Libswresample

Libswresample (lswr) is a library that handles audio resampling, sample format conversion and mixing. More...

Files
file	swresample.h
	libswresample public header

Macros
#define	SWR_CH_MAX   32
	Maximum number of channels.

Functions
const AVClass *	swr_get_class (void)
	Get the AVClass for SwrContext.

Option constants
These constants are used for the AVOptions interface for lswr.
enum	SwrDitherType {   SWR_DITHER_NONE = 0, SWR_DITHER_RECTANGULAR, SWR_DITHER_TRIANGULAR, SWR_DITHER_TRIANGULAR_HIGHPASS,   SWR_DITHER_NS = 64, SWR_DITHER_NS_LIPSHITZ, SWR_DITHER_NS_F_WEIGHTED, SWR_DITHER_NS_MODIFIED_E_WEIGHTED,   SWR_DITHER_NS_IMPROVED_E_WEIGHTED, SWR_DITHER_NS_SHIBATA, SWR_DITHER_NS_LOW_SHIBATA, SWR_DITHER_NS_HIGH_SHIBATA,   SWR_DITHER_NB }
	Dithering algorithms. More...
enum	SwrEngine { SWR_ENGINE_SWR, SWR_ENGINE_SOXR, SWR_ENGINE_NB }
	Resampling Engines. More...
enum	SwrFilterType { SWR_FILTER_TYPE_CUBIC, SWR_FILTER_TYPE_BLACKMAN_NUTTALL, SWR_FILTER_TYPE_KAISER }
	Resampling Filter Types. More...
#define	SWR_FLAG_RESAMPLE   1
	Force resampling even if equal sample rate.

SwrContext constructor functions
struct SwrContext *	swr_alloc (void)
	Allocate SwrContext.
int	swr_init (struct SwrContext *s)
	Initialize context after user parameters have been set.
int	swr_is_initialized (struct SwrContext *s)
	Check whether an swr context has been initialized or not.
struct SwrContext *	swr_alloc_set_opts (struct SwrContext *s, int64_t out_ch_layout, enum AVSampleFormat out_sample_fmt, int out_sample_rate, int64_t in_ch_layout, enum AVSampleFormat in_sample_fmt, int in_sample_rate, int log_offset, void *log_ctx)
	Allocate SwrContext if needed and set/reset common parameters.

SwrContext destructor functions
void	swr_free (struct SwrContext **s)
	Free the given SwrContext and set the pointer to NULL.
void	swr_close (struct SwrContext *s)
	Closes the context so that swr_is_initialized() returns 0.

Core conversion functions
int	swr_convert (struct SwrContext *s, uint8_t **out, int out_count, const uint8_t **in, int in_count)
	Convert audio.
int64_t	swr_next_pts (struct SwrContext *s, int64_t pts)
	Convert the next timestamp from input to output timestamps are in 1/(in_sample_rate * out_sample_rate) units.

Low-level option setting functions
These functons provide a means to set low-level options that is not possible with the AVOption API.
int	swr_set_compensation (struct SwrContext *s, int sample_delta, int compensation_distance)
	Activate resampling compensation ("soft" compensation).
int	swr_set_channel_mapping (struct SwrContext *s, const int *channel_map)
	Set a customized input channel mapping.
int	swr_set_matrix (struct SwrContext *s, const double *matrix, int stride)
	Set a customized remix matrix.

Sample handling functions
int	swr_drop_output (struct SwrContext *s, int count)
	Drops the specified number of output samples.
int	swr_inject_silence (struct SwrContext *s, int count)
	Injects the specified number of silence samples.
int64_t	swr_get_delay (struct SwrContext *s, int64_t base)
	Gets the delay the next input sample will experience relative to the next output sample.

Configuration accessors
unsigned	swresample_version (void)
	Return the LIBSWRESAMPLE_VERSION_INT constant.
const char *	swresample_configuration (void)
	Return the swr build-time configuration.
const char *	swresample_license (void)
	Return the swr license.

AVFrame based API
int	swr_convert_frame (SwrContext *swr, AVFrame *output, const AVFrame *input)
	Convert the samples in the input AVFrame and write them to the output AVFrame.
int	swr_config_frame (SwrContext *swr, const AVFrame *out, const AVFrame *in)
	Configure or reconfigure the SwrContext using the information provided by the AVFrames.

