0"
Now we would add the command line of the filter program, but
as we don't want to invoke any filter here, this line only
contains "-", which causes lavpipe to simply output the contents
of the stream.

The second sequence is the actual transistion. So the first
line is "50" (two seconds), the second one "2" (we use both streams).
The following line will be "0 990" (intro.avi will be continued
at frame 990) and then "1 0" follows (epilogue.qt starts with
frame 0).
The next line is the filter command, in our case
"transist.flt -s $o -n $n -o 0 -O 255 -d 50"
The -s $o -n $n parameters equal to the -o $o -f $n parameters
of lav2yuv, -o 0 means that at the beginning of the transistion,
only intro.avi is visible (opacity of epilogue.qt = 0).
As you would have expected, -O 255 means that at the end of
the transistion, only epilogue.qt is visible (opacity of
epilogue.qt = 255 = 100%) - the opacity will be linearly
interpolated inbetween. And finally -d 50 is the duration
of the transistion in frames and should be equal to the
first line (duration in frames) of the sequence in most cases.

The last sequence continues with only epilogue.qt (the last
3870 frames), thus the first line is "3870". The second line
is "1" (only one stream), then "1 50" follows (epilogue.qt,
beginning with frame 50). The filter command line is "-" again.

Finally, our Pipe List file should look like this:

--------------------< trans.pli >--------------------------
LAV Pipe List
2
lav2yuv -o $o -f $n -n 1 intro.avi
lav2yuv -o $o -f $n epilogue.qt
990
1
0 0
-
50
2
0 990
1 0
transist.flt -s $o -n $n -o 0 -O 255 -d 25
3870
1
1 50
-
--------------------< end of file >------------------------

Remember the call? "lavpipe trans.pli | yuv2lav -o result.avi"
should now produce a nice avi file with a nice transistion.

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3. And what else can we do with it?                                          /
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Example two: Blend one movie over another one, using a third one's luminance
             channel as a matte (alpha channel) for the second one.

matteblend.flt has no parameters until now, as its output is independent of
the actual position in the stream (only depends on the input frames it is fed).

If you read the first example and have understood the pipe list format, it
will be easy to write a pipe list for this task. As there still is no
bluescreen.flt filter, and it is very time consuming to build an animated
matte channel for a given input movie by hand, I will only describe how to
blend a static picture (a title or static logo) over a movie.

For this you need your input.avi, a picture with an alpha channel. Use for
example the GIMP to save the image as plain yuv (pic.yuv) and save its
alpha channel as a grayscale plain yuv (matte.yuv - its chrominance channels
will be ignored). Of course the must be of the right size.
Now create this simple shell script that will output an infinite yuv stream
that only contains the given plain yuv picture:

--------------------< foreveryuv >-------------------------
#!/bin/sh
echo "YUV4MPEG 352 288 3"
while true
do
       echo "FRAME"
       cat $1
done
--------------------< end of file >------------------------

And write the pipe list:

--------------------< title.pli >--------------------------
LAV Pipe List
3
lav2yuv -o $o -f $n input.avi
foreveryuv pic.yuv
foreveryuv matte.yuv
75
3
0 0
1 0
2 0
1000000
1
0 75
matteblend.flt
--------------------< end of file >------------------------

As long as your input.avi is shorter than 1000076 frames,
"lavpipe title.pli | yuv2lav -o result.avi" will output
the whole movie with the given picture blended over it
for the first three seconds.

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4. That's all?! How can we make them do more?                                /
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The solution is of course to code new filter programs. And of course this
is very easy. I want to annote here, that the whole system is not very
fool proof at the moment. So if you feed matteblend.flt the wrong number
of input streams via lavpipe, you will get funny results, if you get
any results at all (without any hint from the programs). Perhaps this
could be improved by adding additional (optional) parameters to the
YUV4MPEG header line.

A filter program consists only of 4 parts:

1. Read input parameters (especially -o and -n, if the output is not only
   dependent on the input frames but also on some variable parameters that
   change over time) - optional.

2. Read in and write out the YUV headers, could look like this:

   int fd_in = 0, fd_out = 1; /* stdin, stdout */
   y4m_stream_info_t istream, ostream;
   int res, width, height, frame_rate_code;

   y4m_init_stream_info(&istream);
   y4m_init_frame_info(&iframe);

   if (y4m_read_stream_header (fd_in, &istream) != Y4M_OK)
      exit (1);

   y4m_init_stream_info(&ostream);
   y4m_copy_stream_info(&ostream,&istream);
   y4m_write_stream_header (fd_out, width, heigth, frame_rate_code);
   
3. Allocate the YUV buffer(s) - one for each input stream and perhaps one
   for the output or an arbitrary number of temporary buffers (no bloated
   code, please ;-) )
   
   char *yuv_buffer[3]; /* this is one yuv buffer */
   yuv_buffer[0] = (char *) malloc(y4m_si_get_plane_length(&istream, 0)); /* Y' */
   yuv_buffer[1] = (char *) malloc(y4m_si_get_plane_length(&istream, 1)); /* Cr */
   yuv_buffer[2] = (char *) malloc(y4m_si_get_plane_length(&istream, 2)); /* Cb */

4. The loop - while (number of frames processed) < (-n parameter)

4.1. Read the input frames, one of those for each input stream (e.g. yuv_buffer[123])

   while (y4m_read_frame (fd_in, &istream, &iframe, yuv_buffer) == Y4M_OK)
         {
4.2. Process the input buffers in any way you want.

5. Write out the result:

         y4m_write_frame(fd_out, &ostream, &iframe, yuv_buffer);
         }

6. Clean up:
     y4m_fini_frame(&iframe);
     y4m_fini_stream_info(&istream);
     y4m_fini_stream_info(&ostream);

That's all. You should in any case have a look at the existing filters,
transist.flt.c and matteblend.flt.c.

- pHilipp Zabel <[e-mail]>