Author: Adam Williams broadcast@earthling.net
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LibMPEG3 decodes the many many derivatives of MPEG standards into uncompressed data suitable for editing and playback.
It currently supports:
MPEG-2 video
MPEG-1 video
mp3 audio
mp2 audio
ac3 audio
MPEG-2 system streams
MPEG-1 system streams
The video output can be in many different color models and frame sizes. The audio output can be in twos compliment or floating point.
STEP 1: Verifying file compatibility
Programs using libmpeg3 must #include "libmpeg3.h"
.
Call mpeg3_check_sig
to verify if the file can be read by
libmpeg3. This returns a 1 if it is compatible and 0 if it isn't.
STEP 2: Open the file
You need an mpeg3_t*
file descriptor:
mpeg3_t* file;
Then you need to open the file:
file = mpeg3_open(char *path);
mpeg3_open
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 file
pointer.
STEP 3: How many CPUs do you want to use?
Call mpeg3_set_cpus(mpeg3_t *file, int cpus)
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.
STEP 4: Get some information about the file.
There are a number of queries for the audio components of the stream:
The audio is presented as a number of streams starting at 0 and
including
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
mpeg3_total_astreams
- 1. Each stream contains a
certain number of channels starting at 0 and including
mpeg3_audio_channels
- 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.
There are also queries for the video components:
The video behavior is the same as with audio, except that video has no
subdivision under streams. Frame rate is a floating point
number of frames per second.
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
STEP 5: Seek to a point in the file
Each audio stream and each video stream has a position in the file
independant of each other stream. The positions are defined by frame
number and sample number.
The audio seeking is handled by:
and the video seeking is handled by:
int mpeg3_set_sample(mpeg3_t *file, long sample, int stream); // Seek
long mpeg3_get_sample(mpeg3_t *file, int stream); // Tell current position
STEP 6: Read the data
int mpeg3_set_frame(mpeg3_t *file, long frame, int stream); // Seek
long mpeg3_get_frame(mpeg3_t *file, int stream); // Tell current position
To read audio data use:
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
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
output_f
and NULL to output_i
. To get an
int16 buffer pass NULL to output_f
and a pre-allocated
buffer to output_i
.
After reading an audio buffer, the current position in the one stream
is advanced. To read each channel you need to first call an
mpeg3_set_sample
and then an mpeg3_read_audio
for each channel.
To read video data use:
The video decoding works like a camcorder taking an illegal copy of a
movie screen. The decoder "sees" a region of the movie screen defined
by
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);
in_x, in_y, in_w, in_h
and transfers it to the frame
buffer defined by **output_rows
. The input values must be
within the boundaries given by mpeg3_video_width
and
mpeg3_video_height
. The size of the frame buffer is
defined by out_w, out_h
. Although the input dimensions
are constrained, the frame buffer can be any size.
color_model
defines which color model the picture should
be decoded to and the possible values are given in libmpeg3.h.
The frame buffer pointed to by output_rows
must have
enough memory allocated to store the color model you select.
mpeg3_read_frame
advances the position in the one stream by 1 frame.
STEP 7: Close the file
Be sure to close the file with mpeg3_close(mpeg3_t *file)
when you're done with it.