Detailed Description

Libswresample (lswr) is a library that handles audio resampling, sample format conversion and mixing.

Interaction with lswr is done through SwrContext, which is allocated with swr_alloc() or swr_alloc_set_opts(). It is opaque, so all parameters must be set with the AVOptions API.

The first thing you will need to do in order to use lswr is to allocate SwrContext. This can be done with swr_alloc() or swr_alloc_set_opts(). If you are using the former, you must set options through the AVOptions API. The latter function provides the same feature, but it allows you to set some common options in the same statement.

For example the following code will setup conversion from planar float sample format to interleaved signed 16-bit integer, downsampling from 48kHz to 44.1kHz and downmixing from 5.1 channels to stereo (using the default mixing matrix). This is using the swr_alloc() function.

SwrContext *swr = swr_alloc();
av_opt_set_channel_layout(swr, "in_channel_layout",  AV_CH_LAYOUT_5POINT1, 0);
av_opt_set_channel_layout(swr, "out_channel_layout", AV_CH_LAYOUT_STEREO,  0);
av_opt_set_int(swr, "in_sample_rate",     48000,                0);
av_opt_set_int(swr, "out_sample_rate",    44100,                0);
av_opt_set_sample_fmt(swr, "in_sample_fmt",  AV_SAMPLE_FMT_FLTP, 0);
av_opt_set_sample_fmt(swr, "out_sample_fmt", AV_SAMPLE_FMT_S16,  0);

The same job can be done using swr_alloc_set_opts() as well:

SwrContext *swr = swr_alloc_set_opts(NULL,  // we're allocating a new context
                      AV_CH_LAYOUT_STEREO,  // out_ch_layout
                      AV_SAMPLE_FMT_S16,    // out_sample_fmt
                      44100,                // out_sample_rate
                      AV_CH_LAYOUT_5POINT1, // in_ch_layout
                      AV_SAMPLE_FMT_FLTP,   // in_sample_fmt
                      48000,                // in_sample_rate
                      0,                    // log_offset
                      NULL);                // log_ctx

Once all values have been set, it must be initialized with swr_init(). If you need to change the conversion parameters, you can change the parameters using AVOptions, as described above in the first example; or by using swr_alloc_set_opts(), but with the first argument the allocated context. You must then call swr_init() again.

The conversion itself is done by repeatedly calling swr_convert(). Note that the samples may get buffered in swr if you provide insufficient output space or if sample rate conversion is done, which requires "future" samples. Samples that do not require future input can be retrieved at any time by using swr_convert() (in_count can be set to 0). At the end of conversion the resampling buffer can be flushed by calling swr_convert() with NULL in and 0 in_count.

The samples used in the conversion process can be managed with the libavutil samples manipulation API, including av_samples_alloc() function used in the following example.

The delay between input and output, can at any time be found by using swr_get_delay().

The following code demonstrates the conversion loop assuming the parameters from above and caller-defined functions get_input() and handle_output():

uint8_t **input;
int in_samples;

while (get_input(&input, &in_samples)) {
    uint8_t *output;
    int out_samples = av_rescale_rnd(swr_get_delay(swr, 48000) +
                                     in_samples, 44100, 48000, AV_ROUND_UP);
    av_samples_alloc(&output, NULL, 2, out_samples,
                     AV_SAMPLE_FMT_S16, 0);
    out_samples = swr_convert(swr, &output, out_samples,
                                     input, in_samples);
    handle_output(output, out_samples);
    av_freep(&output);
}

When the conversion is finished, the conversion context and everything associated with it must be freed with swr_free(). A swr_close() function is also available, but it exists mainly for compatibility with libavresample, and is not required to be called.

There will be no memory leak if the data is not completely flushed before swr_free().

Macro Definition Documentation

#define SWR_CH_MAX   32

Maximum number of channels.

Definition at line 130 of file swresample.h.

#define SWR_FLAG_RESAMPLE   1

Force resampling even if equal sample rate.

Definition at line 140 of file swresample.h.

Referenced by swr_init(), and swr_set_compensation().

Enumeration Type Documentation

enum SwrDitherType

Dithering algorithms.

