GstAudio meta: GStreamer Base Plugins 1.0 Library Reference Manual

## GstAudio meta GstAudio meta — Buffer metadata for audio downmix matrix handling |

## Functions

GstAudioMeta * | gst_buffer_add_audio_meta ()
#define | gst_buffer_get_audio_meta()
GstAudioDownmixMeta * | gst_buffer_add_audio_downmix_meta ()
#define | gst_buffer_get_audio_downmix_meta()
GstAudioDownmixMeta * | gst_buffer_get_audio_downmix_meta_for_channels ()
GstAudioClippingMeta * | gst_buffer_add_audio_clipping_meta ()
#define | gst_buffer_get_audio_clipping_meta()

## Types and Values

struct | GstAudioMeta
struct | GstAudioDownmixMeta
struct | GstAudioClippingMeta

## Includes

    #include <gst/audio/audio.h>

## Description

GstAudioDownmixMeta defines an audio downmix matrix to be send along with audio buffers. These functions in this module help to create and attach the meta as well as extracting it.

## Functions

### gst_buffer_add_audio_meta ()

    GstAudioMeta *
    gst_buffer_add_audio_meta (GstBuffer *buffer,
                               const GstAudioInfo *info,
                               gsize samples,
                               gsize offsets[]);

Allocates and attaches a GstAudioMeta on `buffer` , which must be writable for that purpose. The fields of the GstAudioMeta are directly populated from the arguments of this function.

When `info->layout` is `GST_AUDIO_LAYOUT_NON_INTERLEAVED` and `offsets` is `NULL`, the offsets are calculated with a formula that assumes the planes are tightly packed and in sequence: offsets[channel] = channel * `samples` * sample_stride

It is not allowed for channels to overlap in memory, i.e. for each i in [0, channels), the range [`offsets` [i], `offsets` [i] + `samples` * sample_stride) must not overlap with any other such range. This function will assert if the parameters specified cause this restriction to be violated.

It is, obviously, also not allowed to specify parameters that would cause out-of-bounds memory access on `buffer` . This is also checked, which means that you must add enough memory on the `buffer` before adding this meta.

#### Parameters

buffer | a GstBuffer |
info | the audio properties of the buffer |
samples | the number of valid samples in the buffer |
offsets | the offsets (in bytes) where each channel plane starts in the buffer or `NULL` to calculate it (see below); must be `NULL` also when `info->layout` is `GST_AUDIO_LAYOUT_INTERLEAVED`. | [nullable]

#### Returns

the GstAudioMeta that was attached on the `buffer`

Since: 1.16

### gst_buffer_get_audio_meta()

    #define             gst_buffer_get_audio_meta(b)

### gst_buffer_add_audio_downmix_meta ()

    GstAudioDownmixMeta *
    gst_buffer_add_audio_downmix_meta (GstBuffer *buffer,
                                       const GstAudioChannelPosition *from_position,
                                       gint from_channels,
                                       const GstAudioChannelPosition *to_position,
                                       gint to_channels,
                                       const gfloat **matrix);

Attaches GstAudioDownmixMeta metadata to `buffer` with the given parameters.

`matrix` is an two-dimensional array of `to_channels` times `from_channels` coefficients, i.e. the i-th output channels is constructed by multiplicating the input channels with the coefficients in `matrix` [i] and taking the sum of the results.

#### Parameters

buffer | a GstBuffer |
from_position | the channel positions of the source. | [array length=from_channels]
from_channels | The number of channels of the source |
to_position | the channel positions of the destination. | [array length=to_channels]
to_channels | The number of channels of the destination |
matrix | The matrix coefficients. |

#### Returns

the GstAudioDownmixMeta on `buffer` .

[transfer none]

### gst_buffer_get_audio_downmix_meta()

    #define gst_buffer_get_audio_downmix_meta(b) ((GstAudioDownmixMeta*)gst_buffer_get_meta((b), GST_AUDIO_DOWNMIX_META_API_TYPE))

### gst_buffer_get_audio_downmix_meta_for_channels ()

    GstAudioDownmixMeta *
    gst_buffer_get_audio_downmix_meta_for_channels
                                   (GstBuffer *buffer,
                                    const GstAudioChannelPosition *to_position,
                                    gint to_channels);

Find the GstAudioDownmixMeta on `buffer` for the given destination channel positions.

#### Parameters

buffer | a GstBuffer |
to_position | the channel positions of the destination. | [array length=to_channels]
to_channels | The number of channels of the destination |

#### Returns

the GstAudioDownmixMeta on `buffer` .

