[FFmpeg-devel] Add a new channel layout API

The new API is more extensible and allows for custom layouts.
More accurate information is exported, eg for decoders that do not
set a channel layout, lavc will not make one up for them.

Deprecate the old API working with just uint64_t bitmasks.

Original commit by Anton Khirnov <anton@khirnov.net>.
Expanded and completed by Vittorio Giovara <vittorio.giovara@gmail.com>.
Adapted for FFmpeg by Paul B Mahol <onemda@gmail.com>.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Paul B Mahol <onemda@gmail.com>
---
 libavutil/channel_layout.c | 385 ++++++++++++++++++++++++++++------
 libavutil/channel_layout.h | 411 ++++++++++++++++++++++++++++++++++---
 libavutil/version.h        |   3 +
 3 files changed, 709 insertions(+), 90 deletions(-)

On Mon, Oct 28, 2019 at 02:48:21PM +0100, Paul B Mahol wrote:
> The new API is more extensible and allows for custom layouts.
> More accurate information is exported, eg for decoders that do not
> set a channel layout, lavc will not make one up for them.
> 
> Deprecate the old API working with just uint64_t bitmasks.
> 
> Original commit by Anton Khirnov <anton@khirnov.net>.
> Expanded and completed by Vittorio Giovara <vittorio.giovara@gmail.com>.
> Adapted for FFmpeg by Paul B Mahol <onemda@gmail.com>.
> 
> Signed-off-by: Anton Khirnov <anton@khirnov.net>
> Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
> Signed-off-by: Paul B Mahol <onemda@gmail.com>
> ---
>  libavutil/channel_layout.c | 385 ++++++++++++++++++++++++++++------
>  libavutil/channel_layout.h | 411 ++++++++++++++++++++++++++++++++++---
>  libavutil/version.h        |   3 +
>  3 files changed, 709 insertions(+), 90 deletions(-)
> 
> diff --git a/libavutil/channel_layout.c b/libavutil/channel_layout.c
> index 3bd5ee29b7..9112af32a6 100644
> --- a/libavutil/channel_layout.c
> +++ b/libavutil/channel_layout.c
[...]

./ffmpeg  -v 0 -layouts
looses the channel names 



[...]
> +void av_channel_layout_from_mask(AVChannelLayout *channel_layout,
> +                                 uint64_t mask)
> +{
> +    channel_layout->order       = AV_CHANNEL_ORDER_NATIVE;
> +    channel_layout->nb_channels = av_popcount64(mask);

is this correct in relation to AV_CH_LAYOUT_NATIVE ?


> +    channel_layout->u.mask      = mask;
> +}


[...]
> +/**
> + * Initialize a native channel layout from a bitmask indicating which channels
> + * are present.
> + *
> + * @note channel_layout should be properly allocated as described above.
> + *
> + * @param channel_layout the layout structure to be initialized
> + * @param mask bitmask describing the channel layout
> + */
> +void av_channel_layout_from_mask(AVChannelLayout *channel_layout, uint64_t mask);

This possibly should return a error code
for example a mask of 0 would not be valid

thx

[...]

Paul B Mahol (12019-10-28):
> The new API is more extensible and allows for custom layouts.
> More accurate information is exported, eg for decoders that do not
> set a channel layout, lavc will not make one up for them.
> 
> Deprecate the old API working with just uint64_t bitmasks.
> 
> Original commit by Anton Khirnov <anton@khirnov.net>.
> Expanded and completed by Vittorio Giovara <vittorio.giovara@gmail.com>.
> Adapted for FFmpeg by Paul B Mahol <onemda@gmail.com>.
> 
> Signed-off-by: Anton Khirnov <anton@khirnov.net>
> Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
> Signed-off-by: Paul B Mahol <onemda@gmail.com>
> ---
>  libavutil/channel_layout.c | 385 ++++++++++++++++++++++++++++------
>  libavutil/channel_layout.h | 411 ++++++++++++++++++++++++++++++++++---
>  libavutil/version.h        |   3 +
>  3 files changed, 709 insertions(+), 90 deletions(-)

Just a few quick remarks on the API itself for now. I did not read the
implementation carefully.