Enumerator:

SWR_DITHER_NONE
SWR_DITHER_RECTANGULAR
SWR_DITHER_TRIANGULAR
SWR_DITHER_TRIANGULAR_HIGHPASS
SWR_DITHER_NS	not part of API/ABI
SWR_DITHER_NS_LIPSHITZ
SWR_DITHER_NS_F_WEIGHTED
SWR_DITHER_NS_MODIFIED_E_WEIGHTED
SWR_DITHER_NS_IMPROVED_E_WEIGHTED
SWR_DITHER_NS_SHIBATA
SWR_DITHER_NS_LOW_SHIBATA
SWR_DITHER_NS_HIGH_SHIBATA
SWR_DITHER_NB	not part of API/ABI

Definition at line 145 of file swresample.h.

enum SwrEngine

Resampling Engines.

Enumerator:

SWR_ENGINE_SWR	SW Resampler.
SWR_ENGINE_SOXR	SoX Resampler.
SWR_ENGINE_NB	not part of API/ABI

Definition at line 163 of file swresample.h.

enum SwrFilterType

Resampling Filter Types.

Enumerator:

SWR_FILTER_TYPE_CUBIC	Cubic.
SWR_FILTER_TYPE_BLACKMAN_NUTTALL	Blackman Nuttall Windowed Sinc.
SWR_FILTER_TYPE_KAISER	Kaiser Windowed Sinc.

Definition at line 170 of file swresample.h.

Function Documentation

const AVClass* swr_get_class

(

void

)

Get the AVClass for SwrContext.

It can be used in combination with AV_OPT_SEARCH_FAKE_OBJ for examining options.

See Also

av_opt_find().

Returns

the AVClass of SwrContext

Definition at line 143 of file options.c.

Referenced by opt_default(), resample_child_class_next(), and show_help_default().

struct SwrContext* swr_alloc ( void  )

read

Allocate SwrContext.

If you use this function you will need to set the parameters (manually or with swr_alloc_set_opts()) before calling swr_init().

See Also

swr_alloc_set_opts(), swr_init(), swr_free()

Returns

NULL on error, allocated context otherwise

Definition at line 148 of file options.c.

Referenced by config_audio_output(), init_dict(), main(), open_audio(), opt_default(), opus_decode_init(), and swr_alloc_set_opts().

int swr_init

(

struct SwrContext *

s

)

Initialize context after user parameters have been set.

Note

The context must be configured using the AVOption API.

See Also

av_opt_set_int()

av_opt_set_dict()

Parameters

[in,out]

s

Swr context to initialize

Returns

AVERROR error code in case of failure.

Definition at line 153 of file swresample.c.

Referenced by audio_decode_frame(), config_audio_output(), config_output(), config_props(), init_resampler(), main(), open_audio(), opus_init_resample(), swr_convert_frame(), and swr_set_compensation().

int swr_is_initialized

(

struct SwrContext *

s

)

Check whether an swr context has been initialized or not.

Parameters

[in]

s

Swr context to check

See Also

swr_init()

Returns

positive if it has been initialized, 0 if not initialized

Definition at line 646 of file swresample.c.

Referenced by opus_decode_frame(), opus_decode_subpacket(), swr_convert(), and swr_convert_frame().

struct SwrContext* swr_alloc_set_opts ( struct SwrContext *  s, int64_t  out_ch_layout, enum AVSampleFormat  out_sample_fmt, int  out_sample_rate, int64_t  in_ch_layout, enum AVSampleFormat  in_sample_fmt, int  in_sample_rate, int  log_offset, void *  log_ctx  )

read

Allocate SwrContext if needed and set/reset common parameters.

This function does not require s to be allocated with swr_alloc(). On the other hand, swr_alloc() can use swr_alloc_set_opts() to set the parameters on the allocated context.

Parameters

s	existing Swr context if available, or NULL if not
out_ch_layout	output channel layout (AV_CH_LAYOUT_*)
out_sample_fmt	output sample format (AV_SAMPLE_FMT_*).
out_sample_rate	output sample rate (frequency in Hz)
in_ch_layout	input channel layout (AV_CH_LAYOUT_*)
in_sample_fmt	input sample format (AV_SAMPLE_FMT_*).
in_sample_rate	input sample rate (frequency in Hz)
log_offset	logging level offset
log_ctx	parent logging context, can be NULL

See Also

swr_init(), swr_free()

Returns

NULL on error, allocated context otherwise

Definition at line 58 of file swresample.c.

