<TITLE>LibMPEG3</TITLE>

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<FONT FACE=HELVETICA SIZE=+4><B>Using LibMPEG3 to make your own MPEG applications</B></FONT><P>

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Author: Adam Williams    [email protected]<BR>
Homepage: heroinewarrior.com<BR>
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LibMPEG3 decodes the many many derivatives of MPEG standards into
uncompressed data suitable for editing and playback.<P>

libmpeg3 currently decodes:<P>

<BLOCKQUOTE>MPEG-2 video<BR>
MPEG-1 video<BR>
mp3 audio<BR>
mp2 audio<BR>
ac3 audio (not in Squeak)<BR>
MPEG-2 system streams<BR>
MPEG-1 system streams
</BLOCKQUOTE><P>

The video output can be in many different color models and frame
sizes.  The audio output can be in twos compliment or floating
point.<P>









<FONT FACE=HELVETICA SIZE=+4><B>STEP 1: Verifying file compatibility</B></FONT><P>

Programs using libmpeg3 must <CODE>#include "libmpeg3.h"</CODE>.<P>

Call <CODE>mpeg3_check_sig</CODE> to verify if the file can be read by
libmpeg3.  This returns a 1 if it is compatible and 0 if it isn't.<P>











<FONT FACE=HELVETICA SIZE=+4><B>STEP 2: Open the file</B></FONT><P>

You need an <CODE>mpeg3_t*</CODE> file descriptor:<P>
<CODE>
mpeg3_t* file;
</CODE>
<P>

Then you need to open the file:<P>

<CODE>file = mpeg3_open(char *path);</CODE><P>

<CODE>mpeg3_open</CODE> returns a NULL if the file couldn't be opened
for some reason.  Be sure to check this.  Everything you do with
libmpeg3 requires passing the <CODE>file</CODE> pointer.<P>














<FONT FACE=HELVETICA SIZE=+4><B>STEP 3: How many CPUs do you want to use?</B></FONT><P>

Call <CODE>mpeg3_set_cpus(mpeg3_t *file, int cpus)</CODE> to set how
many CPUs should be devoted to video decompression.  LibMPEG3 can use
any number.  If you don't call this right after opening the file, the
CPU number defaults to 1.<P>







<FONT FACE=HELVETICA SIZE=+4><B>STEP 4: Get some information about the file.</B></FONT><P>

There are a number of queries for the audio components of the stream:<P>

<CODE><PRE>
int mpeg3_has_audio(mpeg3_t *file);
int mpeg3_total_astreams(mpeg3_t *file);             // Number of multiplexed audio streams
int mpeg3_audio_channels(mpeg3_t *file, int stream);
int mpeg3_sample_rate(mpeg3_t *file, int stream);
long mpeg3_audio_samples(mpeg3_t *file, int stream); // Total length
</PRE></CODE>

The audio is presented as a number of <B>streams</B> starting at 0 and
including <CODE>mpeg3_total_astreams</CODE> - 1.  Each stream contains a
certain number of <B>channels</B> starting at 0 and including
<CODE>mpeg3_audio_channels</CODE> - 1.

The methodology is first determine if the file has audio, then get
the number of streams in the file, then for each stream get the number
of channels, sample rate, and length.<P>

There are also queries for the video components:<P>

<CODE><PRE>
int mpeg3_has_video(mpeg3_t *file);
int mpeg3_total_vstreams(mpeg3_t *file);            // Number of multiplexed video streams
int mpeg3_video_width(mpeg3_t *file, int stream);
int mpeg3_video_height(mpeg3_t *file, int stream);
float mpeg3_frame_rate(mpeg3_t *file, int stream);  // Frames/sec
long mpeg3_video_frames(mpeg3_t *file, int stream); // Total length
</PRE></CODE>

The video behavior is the same as with audio, except that video has no
subdivision under <B>streams</B>.  Frame rate is a floating point
number of frames per second.<P>







<FONT FACE=HELVETICA SIZE=+4><B>STEP 5: Seeking to a point in the file</B></FONT><P>

Each audio stream and each video stream has a position in the file
independant of each other stream.  A variety of methods are available
for specifying the position of a stream: percentage, frame, sample. 
Which method you use depends on whether you're seeking audio or video
and whether you're seeking all tracks to a percentage of the file.<P>

The preferred seeking method if you're writing a player is:<P>

<CODE><PRE>
int mpeg3_seek_percentage(mpeg3_t *file, double percentage);
double mpeg3_tell_percentage(mpeg3_t *file);
</PRE></CODE>

This seeks all tracks to a percentage of the file length.  The
percentage is from 0 to 1.<P>