> diff --git a/libavutil/channel_layout.c b/libavutil/channel_layout.c
> index 3bd5ee29b7..9112af32a6 100644
> --- a/libavutil/channel_layout.c
> +++ b/libavutil/channel_layout.c
> @@ -37,75 +37,87 @@ struct channel_name {
>  };
>  
>  static const struct channel_name channel_names[] = {
> -     [0] = { "FL",        "front left"            },
> -     [1] = { "FR",        "front right"           },
> -     [2] = { "FC",        "front center"          },
> -     [3] = { "LFE",       "low frequency"         },
> -     [4] = { "BL",        "back left"             },
> -     [5] = { "BR",        "back right"            },
> -     [6] = { "FLC",       "front left-of-center"  },
> -     [7] = { "FRC",       "front right-of-center" },
> -     [8] = { "BC",        "back center"           },
> -     [9] = { "SL",        "side left"             },
> -    [10] = { "SR",        "side right"            },
> -    [11] = { "TC",        "top center"            },
> -    [12] = { "TFL",       "top front left"        },
> -    [13] = { "TFC",       "top front center"      },
> -    [14] = { "TFR",       "top front right"       },
> -    [15] = { "TBL",       "top back left"         },
> -    [16] = { "TBC",       "top back center"       },
> -    [17] = { "TBR",       "top back right"        },
> -    [29] = { "DL",        "downmix left"          },
> -    [30] = { "DR",        "downmix right"         },
> -    [31] = { "WL",        "wide left"             },
> -    [32] = { "WR",        "wide right"            },
> -    [33] = { "SDL",       "surround direct left"  },
> -    [34] = { "SDR",       "surround direct right" },
> -    [35] = { "LFE2",      "low frequency 2"       },
> +    [AV_CHAN_FRONT_LEFT              ] = { "FL" },
> +    [AV_CHAN_FRONT_RIGHT             ] = { "FR" },
> +    [AV_CHAN_FRONT_CENTER            ] = { "FC" },
> +    [AV_CHAN_LOW_FREQUENCY           ] = { "LFE" },
> +    [AV_CHAN_BACK_LEFT               ] = { "BL" },
> +    [AV_CHAN_BACK_RIGHT              ] = { "BR" },
> +    [AV_CHAN_FRONT_LEFT_OF_CENTER    ] = { "FLC" },
> +    [AV_CHAN_FRONT_RIGHT_OF_CENTER   ] = { "FRC" },
> +    [AV_CHAN_BACK_CENTER             ] = { "BC" },
> +    [AV_CHAN_SIDE_LEFT               ] = { "SL" },
> +    [AV_CHAN_SIDE_RIGHT              ] = { "SR" },
> +    [AV_CHAN_TOP_CENTER              ] = { "TC" },
> +    [AV_CHAN_TOP_FRONT_LEFT          ] = { "TFL" },
> +    [AV_CHAN_TOP_FRONT_CENTER        ] = { "TFC" },
> +    [AV_CHAN_TOP_FRONT_RIGHT         ] = { "TFR" },
> +    [AV_CHAN_TOP_BACK_LEFT           ] = { "TBL" },
> +    [AV_CHAN_TOP_BACK_CENTER         ] = { "TBC" },
> +    [AV_CHAN_TOP_BACK_RIGHT          ] = { "TBR" },
> +    [AV_CHAN_STEREO_LEFT             ] = { "DL" },
> +    [AV_CHAN_STEREO_RIGHT            ] = { "DR" },
> +    [AV_CHAN_WIDE_LEFT               ] = { "WL" },
> +    [AV_CHAN_WIDE_RIGHT              ] = { "WR" },
> +    [AV_CHAN_SURROUND_DIRECT_LEFT    ] = { "SDL" },
> +    [AV_CHAN_SURROUND_DIRECT_RIGHT   ] = { "SDR" },
> +    [AV_CHAN_LOW_FREQUENCY_2         ] = { "LFE2" },
> +    [AV_CHAN_SILENCE                 ] = { "PAD" },
>  };
>  
> -static const char *get_channel_name(int channel_id)
> +const char *av_channel_name(enum AVChannel channel_id)
>  {
>      if (channel_id < 0 || channel_id >= FF_ARRAY_ELEMS(channel_names))
>          return NULL;
>      return channel_names[channel_id].name;
>  }
>  
> +int av_channel_from_string(const char *str)
> +{
> +    for (int i = 0; i < FF_ARRAY_ELEMS(channel_names); i++) {
> +        if (channel_names[i].name && !strcmp(str, channel_names[i].name)) {
> +            return i;
> +        }
> +    }
> +    return AVERROR(EINVAL);
> +}
> +
>  static const struct {
>      const char *name;
> -    int         nb_channels;
> -    uint64_t     layout;
> +    AVChannelLayout layout;
>  } channel_layout_map[] = {
> -    { "mono",        1,  AV_CH_LAYOUT_MONO },
> -    { "stereo",      2,  AV_CH_LAYOUT_STEREO },
> -    { "2.1",         3,  AV_CH_LAYOUT_2POINT1 },
> -    { "3.0",         3,  AV_CH_LAYOUT_SURROUND },
> -    { "3.0(back)",   3,  AV_CH_LAYOUT_2_1 },
> -    { "4.0",         4,  AV_CH_LAYOUT_4POINT0 },
> -    { "quad",        4,  AV_CH_LAYOUT_QUAD },
> -    { "quad(side)",  4,  AV_CH_LAYOUT_2_2 },
> -    { "3.1",         4,  AV_CH_LAYOUT_3POINT1 },
> -    { "5.0",         5,  AV_CH_LAYOUT_5POINT0_BACK },
> -    { "5.0(side)",   5,  AV_CH_LAYOUT_5POINT0 },
> -    { "4.1",         5,  AV_CH_LAYOUT_4POINT1 },
> -    { "5.1",         6,  AV_CH_LAYOUT_5POINT1_BACK },
> -    { "5.1(side)",   6,  AV_CH_LAYOUT_5POINT1 },
> -    { "6.0",         6,  AV_CH_LAYOUT_6POINT0 },
> -    { "6.0(front)",  6,  AV_CH_LAYOUT_6POINT0_FRONT },
> -    { "hexagonal",   6,  AV_CH_LAYOUT_HEXAGONAL },
> -    { "6.1",         7,  AV_CH_LAYOUT_6POINT1 },
> -    { "6.1(back)",   7,  AV_CH_LAYOUT_6POINT1_BACK },
> -    { "6.1(front)",  7,  AV_CH_LAYOUT_6POINT1_FRONT },
> -    { "7.0",         7,  AV_CH_LAYOUT_7POINT0 },
> -    { "7.0(front)",  7,  AV_CH_LAYOUT_7POINT0_FRONT },
> -    { "7.1",         8,  AV_CH_LAYOUT_7POINT1 },
> -    { "7.1(wide)",   8,  AV_CH_LAYOUT_7POINT1_WIDE_BACK },