Referenced by audio_decode_frame(), config_output(), config_props(), init_resampler(), and main().

void swr_free

(

struct SwrContext **

s

)

Free the given SwrContext and set the pointer to NULL.

Parameters

[in]

s

a pointer to a pointer to Swr context

Definition at line 138 of file swresample.c.

Referenced by audio_decode_frame(), close_stream(), init_resampler(), main(), opt_default(), opus_decode_close(), stream_component_close(), swr_alloc_set_opts(), and uninit().

void swr_close

(

struct SwrContext *

s

)

Closes the context so that swr_is_initialized() returns 0.

The context can be brought back to life by running swr_init(), swr_init() can also be used without swr_close(). This function is mainly provided for simplifying the usecase where one tries to support libavresample and libswresample.

Parameters

[in,out]

s

Swr context to be closed

Definition at line 149 of file swresample.c.

Referenced by opus_decode_flush(), opus_decode_subpacket(), swr_config_frame(), and swr_convert_frame().

int swr_convert	(	struct SwrContext *	s,
		uint8_t **	out,
		int	out_count,
		const uint8_t **	in,
		int	in_count
	)

Convert audio.

in and in_count can be set to 0 to flush the last few samples out at the end.

If more input is provided than output space then the input will be buffered. You can avoid this buffering by providing more output space than input. Conversion will run directly without copying whenever possible.

Parameters

s	allocated Swr context, with parameters set
out	output buffers, only the first one need be set in case of packed audio
out_count	amount of space available for output in samples per channel
in	input buffers, only the first one need to be set in case of packed audio
in_count	number of input samples available in one channel

Returns

number of samples output per channel, negative value on error

int64_t swr_next_pts	(	struct SwrContext *	s,
		int64_t	pts
	)

Convert the next timestamp from input to output timestamps are in 1/(in_sample_rate * out_sample_rate) units.

Note

There are 2 slightly differently behaving modes.

• When automatic timestamp compensation is not used, (min_compensation >= FLT_MAX) in this case timestamps will be passed through with delays compensated
• When automatic timestamp compensation is used, (min_compensation < FLT_MAX) in this case the output timestamps will match output sample numbers. See ffmpeg-resampler(1) for the two modes of compensation.

Parameters

s[in]	initialized Swr context
pts[in]	timestamp for the next input sample, INT64_MIN if unknown

See Also

swr_set_compensation(), swr_drop_output(), and swr_inject_silence() are function used internally for timestamp compensation.

Returns

the output timestamp for the next output sample

Definition at line 823 of file swresample.c.

Referenced by filter_frame(), and flush_frame().

int swr_set_compensation	(	struct SwrContext *	s,
		int	sample_delta,
		int	compensation_distance
	)

Activate resampling compensation ("soft" compensation).

This function is internally called when needed in swr_next_pts().

Parameters

[in,out]	s	allocated Swr context. If it is not initialized, or SWR_FLAG_RESAMPLE is not set, swr_init() is called with the flag set.
[in]	sample_delta	delta in PTS per sample
[in]	compensation_distance	number of samples to compensate for

Returns

>= 0 on success, AVERROR error codes if:

• s is NULL,
• compensation_distance is less than 0,
• compensation_distance is 0 but sample_delta is not,
• compensation unsupported by resampler, or
• swr_init() fails when called.

Definition at line 803 of file swresample.c.

Referenced by audio_decode_frame(), and swr_next_pts().

int swr_set_channel_mapping	(	struct SwrContext *	s,
		const int *	channel_map
	)

Set a customized input channel mapping.

Parameters

[in,out]	s	allocated Swr context, not yet initialized
[in]	channel_map	customized input channel mapping (array of channel indexes, -1 for a muted channel)

Returns

>= 0 on success, or AVERROR error code in case of failure.

Definition at line 51 of file swresample.c.

Referenced by config_props().

int swr_set_matrix	(	struct SwrContext *	s,
		const double *	matrix,
		int	stride
	)

Set a customized remix matrix.

Parameters

s	allocated Swr context, not yet initialized
matrix	remix coefficients; matrix[i + stride * o] is the weight of input channel i in output channel o
stride	offset between lines of the matrix

Returns

>= 0 on success, or AVERROR error code in case of failure.

Definition at line 61 of file rematrix.c.

