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Working with media files becomes relatively painless when you use the BMediaFile and BMediaTrack classes. This section looks at a sample program that converts a media file from one format to another, using these classes.
void BuildMediaFormat(int32 width, int32 height, color_space cspace,
media_format *format) {
media_raw_video_format *rvf = &format->u.raw_video;
memset(format, 0, sizeof(*format));
format->type = B_MEDIA_RAW_VIDEO;
rvf->last_active = (uint32)(height - 1);
rvf->orientation = B_VIDEO_TOP_LEFT_RIGHT;
rvf->pixel_width_aspect = 1;
rvf->pixel_height_aspect = 3;
rvf->display.format = cspace;
rvf->display.line_width = (int32)width;
rvf->display.line_count = (int32)height;
if (cspace == B_RGB32)
rvf->display.bytes_per_row = 4 * width;
else {
printf("can>t build the format!n");
exit(5);
}
}
BuildMediaFormat() accepts as input parameters describing a video format—the width, height, and color space of the video—and returns a media_format structure describing that format. For our purposes, we require a B_RGB32 colorspace, and the frames will be in raw video format.
The transcode() function below handles actually converting a media file into another format, writing the newly-converted media into a new file. We'll look at it in chunks to lighten the load.
transcode() accepts as input a BMediaTrack referring to the video track from the original file, and another BMediaTrack referring to the audio track. output is the name of the new file to be created.
family_name specifies the file format family to be used when creating the new file, and video_name and audio_name indicate by name which encoders should be used for the video and audio tracks.
void transcode(BMediaTrack *vidtrack, BMediaTrack *audtrack,
char *output, char *family_name, char *video_name,
char *audio_name) {
char *chunk;
char *bitmap = NULL, *sound_buffer = NULL;
bool found_video_encoder = false, found_audio_encoder = false;
bool found_family;
int32 i, sz, cookie;
int64 numFrames, j;
int64 framesize;
status_t err;
entry_ref ref;
BMediaFile out;
BMediaTrack vid = NULL, *aud = NULL;
media_format format, outfmt;
media_codec_info mci;
media_file_format mfi;
media_header mh;
err = get_ref_for_path(output, &ref);
if (err) {
printf("problem with get_ref_for_path() -- %sn", strerror(err));
return;
}
The function begins by creating an entry_ref for the output file. If an error occurs, the function terminates after displaying an error message.
cookie = 0;
while((err = get_next_file_format(&cookie, &mfi)) == B_OK) {
if (strcmp(mfi.short_name, family_name) == 0)
break;
}
if (err != B_OK) {
printf("failed to find a file format handler !n");
return;
}
Next, a loop is used, calling get_next_file_format() to find the appropriate handler for the requested file format family. If the specified family_name doesn't match the short_name field of any of the media file formats available, an error is printed and the function aborts. Otherwise, mfi contains a description of the file format.
out = new BMediaFile(&ref, &mfi);
err = out->InitCheck();
if (err != B_OK) {
printf("failed to properly init the output file... (%s)n", strerror(err));
delete out;
return;
}
Once the appropriate media file format has been determined, a new BMediaFile is created. The entry_ref of the output file is specified, and the media file format we selected is provided. This will create the new media file, with no tracks. If this fails (BMediaFile::InitCheck() returns something other than B_OK), an error message is displayed, the BMediaFile is deleted, and the function returns.
The next chunk of code handles creating the new video track. Note that this only runs if vidtrack, the reference to the original BMediaTrack, isn't NULL. If it's NULL, the file is assumed not to have a video track.
if (vidtrack) {
vidtrack->EncodedFormat(&format);
if (video_name) {
int width, height;
width = format.u.encoded_video.output.display.line_width;
height = format.u.encoded_video.output.display.line_count;
memset(&format, 0, sizeof(format));
BuildMediaFormat(width, height, B_RGB32, &format);
vidtrack->DecodedFormat(&format);
bitmap = (char *)malloc(width * height * 4);
cookie = 0;
while (get_next_encoder(&cookie, &mfi, &format, &outfmt, &mci) == B_OK) {
printf("found encoder %s (%d)n", mci.pretty_name, mci.id);
if (strcmp(video_name, mci.short_name) == 0) {
found_video_encoder = true;
break;
}
}
}
If the video track exists, we determine the encoded format by calling BMediaTrack::EncodedFormat(). The returned media_format describes the format of the video frames in the original file.