> -    { "7.1(wide-side)",   8,  AV_CH_LAYOUT_7POINT1_WIDE },
> -    { "octagonal",   8,  AV_CH_LAYOUT_OCTAGONAL },
> -    { "hexadecagonal", 16, AV_CH_LAYOUT_HEXADECAGONAL },
> -    { "downmix",     2,  AV_CH_LAYOUT_STEREO_DOWNMIX, },
> +    { "mono",           AV_CHANNEL_LAYOUT_MONO                },
> +    { "stereo",         AV_CHANNEL_LAYOUT_STEREO              },
> +    { "2.1",            AV_CHANNEL_LAYOUT_2POINT1             },
> +    { "3.0",            AV_CHANNEL_LAYOUT_SURROUND            },
> +    { "3.0(back)",      AV_CHANNEL_LAYOUT_2_1                 },
> +    { "4.0",            AV_CHANNEL_LAYOUT_4POINT0             },
> +    { "quad",           AV_CHANNEL_LAYOUT_QUAD                },
> +    { "quad(side)",     AV_CHANNEL_LAYOUT_2_2                 },
> +    { "3.1",            AV_CHANNEL_LAYOUT_3POINT1             },
> +    { "5.0",            AV_CHANNEL_LAYOUT_5POINT0_BACK        },
> +    { "5.0(side)",      AV_CHANNEL_LAYOUT_5POINT0             },
> +    { "4.1",            AV_CHANNEL_LAYOUT_4POINT1             },
> +    { "5.1",            AV_CHANNEL_LAYOUT_5POINT1_BACK        },
> +    { "5.1(side)",      AV_CHANNEL_LAYOUT_5POINT1             },
> +    { "6.0",            AV_CHANNEL_LAYOUT_6POINT0             },
> +    { "6.0(front)",     AV_CHANNEL_LAYOUT_6POINT0_FRONT       },
> +    { "hexagonal",      AV_CHANNEL_LAYOUT_HEXAGONAL           },
> +    { "6.1",            AV_CHANNEL_LAYOUT_6POINT1             },
> +    { "6.1(back)",      AV_CHANNEL_LAYOUT_6POINT1_BACK        },
> +    { "6.1(front)",     AV_CHANNEL_LAYOUT_6POINT1_FRONT       },
> +    { "7.0",            AV_CHANNEL_LAYOUT_7POINT0             },
> +    { "7.0(front)",     AV_CHANNEL_LAYOUT_7POINT0_FRONT       },
> +    { "7.1",            AV_CHANNEL_LAYOUT_7POINT1             },
> +    { "7.1(wide)",      AV_CHANNEL_LAYOUT_7POINT1_WIDE        },
> +    { "7.1(wide-back)", AV_CHANNEL_LAYOUT_7POINT1_WIDE_BACK   },
> +    { "octagonal",      AV_CHANNEL_LAYOUT_OCTAGONAL           },
> +    { "hexadecagonal",  AV_CHANNEL_LAYOUT_HEXADECAGONAL       },
> +    { "downmix",        AV_CHANNEL_LAYOUT_STEREO_DOWNMIX,     },
>  };
>  
> +#if FF_API_OLD_CHANNEL_LAYOUT
> +FF_DISABLE_DEPRECATION_WARNINGS
>  static uint64_t get_channel_layout_single(const char *name, int name_len)
>  {
>      int i;
> @@ -115,7 +127,7 @@ static uint64_t get_channel_layout_single(const char *name, int name_len)
>      for (i = 0; i < FF_ARRAY_ELEMS(channel_layout_map); i++) {
>          if (strlen(channel_layout_map[i].name) == name_len &&
>              !memcmp(channel_layout_map[i].name, name, name_len))
> -            return channel_layout_map[i].layout;
> +            return channel_layout_map[i].layout.u.mask;
>      }
>      for (i = 0; i < FF_ARRAY_ELEMS(channel_names); i++)
>          if (channel_names[i].name &&
> @@ -183,8 +195,8 @@ void av_bprint_channel_layout(struct AVBPrint *bp,
>          nb_channels = av_get_channel_layout_nb_channels(channel_layout);
>  
>      for (i = 0; i < FF_ARRAY_ELEMS(channel_layout_map); i++)
> -        if (nb_channels    == channel_layout_map[i].nb_channels &&
> -            channel_layout == channel_layout_map[i].layout) {
> +        if (nb_channels    == channel_layout_map[i].layout.nb_channels &&
> +            channel_layout == channel_layout_map[i].layout.u.mask) {
>              av_bprintf(bp, "%s", channel_layout_map[i].name);
>              return;
>          }
> @@ -195,7 +207,7 @@ void av_bprint_channel_layout(struct AVBPrint *bp,
>          av_bprintf(bp, " (");
>          for (i = 0, ch = 0; i < 64; i++) {
>              if ((channel_layout & (UINT64_C(1) << i))) {
> -                const char *name = get_channel_name(i);
> +                const char *name = av_channel_name(i);
>                  if (name) {
>                      if (ch > 0)
>                          av_bprintf(bp, "+");
> @@ -225,8 +237,8 @@ int av_get_channel_layout_nb_channels(uint64_t channel_layout)
>  int64_t av_get_default_channel_layout(int nb_channels) {
>      int i;
>      for (i = 0; i < FF_ARRAY_ELEMS(channel_layout_map); i++)
> -        if (nb_channels == channel_layout_map[i].nb_channels)
> -            return channel_layout_map[i].layout;
> +        if (nb_channels == channel_layout_map[i].layout.nb_channels)
> +            return channel_layout_map[i].layout.u.mask;
>      return 0;
>  }
>  
> @@ -247,7 +259,7 @@ const char *av_get_channel_name(uint64_t channel)
>          return NULL;
>      for (i = 0; i < 64; i++)
>          if ((1ULL<<i) & channel)
> -            return get_channel_name(i);
> +            return av_channel_name(i);
>      return NULL;
>  }
>  
> @@ -281,7 +293,252 @@ int av_get_standard_channel_layout(unsigned index, uint64_t *layout,
>  {
>      if (index >= FF_ARRAY_ELEMS(channel_layout_map))
>          return AVERROR_EOF;
> -    if (layout) *layout = channel_layout_map[index].layout;
> +    if (layout) *layout = channel_layout_map[index].layout.u.mask;
>      if (name)   *name   = channel_layout_map[index].name;
>      return 0;
>  }
> +
> +FF_ENABLE_DEPRECATION_WARNINGS
