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.
libmpeg3 currently decodes:
MPEG-2 video
MPEG-1 video
mp3 audio
mp2 audio
ac3 audio (not in Squeak)
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:
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:
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: Seeking to a point in the file
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.
The preferred seeking method if you're writing a player is:
int mpeg3_seek_percentage(mpeg3_t *file, double percentage);
double mpeg3_tell_percentage(mpeg3_t *file);
The alternative is absolute seeking. The audio 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
int mpeg3_set_frame(mpeg3_t *file, long frame, int stream); // Seek
long mpeg3_get_frame(mpeg3_t *file, int stream); // Tell current position
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
mpeg3_audio_samples and mpeg3_video_frames
commands don't work in percentage seeking. Instead use
double mpeg3_get_time(mpeg3_t *file);
STEP 6: Read the data
To read audio data use:
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. How then, do you read more than one channel of audio
data? Use
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);
To read video data there are two methods. RGB frames or YUV
frames. To get an RGB frame use:
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 RGB 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.
You must allocate 4 extra bytes in the last output_row. This is scratch area for the MMX routines.
mpeg3_read_frame advances the position in the one stream by 1 frame.
The alternative is YUV frames:
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);
STEP 7: Close the file
Be sure to close the file with mpeg3_close(mpeg3_t *file)
when you're done with it.