[transfer none]

### gst_buffer_add_audio_clipping_meta ()

    GstAudioClippingMeta *
    gst_buffer_add_audio_clipping_meta (GstBuffer *buffer,
                                        GstFormat format,
                                        guint64 start,
                                        guint64 end);

Attaches GstAudioClippingMeta metadata to `buffer` with the given parameters.

#### Parameters

buffer | a GstBuffer |
format | GstFormat of `start` and `stop` , GST_FORMAT_DEFAULT is samples |
start | Amount of audio to clip from start of buffer |
end | Amount of to clip from end of buffer |

#### Returns

the GstAudioClippingMeta on `buffer` .

[transfer none]

Since: 1.8

### gst_buffer_get_audio_clipping_meta()

    #define gst_buffer_get_audio_clipping_meta(b) ((GstAudioClippingMeta*)gst_buffer_get_meta((b), GST_AUDIO_CLIPPING_META_API_TYPE))

## Types and Values

### struct GstAudioMeta

    struct GstAudioMeta {
      GstMeta      meta;

      GstAudioInfo info;
      gsize        samples;
      gsize        *offsets;
    };

Buffer metadata describing how data is laid out inside the buffer. This is useful for non-interleaved (planar) buffers, where it is necessary to have a place to store where each plane starts and how long each plane is.

It is a requirement for non-interleaved buffers to have this metadata attached and to be mapped with `gst_audio_buffer_map()` in order to ensure correct handling of clipping and channel reordering.

The different channels in `offsets` are always in the GStreamer channel order. Zero-copy channel reordering can be implemented by swapping the values in `offsets` .

It is not allowed for channels to overlap in memory, i.e. for each i in [0, channels), the range [`offsets` [i], `offsets` [i] + `samples` * sample_stride) must not overlap with any other such range.

It is, however, allowed to have parts of the buffer memory unused, by using `offsets` and `samples` in such a way that leave gaps on it. This is used to implement zero-copy clipping in non-interleaved buffers.

Obviously, due to the above, it is not safe to infer the number of valid samples from the size of the buffer. You should always use the `samples` variable of this metadata.

Note that for interleaved audio it is not a requirement to have this metadata attached and at the moment of writing, there is actually no use case to do so. It is, however, allowed to attach it, for some potential future use case.

#### Members

GstMeta `meta`; | parent GstMeta |
GstAudioInfo `info`; | the audio properties of the buffer |
gsize `samples`; | the number of valid samples in the buffer |
gsize *`offsets`; | the offsets (in bytes) where each channel plane starts in the buffer or `NULL` if the buffer has interleaved layout; if not `NULL`, this is guaranteed to be an array of `info.channels` elements |

Since: 1.16

### struct GstAudioDownmixMeta

    struct GstAudioDownmixMeta {
      GstMeta      meta;

      GstAudioChannelPosition *from_position;
      GstAudioChannelPosition *to_position;
      gint        from_channels, to_channels;
      gfloat       **matrix;
    };

Extra buffer metadata describing audio downmixing matrix. This metadata is attached to audio buffers and contains a matrix to downmix the buffer number of channels to `channels` .

`matrix` is an two-dimensional array of `to_channels` times `from_channels` coefficients, i.e. the i-th output channels is constructed by multiplicating the input channels with the coefficients in `matrix` [i] and taking the sum of the results.

#### Members

GstMeta `meta`; | parent GstMeta |
GstAudioChannelPosition *`from_position`; | the channel positions of the source |
GstAudioChannelPosition *`to_position`; | the channel positions of the destination |
gint `from_channels`; | the number of channels of the source |
gint `to_channels`; | the number of channels of the destination |
gfloat **`matrix`; | the matrix coefficients. |

### struct GstAudioClippingMeta

    struct GstAudioClippingMeta {
      GstMeta   meta;

      GstFormat format;
      guint64   start;
      guint64   end;
    };

Extra buffer metadata describing how much audio has to be clipped from the start or end of a buffer. This is used for compressed formats, where the first frame usually has some additional samples due to encoder and decoder delays, and the last frame usually has some additional samples to be able to fill the complete last frame.

This is used to ensure that decoded data in the end has the same amount of samples, and multiply decoded streams can be gaplessly concatenated.

Note: If clipping of the start is done by adjusting the segment, this meta has to be dropped from buffers as otherwise clipping could happen twice.

#### Members

GstMeta `meta`; | parent GstMeta |
GstFormat `format`; | GstFormat of `start` and `stop` , GST_FORMAT_DEFAULT is samples |
guint64 `start`; | Amount of audio to clip from start of buffer |
guint64 `end`; | Amount of to clip from end of buffer |

Since: 1.8