> +#endif
> +
> +void av_channel_layout_from_mask(AVChannelLayout *channel_layout,
> +                                 uint64_t mask)
> +{
> +    channel_layout->order       = AV_CHANNEL_ORDER_NATIVE;
> +    channel_layout->nb_channels = av_popcount64(mask);
> +    channel_layout->u.mask      = mask;
> +}
> +
> +int av_channel_layout_from_string(AVChannelLayout *channel_layout,
> +                                  const char *str)
> +{
> +    int i, channels;
> +    const char *dup = str;
> +    uint64_t mask = 0;
> +
> +    /* channel layout names */
> +    for (i = 0; i < FF_ARRAY_ELEMS(channel_layout_map); i++) {
> +        if (channel_layout_map[i].name && !strcmp(str, channel_layout_map[i].name)) {
> +            *channel_layout = channel_layout_map[i].layout;
> +            return 0;
> +        }
> +    }
> +
> +    /* channel names */
> +    while (*dup) {
> +        char *chname = av_get_token(&dup, "+");
> +        if (!chname)
> +            return AVERROR(ENOMEM);
> +        if (*dup)
> +            dup++; // skip separator
> +        for (i = 0; i < FF_ARRAY_ELEMS(channel_names); i++) {
> +            if (channel_names[i].name && !strcmp(chname, channel_names[i].name)) {
> +                mask |= 1ULL << i;
> +            }
> +        }
> +        av_free(chname);
> +    }
> +    if (mask) {
> +        av_channel_layout_from_mask(channel_layout, mask);
> +        return 0;
> +    }
> +
> +    /* channel layout mask */
> +    if (!strncmp(str, "0x", 2) && sscanf(str, "%"PRIx64, &mask) == 1) {
> +        av_channel_layout_from_mask(channel_layout, mask);
> +        return 0;
> +    }
> +
> +    /* number of channels */
> +    if (sscanf(str, "%d", &channels) == 1) {
> +        av_channel_layout_default(channel_layout, channels);
> +        return 0;
> +    }
> +
> +    /* number of unordered channels */
> +    if (sscanf(str, "%d channels", &channel_layout->nb_channels) == 1) {
> +        channel_layout->order = AV_CHANNEL_ORDER_UNSPEC;
> +        return 0;
> +    }
> +
> +    return AVERROR_INVALIDDATA;
> +}
> +
> +void av_channel_layout_uninit(AVChannelLayout *channel_layout)
> +{
> +    if (channel_layout->order == AV_CHANNEL_ORDER_CUSTOM)
> +        av_freep(&channel_layout->u.map);
> +    memset(channel_layout, 0, sizeof(*channel_layout));
> +}
> +
> +int av_channel_layout_copy(AVChannelLayout *dst, const AVChannelLayout *src)
> +{
> +    av_channel_layout_uninit(dst);
> +    *dst = *src;
> +    if (src->order == AV_CHANNEL_ORDER_CUSTOM) {
> +        dst->u.map = av_malloc(src->nb_channels * sizeof(*dst->u.map));
> +        if (!dst->u.map)
> +            return AVERROR(ENOMEM);
> +        memcpy(dst->u.map, src->u.map, src->nb_channels * sizeof(*src->u.map));
> +    }
> +    return 0;
> +}
> +
> +char *av_channel_layout_describe(const AVChannelLayout *channel_layout)
> +{
> +    int i;
> +
> +    switch (channel_layout->order) {
> +    case AV_CHANNEL_ORDER_NATIVE:
> +        for (i = 0; channel_layout_map[i].name; i++)
> +            if (channel_layout->u.mask == channel_layout_map[i].layout.u.mask)
> +                return av_strdup(channel_layout_map[i].name);
> +        // fall-through
> +    case AV_CHANNEL_ORDER_CUSTOM: {
> +        // max 4 bytes for channel name + a separator
> +        int size = 5 * channel_layout->nb_channels + 1;
> +        char *ret;
> +
> +        ret = av_mallocz(size);
> +        if (!ret)
> +            return NULL;
> +
> +        for (i = 0; i < channel_layout->nb_channels; i++) {
> +            enum AVChannel ch = av_channel_layout_get_channel(channel_layout, i);
> +            const char *ch_name = av_channel_name(ch);
> +
> +            if (i)
> +                av_strlcat(ret, "+", size);
> +            av_strlcat(ret, ch_name, size);
> +        }
> +        return ret;
> +        }
> +    case AV_CHANNEL_ORDER_UNSPEC: {
> +        char buf[64];
> +        snprintf(buf, sizeof(buf), "%d channels", channel_layout->nb_channels);
> +        return av_strdup(buf);
> +        }
> +    default:
> +        return NULL;
> +    }
> +}
> +
> +int av_channel_layout_get_channel(const AVChannelLayout *channel_layout, int idx)
> +{
> +    int i;
> +
> +    if (idx < 0 || idx >= channel_layout->nb_channels)
> +        return AVERROR(EINVAL);
> +
> +    switch (channel_layout->order) {
> +    case AV_CHANNEL_ORDER_CUSTOM:
> +        return channel_layout->u.map[idx];
> +    case AV_CHANNEL_ORDER_NATIVE:
> +        for (i = 0; i < 64; i++) {
> +            if ((1ULL << i) & channel_layout->u.mask && !idx--)
> +                return i;
> +        }
> +    default:
> +        return AVERROR(EINVAL);
> +    }
> +}
> +
> +int av_channel_layout_channel_index(const AVChannelLayout *channel_layout,
> +                                    enum AVChannel channel)
> +{
> +    int i;
> +
> +    switch (channel_layout->order) {
> +    case AV_CHANNEL_ORDER_CUSTOM:
> +        for (i = 0; i < channel_layout->nb_channels; i++)
> +            if (channel_layout->u.map[i] == channel)
> +                return i;
> +        return AVERROR(EINVAL);