If a video encoder for the output was specified, a media_format is constructed using the BuildMediaFormat() function we implemented previously. This format will be used for reading the frames from the original file into a raw, unencoded video buffer. We call BMediaTrack::DecodedFormat() on the original video track, specifying that this format should be used for outputting frames. From this point onward, frames delivered by vidtrack will be in raw video format.
A bitmap buffer is created. This will contain each frame of raw video while it's being transcoded, after being decoded and before it's encoded into the new file.
Another loop is used to locate an encoder that can convert raw video frames into the desired output format. This is done by calling get_next_encoder() in a loop, looking for an encoder that can accept data in the media_file_format specified by mfi, converting the frames into the media_format specified by format. A description of the encoder is returned in mci. When a match is found, we check to see if it matches the encoder name, video_name, requested by the input arguments. If it does, we accept it by setting the found_video_encoder flag to true.
if (found_video_encoder)
vid = out->CreateTrack(&format, &mci);
else
vid = out->CreateTrack(&format);
if (vid == NULL) {
printf("Failed to create video trackn");
delete out;
return;
}
}
If a video encoder has been found, a new video track is created, using the found encoder, mci. Otherwise, a raw video track is created. This covers the case where the user indicates that he doesn't want any encoding (if they specify a NULL video_name). A pointer to the new BMediaTrack is kept in vid.
If the track can't be created, an error message is displayed, the BMediaFile for the output is deleted, and the function returns.
Next, the audio track is prepared in the same way: the encoded format is determined, the decoded format is specified, and an encoder is located, followed by the creation of the new track. A sound_buffer is allocated to contain the amount of audio data that will be stuffed into each buffer returned as the source track is read.
if (audtrack) {
audtrack->EncodedFormat(&format);
audtrack->DecodedFormat(&format);
sound_buffer = (char*)malloc(format.u.raw_audio.buffer_size);
framesize = (format.u.raw_audio.format&15)*format.u.raw_audio.channel_count;
if (audio_name) {
cookie = 0;
while (get_next_encoder(&cookie, &mfi, &format, &outfmt, &mci) == B_OK) {
printf("found encoder %s (%d)n", mci.pretty_name, mci.id);
if (strcmp(audio_name, mci.short_name) == 0) {
found_audio_encoder = true;
break;
}
}
}
if (found_audio_encoder)
aud = out->CreateTrack(&format, &mci);
else
aud = out->CreateTrack(&format);
if (aud == NULL) {
printf("Failed to create audio trackn");
delete out;
return;
}
}
Final touches are then put on the new file's header.
// Add the copyright and commit the header
out->AddCopyright("Copyright 1999 Be Incorporated");
out->CommitHeader();
In this example, a copyright notice is added to the file by calling BMediaFile::AddCopyright(). Once everything's ready, we call BMediaFile::CommitHeader() to indicate that we're about to begin writing media data into the file.
Then writing of the video track begins, if there is one.
// Process the video track, if any
if (vidtrack) {
int is_key_frame = 0;
if (found_video_encoder) {
numFrames = vidtrack->CountFrames();
for(j = 0; j < numFrames; j++) {
int64 framecount = 1;
printf(" r");
printf("processing frame: %5Ld", j);
fflush(stdout);
err = vidtrack->ReadFrames(bitmap, &framecount, &mh);
if (err) {
printf("video: GetNextChunk error -- %sn", strerror(err));
break;
}
err = vid->WriteFrames(bitmap, 1, mh.u.encoded_video.field_flags);
if (err) {
printf("err %s (0x%x) writing video frame %Ldn",
strerror(err), err, j);
break;
}
}
If there's a video encoder in use (the output data will be encoded), we count up the number of frames in the video track by calling BMediaTrack::CountFrames(), then loop over all of the frames.
For each frame, we display a status notice to let the user know what we're doing, then read in the next frame from the source file by calling BMediaTrack::ReadFrames(). We specify the bitmap buffer as the buffer into which the frame should be read, and framecount will contain the number of read frames (it should always be 1 for video tracks). mh, a media_header, will receive a description of the data in the buffer.