Referenced by config_props().

int swr_drop_output	(	struct SwrContext *	s,
		int	count
	)

Drops the specified number of output samples.

This function, along with swr_inject_silence(), is called by swr_next_pts() if needed for "hard" compensation.

Parameters

s	allocated Swr context
count	number of samples to be dropped

Returns

>= 0 on success, or a negative AVERROR code on failure

Definition at line 756 of file swresample.c.

Referenced by swr_next_pts().

int swr_inject_silence	(	struct SwrContext *	s,
		int	count
	)

Injects the specified number of silence samples.

This function, along with swr_drop_output(), is called by swr_next_pts() if needed for "hard" compensation.

Parameters

s	allocated Swr context
count	number of samples to be dropped

Returns

>= 0 on success, or a negative AVERROR code on failure

Definition at line 767 of file swresample.c.

Referenced by swr_inject_silence(), and swr_next_pts().

int64_t swr_get_delay	(	struct SwrContext *	s,
		int64_t	base
	)

Gets the delay the next input sample will experience relative to the next output sample.

Swresample can buffer data if more input has been provided than available output space, also converting between sample rates needs a delay. This function returns the sum of all such delays. The exact delay is not necessarily an integer value in either input or output sample rate. Especially when downsampling by a large value, the output sample rate may be a poor choice to represent the delay, similarly for upsampling and the input sample rate.

Parameters

s

swr context

base

timebase in which the returned delay will be: if it's set to 1 the returned delay is in seconds if it's set to 1000 the returned delay is in milliseconds if it's set to the input sample rate then the returned delay is in input samples if it's set to the output sample rate then the returned delay is in output samples if it's the least common multiple of in_sample_rate and out_sample_rate then an exact rounding-free delay will be returned

Returns

the delay in 1 / base units.

Definition at line 795 of file swresample.c.

Referenced by filter_frame(), main(), swr_convert_frame(), swr_next_pts(), and write_audio_frame().

unsigned swresample_version

(

void

)

Return the LIBSWRESAMPLE_VERSION_INT constant.

This is useful to check if the build-time libswresample has the same version as the run-time one.

Returns

the unsigned int-typed version

Definition at line 34 of file swresample.c.

const char* swresample_configuration

(

void

)

Return the swr build-time configuration.

Returns

the build-time ./configure flags

Definition at line 40 of file swresample.c.

const char* swresample_license

(

void

)

Return the swr license.

Returns

the license of libswresample, determined at build-time

Definition at line 45 of file swresample.c.

int swr_convert_frame	(	SwrContext *	swr,
		AVFrame *	output,
		const AVFrame *	input
	)

Convert the samples in the input AVFrame and write them to the output AVFrame.

Input and output AVFrames must have channel_layout, sample_rate and format set.

If the output AVFrame does not have the data pointers allocated the nb_samples field will be set using av_frame_get_buffer() is called to allocate the frame.

The output AVFrame can be NULL or have fewer allocated samples than required. In this case, any remaining samples not written to the output will be added to an internal FIFO buffer, to be returned at the next call to this function or to swr_convert().

If converting sample rate, there may be data remaining in the internal resampling delay buffer. swr_get_delay() tells the number of remaining samples. To get this data as output, call this function or swr_convert() with NULL input.

If the SwrContext configuration does not match the output and input AVFrame settings the conversion does not take place and depending on which AVFrame is not matching AVERROR_OUTPUT_CHANGED, AVERROR_INPUT_CHANGED or the result of a bitwise-OR of them is returned.

See Also

swr_delay()

swr_convert()

swr_get_delay()

Parameters

swr	audio resample context
output	output AVFrame
input	input AVFrame

Returns

0 on success, AVERROR on failure or nonmatching configuration.

Definition at line 123 of file swresample_frame.c.

int swr_config_frame	(	SwrContext *	swr,
		const AVFrame *	out,
		const AVFrame *	in
	)

Configure or reconfigure the SwrContext using the information provided by the AVFrames.

The original resampling context is reset even on failure. The function calls swr_close() internally if the context is open.

See Also

swr_close();

Parameters

swr	audio resample context
output	output AVFrame
input	input AVFrame

Returns

0 on success, AVERROR on failure.

Definition at line 26 of file swresample_frame.c.

Referenced by swr_convert_frame().