> +    case AV_CHANNEL_ORDER_NATIVE: {
> +        uint64_t mask = channel_layout->u.mask;
> +        if (!(mask & (1ULL << channel)))
> +            return AVERROR(EINVAL);
> +        mask &= (1ULL << channel) - 1;
> +        return av_popcount64(mask);
> +        }
> +    default:
> +        return AVERROR(EINVAL);
> +    }
> +}
> +
> +int av_channel_layout_check(const AVChannelLayout *channel_layout)
> +{
> +    if (!channel_layout || channel_layout->nb_channels <= 0)
> +        return 0;
> +
> +    switch (channel_layout->order) {
> +    case AV_CHANNEL_ORDER_NATIVE:
> +        return av_popcount64(channel_layout->u.mask) == channel_layout->nb_channels;
> +    case AV_CHANNEL_ORDER_CUSTOM:
> +        return !!channel_layout->u.map;
> +    case AV_CHANNEL_ORDER_UNSPEC:
> +        return 1;
> +    default:
> +        return 0;
> +    }
> +}
> +
> +int av_channel_layout_compare(const AVChannelLayout *chl, const AVChannelLayout *chl1)
> +{
> +    /* different channel counts -> not equal */
> +    if (chl->nb_channels != chl1->nb_channels && chl->order == chl1->order)
> +        return (chl->nb_channels - chl1->nb_channels > 0) - (chl1->nb_channels - chl->nb_channels > 0);
> +
> +    /* if only one is unspecified -> not equal */
> +    if ((chl->order  == AV_CHANNEL_ORDER_UNSPEC) !=
> +        (chl1->order == AV_CHANNEL_ORDER_UNSPEC))
> +        return -2;
> +    /* both are unspecified -> equal */
> +    else if (chl->order == AV_CHANNEL_ORDER_UNSPEC)
> +        return 0;
> +
> +    /* can compare masks directly */
> +    if (chl->order != AV_CHANNEL_ORDER_CUSTOM &&
> +        chl->order == chl1->order)
> +        return chl->u.mask == chl1->u.mask ? 0 : -2;
> +
> +    /* compare channel by channel */
> +    for (int i = 0; i < chl->nb_channels; i++)
> +        if (av_channel_layout_get_channel(chl,  i) !=
> +            av_channel_layout_get_channel(chl1, i))
> +            return -2;
> +    return 0;
> +}
> +
> +void av_channel_layout_default(AVChannelLayout *ch_layout, int nb_channels)
> +{
> +    switch (nb_channels) {
> +    case 1: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_MONO;     break;
> +    case 2: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO;   break;
> +    case 3: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_SURROUND; break;
> +    case 4: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_QUAD;     break;
> +    case 5: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT0;  break;
> +    case 6: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT1;  break;
> +    case 7: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_6POINT1;  break;
> +    case 8: *ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_7POINT1;  break;
> +    default:
> +        ch_layout->order       = AV_CHANNEL_ORDER_UNSPEC;
> +        ch_layout->nb_channels = nb_channels;
> +    }
> +}
> +
> +uint64_t av_channel_layout_subset(const AVChannelLayout *channel_layout,
> +                                  uint64_t mask)
> +{
> +    uint64_t ret = 0;
> +    int i;
> +
> +    if (channel_layout->order == AV_CHANNEL_ORDER_NATIVE)
> +        return channel_layout->u.mask & mask;
> +
> +    for (i = 0; i < 64; i++)
> +        if (mask & (1ULL << i) && av_channel_layout_channel_index(channel_layout, i) >= 0)
> +            ret |= (1ULL << i);
> +
> +    return ret;
> +}
> diff --git a/libavutil/channel_layout.h b/libavutil/channel_layout.h
> index 50bb8f03c5..a4d76dc4f0 100644
> --- a/libavutil/channel_layout.h
> +++ b/libavutil/channel_layout.h
> @@ -24,6 +24,9 @@
>  
>  #include <stdint.h>
>  
> +#include "version.h"
> +#include "attributes.h"
> +
>  /**
>   * @file
>   * audio channel layout utility functions
> @@ -34,6 +37,60 @@
>   * @{
>   */
>  
> +enum AVChannel {
> +    AV_CHAN_FRONT_LEFT,
> +    AV_CHAN_FRONT_RIGHT,
> +    AV_CHAN_FRONT_CENTER,
> +    AV_CHAN_LOW_FREQUENCY,
> +    AV_CHAN_BACK_LEFT,
> +    AV_CHAN_BACK_RIGHT,
> +    AV_CHAN_FRONT_LEFT_OF_CENTER,
> +    AV_CHAN_FRONT_RIGHT_OF_CENTER,
> +    AV_CHAN_BACK_CENTER,
> +    AV_CHAN_SIDE_LEFT,
> +    AV_CHAN_SIDE_RIGHT,
> +    AV_CHAN_TOP_CENTER,
> +    AV_CHAN_TOP_FRONT_LEFT,
> +    AV_CHAN_TOP_FRONT_CENTER,
> +    AV_CHAN_TOP_FRONT_RIGHT,
> +    AV_CHAN_TOP_BACK_LEFT,
> +    AV_CHAN_TOP_BACK_CENTER,
> +    AV_CHAN_TOP_BACK_RIGHT,
> +    /** Stereo downmix. */
> +    AV_CHAN_STEREO_LEFT = 29,
> +    /** See above. */
> +    AV_CHAN_STEREO_RIGHT,
> +    AV_CHAN_WIDE_LEFT,
> +    AV_CHAN_WIDE_RIGHT,
> +    AV_CHAN_SURROUND_DIRECT_LEFT,
> +    AV_CHAN_SURROUND_DIRECT_RIGHT,
> +    AV_CHAN_LOW_FREQUENCY_2,
> +
> +    /** Channel is empty can be safely skipped. */
> +    AV_CHAN_SILENCE = 64,
> +};
> +
> +enum AVChannelOrder {
> +    /**
> +     * The native channel order, i.e. the channels are in the same order in
> +     * which they are defined in the AVChannel enum. This supports up to 63
> +     * different channels.
> +     */
> +    AV_CHANNEL_ORDER_NATIVE,
> +    /**
> +     * The channel order does not correspond to any other predefined order and
> +     * is stored as an explicit map. For example, this could be used to support