If an error occurs reading the frame, an error message is printed, and the video conversion is terminated.
Otherwise, the output video track, vid, is written into by calling BMediaTrack::WriteFrames(). The new frame is automatically encoded and appended to the new file. If an error occurs doing this, an error message is displayed and processing is terminated.
If there's no encoder being used (the output file is going to be written as raw or unknown-format data), the following code is used:
} else {
numFrames = vidtrack->CountFrames();
for(j = 0; j < numFrames; j++) {
printf(" r");
printf("processing frame: %5Ld", j);
fflush(stdout);
err = vidtrack->ReadChunk(&chunk, &sz, &mh);
if (err) {
printf("video: GetNextChunk error -- %sn", strerror(err));
break;
}
err = vid->WriteChunk(chunk, sz, mh.u.encoded_video.field_flags);
if (err) {
printf("err %s (0x%x) writing video frame %Ldn",
strerror(err), err, j);
break;
}
}
}
printf("r r");
}
The only real difference here is that instead of using BMediaTrack::ReadFrames() and BMediaTrack::WriteFrames(), we use BMediaTrack::ReadChunk() and BMediaTrack::WriteChunk(). These work with media data without attempting to interpret the data in any way.
One or the other of these loops will continue until either an error occurs, or the entire video track is converted.
Next, the audio track is converted, if there is one. As you can see, this is done in almost exactly the same manner, except that each buffer we receive will have more than one frame of audio in it. Note how the loop that iterates over the frames adds framecount to its counter variable each pass; framecount indicates the number of frames returned by the last call to ReadFrames() or ReadChunk().
// Process the audio track, if any
if (audtrack) {
int64 framecount = 0;
if (found_audio_encoder) {
// Decode and encode all the frames
numFrames = audtrack->CountFrames();
printf("Total frame count : %Ldn", numFrames);
for (j = 0; j < numFrames; j+=framecount) {
err = audtrack->ReadFrames(sound_buffer, &framecount, &mh);
if (err) {
printf("video: GetNextChunk error -- %sn", strerror(err));
break;
}
err = aud->WriteFrames(sound_buffer, framecount);
if (err) {
printf("err %s (0x%x) writing audio frame %Ldn",
strerror(err), err, j);
break;
}
}
} else {
printf("processing chunks...n");
while (true) {
err = audtrack->ReadFrames(sound_buffer, &framecount, &mh);
if (err) {
printf("audio: GetNextChunk error -- %sn", strerror(err));
break;
}
err = aud->WriteChunk(sound_buffer, framecount*framesize);
if (err) {
printf("err %s (0x%x) writing audio chunk %Ldn",
strerror(err), err, j);
break;
}
}
}
printf("r r");
}
Once converting the audio is done, there's very little left to do but release the video and audio buffers we've allocated, close the output BMediaFile, and delete it.
if (bitmap) free(bitmap); if (sound_buffer) free(sound_buffer); out->CloseFile(); delete out; out = NULL; }
After this function returns, the caller is responsible for deleting the source BMediaFile.
Let's look at a main() that uses the transcode() function to provide a command-line utility for converting media files from one format to another.
int main(int argc, char **argv) {
status_t err;
entry_ref ref;
media_format format;
BMediaFile mediaFile;
BMediaTrack *track = NULL, *vidtrack = NULL, *audtrack = NULL;
int32 i, numTracks;
char *input = NULL, *output = NULL;
char *video_encoder_name = NULL, *audio_encoder_name = NULL;
char *family_name = NULL;
if (argc < 2) {
printf("usage: %s [-info][-avi|-qt][-wav][-aiff][-v <encoder_name>][-a <encoder_name>] <filename> [<output>]n", argv[0]);
return 1;
}
If the number of arguments is less than 2, a usage notice is printed.