The alternative is absolute seeking.  The audio seeking is handled
by:<P>

<CODE><PRE>
int mpeg3_set_sample(mpeg3_t *file, long sample, int stream);    // Seek
long mpeg3_get_sample(mpeg3_t *file, int stream);    // Tell current position
</PRE></CODE>

and the video seeking is handled by:<P>

<CODE><PRE>
int mpeg3_set_frame(mpeg3_t *file, long frame, int stream); // Seek
long mpeg3_get_frame(mpeg3_t *file, int stream);            // Tell current position
</PRE></CODE>


You can either perform percentage seeking or absolute seeking but not
both on the same file handle.  Once you perform either method, the file
becomes configured for that method.<P>

If you're in percentage seeking mode and you want the current time
stamp in the file you can't use mpeg3_tell_percentage because you don't
know how many seconds the total length is.  The
<CODE>mpeg3_audio_samples</CODE> and <CODE>mpeg3_video_frames</CODE>
commands don't work in percentage seeking.  Instead use

<CODE><PRE>
double mpeg3_get_time(mpeg3_t *file);
</PRE></CODE>

which gives you the last timecode read in seconds.  The MPEG standard
specifies timecodes being placed in the streams.<P>











<FONT FACE=HELVETICA SIZE=+4><B>STEP 6: Read the data</B></FONT><P>

To read <B>audio</B> data use:<P>

<CODE><PRE>
int mpeg3_read_audio(mpeg3_t *file, 
		float *output_f,      // Pointer to pre-allocated buffer of floats
		short *output_i,      // Pointer to pre-allocated buffer if int16's
		int channel,          // Channel to decode
		long samples,         // Number of samples to decode
		int stream);          // Stream containing the channel
</PRE></CODE>

This decodes a buffer of sequential floats or int16's for a single
channel, depending on which *output... parameter has a nonzero
argument.  To get a floating point buffer pass a pre-allocated buffer
to <CODE>output_f</CODE> and NULL to <CODE>output_i</CODE>. To get an
int16 buffer pass NULL to <CODE>output_f</CODE> and a pre-allocated
buffer to <CODE>output_i</CODE>.<P>

After reading an audio buffer, the current position in the one stream
is advanced.  How then, do you read more than one channel of audio
data?  Use

<CODE><PRE>
mpeg3_reread_audio(mpeg3_t *file, 
		float *output_f,      /* Pointer to pre-allocated buffer of floats */
		short *output_i,      /* Pointer to pre-allocated buffer of int16's */
		int channel,          /* Channel to decode */
		long samples,         /* Number of samples to decode */
		int stream);
</PRE></CODE>

to read each remaining channel after the first channel.<P>

To read <B>video</B> data there are two methods.  RGB frames or YUV
frames.  To get an RGB frame use:<BR>

<CODE><PRE>
int mpeg3_read_frame(mpeg3_t *file, 
		unsigned char **output_rows, // Array of pointers to the start of each output row
		int in_x,                    // Location in input frame to take picture
		int in_y, 
		int in_w, 
		int in_h, 
		int out_w,                   // Dimensions of output_rows
		int out_h, 
		int color_model,             // One of the color model #defines given above.
		int stream);
</PRE></CODE>

The video decoding works like a camcorder taking copy of a movie
screen.  The decoder "sees" a region of the movie screen defined by
<CODE>in_x, in_y, in_w, in_h</CODE> and transfers it to the frame
buffer defined by <CODE>**output_rows</CODE>.  The input values must be
within the boundaries given by <CODE>mpeg3_video_width</CODE> and
<CODE>mpeg3_video_height</CODE>.  The size of the frame buffer is
defined by <CODE>out_w, out_h</CODE>.  Although the input dimensions
are constrained, the frame buffer can be any size.<P>

<CODE>color_model</CODE> defines which RGB color model the picture
should be decoded to and the possible values are given in
<B>libmpeg3.h</B>.  The frame buffer pointed to by
<CODE>output_rows</CODE> must have enough memory allocated to store the
color model you select.<P>

<B>You must allocate 4 extra bytes in the last output_row.</B>  This is
scratch area for the MMX routines.<P>

<CODE>mpeg3_read_frame</CODE> advances the position in the one stream by 1 frame.<P>

The alternative is YUV frames:<BR>

<CODE><PRE>
int mpeg3_read_yuvframe(mpeg3_t *file,
		char *y_output,
		char *u_output,
		char *v_output,
		int in_x,
		int in_y,
		int in_w,
		int in_h,
		int stream);
</PRE></CODE>

The behavior of in_x, in_y, in_w, in_h is identical to mpeg3_read_frame
except here you have no control over the output frame size.  <B>You
must allocate in_w * in_h for the y_output, and in_w * in_h / 4 for the
chroma outputs.</B><P>





<FONT FACE=HELVETICA SIZE=+4><B>STEP 7: Close the file</B></FONT><P>

Be sure to close the file with <CODE>mpeg3_close(mpeg3_t *file)</CODE>
when you're done with it.