> +     * layouts with 64 or more channels, or with channels that could be skipped.

"or with channels that could be skipped"? What does that mean. If a
channel is skipped, it is not present, that can be expressed in the
mask. Is this a mistake for "duplicated"?

> +     */
> +    AV_CHANNEL_ORDER_CUSTOM,
> +    /**
> +     * Only the channel count is specified, without any further information
> +     * about the channel order.
> +     */
> +    AV_CHANNEL_ORDER_UNSPEC,
> +};
> +
> +
>  /**
>   * @defgroup channel_masks Audio channel masks
>   *
> @@ -46,36 +103,41 @@
>   *
>   * @{
>   */
> -#define AV_CH_FRONT_LEFT             0x00000001
> -#define AV_CH_FRONT_RIGHT            0x00000002
> -#define AV_CH_FRONT_CENTER           0x00000004
> -#define AV_CH_LOW_FREQUENCY          0x00000008
> -#define AV_CH_BACK_LEFT              0x00000010
> -#define AV_CH_BACK_RIGHT             0x00000020
> -#define AV_CH_FRONT_LEFT_OF_CENTER   0x00000040
> -#define AV_CH_FRONT_RIGHT_OF_CENTER  0x00000080
> -#define AV_CH_BACK_CENTER            0x00000100
> -#define AV_CH_SIDE_LEFT              0x00000200
> -#define AV_CH_SIDE_RIGHT             0x00000400
> -#define AV_CH_TOP_CENTER             0x00000800
> -#define AV_CH_TOP_FRONT_LEFT         0x00001000
> -#define AV_CH_TOP_FRONT_CENTER       0x00002000
> -#define AV_CH_TOP_FRONT_RIGHT        0x00004000
> -#define AV_CH_TOP_BACK_LEFT          0x00008000
> -#define AV_CH_TOP_BACK_CENTER        0x00010000
> -#define AV_CH_TOP_BACK_RIGHT         0x00020000
> -#define AV_CH_STEREO_LEFT            0x20000000  ///< Stereo downmix.
> -#define AV_CH_STEREO_RIGHT           0x40000000  ///< See AV_CH_STEREO_LEFT.
> -#define AV_CH_WIDE_LEFT              0x0000000080000000ULL
> -#define AV_CH_WIDE_RIGHT             0x0000000100000000ULL
> -#define AV_CH_SURROUND_DIRECT_LEFT   0x0000000200000000ULL
> -#define AV_CH_SURROUND_DIRECT_RIGHT  0x0000000400000000ULL
> -#define AV_CH_LOW_FREQUENCY_2        0x0000000800000000ULL
> +#define AV_CH_FRONT_LEFT             (1ULL << AV_CHAN_FRONT_LEFT           )
> +#define AV_CH_FRONT_RIGHT            (1ULL << AV_CHAN_FRONT_RIGHT          )
> +#define AV_CH_FRONT_CENTER           (1ULL << AV_CHAN_FRONT_CENTER         )
> +#define AV_CH_LOW_FREQUENCY          (1ULL << AV_CHAN_LOW_FREQUENCY        )
> +#define AV_CH_BACK_LEFT              (1ULL << AV_CHAN_BACK_LEFT            )
> +#define AV_CH_BACK_RIGHT             (1ULL << AV_CHAN_BACK_RIGHT           )
> +#define AV_CH_FRONT_LEFT_OF_CENTER   (1ULL << AV_CHAN_FRONT_LEFT_OF_CENTER )
> +#define AV_CH_FRONT_RIGHT_OF_CENTER  (1ULL << AV_CHAN_FRONT_RIGHT_OF_CENTER)
> +#define AV_CH_BACK_CENTER            (1ULL << AV_CHAN_BACK_CENTER          )
> +#define AV_CH_SIDE_LEFT              (1ULL << AV_CHAN_SIDE_LEFT            )
> +#define AV_CH_SIDE_RIGHT             (1ULL << AV_CHAN_SIDE_RIGHT           )
> +#define AV_CH_TOP_CENTER             (1ULL << AV_CHAN_TOP_CENTER           )
> +#define AV_CH_TOP_FRONT_LEFT         (1ULL << AV_CHAN_TOP_FRONT_LEFT       )
> +#define AV_CH_TOP_FRONT_CENTER       (1ULL << AV_CHAN_TOP_FRONT_CENTER     )
> +#define AV_CH_TOP_FRONT_RIGHT        (1ULL << AV_CHAN_TOP_FRONT_RIGHT      )
> +#define AV_CH_TOP_BACK_LEFT          (1ULL << AV_CHAN_TOP_BACK_LEFT        )
> +#define AV_CH_TOP_BACK_CENTER        (1ULL << AV_CHAN_TOP_BACK_CENTER      )
> +#define AV_CH_TOP_BACK_RIGHT         (1ULL << AV_CHAN_TOP_BACK_RIGHT       )
> +#define AV_CH_STEREO_LEFT            (1ULL << AV_CHAN_STEREO_LEFT          )
> +#define AV_CH_STEREO_RIGHT           (1ULL << AV_CHAN_STEREO_RIGHT         )
> +#define AV_CH_WIDE_LEFT              (1ULL << AV_CHAN_WIDE_LEFT            )
> +#define AV_CH_WIDE_RIGHT             (1ULL << AV_CHAN_WIDE_RIGHT           )
> +#define AV_CH_SURROUND_DIRECT_LEFT   (1ULL << AV_CHAN_SURROUND_DIRECT_LEFT )
> +#define AV_CH_SURROUND_DIRECT_RIGHT  (1ULL << AV_CHAN_SURROUND_DIRECT_RIGHT)
> +#define AV_CH_LOW_FREQUENCY_2        (1ULL << AV_CHAN_LOW_FREQUENCY_2      )
>  
> +#if FF_API_OLD_CHANNEL_LAYOUT
>  /** Channel mask value used for AVCodecContext.request_channel_layout
>      to indicate that the user requests the channel order of the decoder output
> -    to be the native codec channel order. */
> +    to be the native codec channel order.
> +    @deprecated channel order is now indicated in a special field in
> +                AVChannelLayout
> +    */
>  #define AV_CH_LAYOUT_NATIVE          0x8000000000000000ULL
> +#endif
>  
>  /**
>   * @}
> @@ -122,6 +184,135 @@ enum AVMatrixEncoding {
>      AV_MATRIX_ENCODING_NB
>  };
>  
> +/**
> + * An AVChannelLayout holds information about the channel layout of audio data.
> + *
> + * A channel layout here is defined as a set of channels ordered in a specific
> + * way (unless the channel order is AV_CHANNEL_ORDER_UNSPEC, in which case an
> + * AVChannelLayout carries only the channel count).
> + *