for (i=1; i < argc; i++) {
if (strcmp(&argv[i][0], "-info") == 0) {
dump_info();
exit(0);
} else if (strcmp(&argv[i][0], "-avi") == 0 ||
strcmp(&argv[i][0], "-wav") == 0 ||
strcmp(&argv[i][0], "-aiff") == 0 ||
strcmp(&argv[i][0], "-quicktime") == 0) {
family_name = &argv[i][1];
} else if (strcmp(&argv[i][0], "-qt") == 0) {
family_name = "quicktime";
} else if (strcmp(&argv[i][0], "-v") == 0 && argv[i+1]) {
video_encoder_name = argv[i+1];
i++;
} else if (strcmp(&argv[i][0], "-a") == 0 && argv[i+1]) {
audio_encoder_name = argv[i+1];
i++;
} else if (input == NULL) {
input = &argv[i][0];
} else if (output == NULL) {
output = &argv[i][0];
} else {
printf("%s: extra argument %sn", &argv[0][0], &argv[i][0]);
}
}
The arguments are interpreted here. The arguments are:
input is the input file's name, and output is the name of the new media file to be created.
if (output == NULL)
output = "output";
err = get_ref_for_path(input, &ref);
if (err) {
printf("problem with get_ref_for_path() -- %sn", strerror(err));
return 1;
}
If no output file name is specified, the name "output" is assumed. An entry_ref to the input file is constructed; if the file isn't found, an error message is printed and the program exits.
mediaFile = new BMediaFile(&ref);
err = mediaFile->InitCheck();
if (err) {
printf("cannot contruct BMediaFile object -- %sn", strerror(err));
return 1;
}
A BMediaFile is then instantiated, referencing the input file. If an error occurs, an error is displayed and the program terminates.
numTracks = mediaFile->CountTracks();
printf("%s has %d media tracksn", input, numTracks);
const char *copyright = mediaFile->Copyright();
if (copyright)
printf("#### copyright info: %sn", copyright);
The number of tracks in the source file is obtained by calling BMediaFile::CountTracks(), and the file's copyright notice is obtained by calling BMediaFile::Copyright(). This information is printed for the user to read.
for(i=0; i < numTracks; i++) {
track = mediaFile->TrackAt(i);
if (!track) {
printf("cannot get track %d?!?n", i);
return 1;
}
// get the encoded format
err = track->EncodedFormat(&format);
if (err) {
printf("BMediaTrack::EncodedFormat error -- %sn", strerror(err));
return 1;
}
if (format.type == B_MEDIA_RAW_VIDEO ||
format.type == B_MEDIA_ENCODED_VIDEO) {
vidtrack = track;
} else if (format.type == B_MEDIA_RAW_AUDIO ||
format.type == B_MEDIA_ENCODED_AUDIO) {
audtrack = track;
} else {
mediaFile->ReleaseTrack(track);
track = NULL;
}
}
Next, a loop iterates over all the tracks in the file by calling BMediaFile::TrackAt() to obtain BMediaTrack objects referencing them, one by one. Each track's encoded format is obtained, and is checked to see if the track represents a video or audio track. If it's a video track, it's kept in the vidtrack variable. If it's an audio track, it's kept in audtrack. If it's neither, the track is released.
This serves to search all the tracks for a video and an audio track to be converted; most movies will only have one of each, but may have other informational tracks, which need to be ignored for our purposes.
if (vidtrack == NULL && audtrack == NULL) {
printf("%s has no audio or video tracks?!?n", input);
return 1;
}
If there's neither a video nor an audio track, the source file is empty and isn't worth converting, so an error message is displayed and the program terminates.
if (family_name == NULL && vidtrack == NULL) family_name = "wav"; else if (family_name == NULL) family_name = "quicktime";
If the user didn't specify an output family, and the file has no video track, the WAV file format is assumed. You can change the program to assume AIFF for audio-only files by changing "wav" to "aiff" here.
For any file that has a video track, if no family is specified by the user, QuickTime format is assumed. Again, if you want the default to be another format, you can change that here.
transcode(vidtrack, audtrack, output, family_name, video_encoder_name, audio_encoder_name);
The conversion is performed by passing all these parameters to transcode() to do the real work.
delete mediaFile; return 0; }
Once transcode() returns, we simply delete the source file and exit the program. The file has been converted (assuming no errors occurred in transcode().
When creating a real application, you'll need to provide a way for the user to review a list of the various media file formats available, as well as the encoders provided for each format. The dump_info() sample function discussed in the description of the get_next_file_format() function shows how this can be done.
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