> + * Unlike most structures in Libav, sizeof(AVChannelLayout) is a part of the
> + * public ABI and may be used by the caller. E.g. it may be allocated on stack.
> + * In particular, this structure can be initialized as follows:
> + * - default initialization with {0} or by setting all used fields correctly
> + * - with predefined layout as initializer (AV_CHANNEL_LAYOUT_STEREO, etc.)
> + * - with a constructor function such as av_channel_layout_default()
> + * On that note, this also applies:
> + * - copy via assigning is forbidden, av_channel_layout_copy() must be used
> + *   instead (and its return value should be checked)
> + * - if order is AV_CHANNEL_ORDER_CUSTOM, then it must be uninitialized with
> + *   av_channel_layout_uninit().
> + *
> + * No new fields may be added to it without a major version bump, except for
> + * new elements of the union fitting in sizeof(uint64_t).

This is a significant difference from the other APIs, and we need to be
really sure we can live with it.

I am rather for this version, but I am afraid we will find cases that
need extending the structure. After all, the trial by fire for an API is
to be used, this one has not yet had it.

I suggest we explicitly declare this API unstable for a few months,
during which we consider acceptable to add fields.

> + *
> + * An AVChannelLayout can be constructed using the convenience function
> + * av_channel_layout_from_mask() / av_channel_layout_from_string(), or it can be
> + * built manually by the caller.
> + */
> +typedef struct AVChannelLayout {
> +    /**
> +     * Channel order used in this layout.
> +     * This is a mandatory field, will default to AV_CHANNEL_ORDER_NATIVE.
> +     */
> +    enum AVChannelOrder order;
> +
> +    /**
> +     * Number of channels in this layout. Mandatory field.
> +     */

> +    int nb_channels;

unsigned.

> +
> +    /**
> +     * Details about which channels are present in this layout.
> +     * For AV_CHANNEL_ORDER_UNSPEC, this field is undefined and must not be
> +     * used.
> +     */
> +    union {
> +        /**
> +         * This member must be used for AV_CHANNEL_ORDER_NATIVE.
> +         * It is a bitmask, where the position of each set bit means that the
> +         * AVChannel with the corresponding value is present.
> +         *
> +         * I.e. when (mask & (1 << AV_CHAN_FOO)) is non-zero, then AV_CHAN_FOO
> +         * is present in the layout. Otherwise it is not present.
> +         *
> +         * @note when a channel layout using a bitmask is constructed or
> +         * modified manually (i.e.  not using any of the av_channel_layout_*
> +         * functions), the code doing it must ensure that the number of set bits
> +         * is equal to nb_channels.
> +         */
> +        uint64_t mask;
> +        /**
> +         * This member must be used when the channel order is
> +         * AV_CHANNEL_ORDER_CUSTOM. It is a nb_channels-sized array, with each
> +         * element signalling the presend of the AVChannel with the
> +         * corresponding value.
> +         *
> +         * I.e. when map[i] is equal to AV_CHAN_FOO, then AV_CH_FOO is the i-th
> +         * channel in the audio data.
> +         */
> +        enum AVChannel *map;
> +    } u;
> +} AVChannelLayout;
> +

> +#define AV_CHANNEL_LAYOUT_MONO \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 1,  .u = { .mask = AV_CH_LAYOUT_MONO }}

These macros can only be used to init structures. This is unusual for
macros in ALL_CAPS, and therefore it needs to be documented.

> +#define AV_CHANNEL_LAYOUT_STEREO \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 2,  .u = { .mask = AV_CH_LAYOUT_STEREO }}
> +#define AV_CHANNEL_LAYOUT_2POINT1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 3,  .u = { .mask = AV_CH_LAYOUT_2POINT1 }}
> +#define AV_CHANNEL_LAYOUT_2_1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 3,  .u = { .mask = AV_CH_LAYOUT_2_1 }}
> +#define AV_CHANNEL_LAYOUT_SURROUND \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 3,  .u = { .mask = AV_CH_LAYOUT_SURROUND }}
> +#define AV_CHANNEL_LAYOUT_3POINT1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 4,  .u = { .mask = AV_CH_LAYOUT_3POINT1 }}
> +#define AV_CHANNEL_LAYOUT_4POINT0 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 4,  .u = { .mask = AV_CH_LAYOUT_4POINT0 }}
> +#define AV_CHANNEL_LAYOUT_4POINT1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 5,  .u = { .mask = AV_CH_LAYOUT_4POINT1 }}
> +#define AV_CHANNEL_LAYOUT_2_2 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 4,  .u = { .mask = AV_CH_LAYOUT_2_2 }}
> +#define AV_CHANNEL_LAYOUT_QUAD \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 4,  .u = { .mask = AV_CH_LAYOUT_QUAD }}
> +#define AV_CHANNEL_LAYOUT_5POINT0 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 5,  .u = { .mask = AV_CH_LAYOUT_5POINT0 }}
> +#define AV_CHANNEL_LAYOUT_5POINT1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 6,  .u = { .mask = AV_CH_LAYOUT_5POINT1 }}
> +#define AV_CHANNEL_LAYOUT_5POINT0_BACK \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 5,  .u = { .mask = AV_CH_LAYOUT_5POINT0_BACK }}
> +#define AV_CHANNEL_LAYOUT_5POINT1_BACK \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 6,  .u = { .mask = AV_CH_LAYOUT_5POINT1_BACK }}
> +#define AV_CHANNEL_LAYOUT_6POINT0 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 6,  .u = { .mask = AV_CH_LAYOUT_6POINT0 }}
> +#define AV_CHANNEL_LAYOUT_6POINT0_FRONT \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 6,  .u = { .mask = AV_CH_LAYOUT_6POINT0_FRONT }}
> +#define AV_CHANNEL_LAYOUT_HEXAGONAL \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 6,  .u = { .mask = AV_CH_LAYOUT_HEXAGONAL }}
> +#define AV_CHANNEL_LAYOUT_6POINT1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 7,  .u = { .mask = AV_CH_LAYOUT_6POINT1 }}
> +#define AV_CHANNEL_LAYOUT_6POINT1_BACK \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 7,  .u = { .mask = AV_CH_LAYOUT_6POINT1_BACK }}
> +#define AV_CHANNEL_LAYOUT_6POINT1_FRONT \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 7,  .u = { .mask = AV_CH_LAYOUT_6POINT1_FRONT }}
> +#define AV_CHANNEL_LAYOUT_7POINT0 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 7,  .u = { .mask = AV_CH_LAYOUT_7POINT0 }}
> +#define AV_CHANNEL_LAYOUT_7POINT0_FRONT \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 7,  .u = { .mask = AV_CH_LAYOUT_7POINT0_FRONT }}
> +#define AV_CHANNEL_LAYOUT_7POINT1 \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 8,  .u = { .mask = AV_CH_LAYOUT_7POINT1 }}
> +#define AV_CHANNEL_LAYOUT_7POINT1_WIDE \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 8,  .u = { .mask = AV_CH_LAYOUT_7POINT1_WIDE }}
> +#define AV_CHANNEL_LAYOUT_7POINT1_WIDE_BACK \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 8,  .u = { .mask = AV_CH_LAYOUT_7POINT1_WIDE_BACK }}
> +#define AV_CHANNEL_LAYOUT_OCTAGONAL \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 8,  .u = { .mask = AV_CH_LAYOUT_OCTAGONAL }}
> +#define AV_CHANNEL_LAYOUT_HEXADECAGONAL \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 16, .u = { .mask = AV_CH_LAYOUT_HEXAGONAL }}
> +#define AV_CHANNEL_LAYOUT_STEREO_DOWNMIX \
> +    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = 2,  .u = { .mask = AV_CH_LAYOUT_STEREO_DOWNMIX }}
> +
> +#if FF_API_OLD_CHANNEL_LAYOUT
>  /**
>   * Return a channel layout id that matches name, or 0 if no match is found.
>   *
> @@ -138,7 +329,10 @@ enum AVMatrixEncoding {
>   *   AV_CH_* macros).
>   *
>   * Example: "stereo+FC" = "2c+FC" = "2c+1c" = "0x7"
> + *
> + * @deprecated use av_channel_layout_from_string()
>   */
> +attribute_deprecated
>  uint64_t av_get_channel_layout(const char *name);
>  
>  /**
> @@ -161,7 +355,9 @@ int av_get_extended_channel_layout(const char *name, uint64_t* channel_layout, i
>   *
>   * @param buf put here the string containing the channel layout
>   * @param buf_size size in bytes of the buffer
> + * @deprecated use av_channel_layout_describe()
>   */
> +attribute_deprecated
>  void av_get_channel_layout_string(char *buf, int buf_size, int nb_channels, uint64_t channel_layout);
>  
>  struct AVBPrint;
> @@ -172,12 +368,17 @@ void av_bprint_channel_layout(struct AVBPrint *bp, int nb_channels, uint64_t cha
>  
>  /**
>   * Return the number of channels in the channel layout.
> + * @deprecated use AVChannelLayout.nb_channels
>   */
> +attribute_deprecated
>  int av_get_channel_layout_nb_channels(uint64_t channel_layout);
>  
>  /**
>   * Return default channel layout for a given number of channels.
> + *
> + * @deprecated use av_channel_layout_default()
>   */
> +attribute_deprecated
>  int64_t av_get_default_channel_layout(int nb_channels);
>  
>  /**
> @@ -188,20 +389,28 @@ int64_t av_get_default_channel_layout(int nb_channels);
>   *
>   * @return index of channel in channel_layout on success, a negative AVERROR
>   *         on error.
> + *
> + * @deprecated use av_channel_layout_channel_index()
>   */
> +attribute_deprecated
>  int av_get_channel_layout_channel_index(uint64_t channel_layout,
>                                          uint64_t channel);
>  
>  /**
>   * Get the channel with the given index in channel_layout.
> + * @deprecated use av_channel_layout_get_channel()
>   */
> +attribute_deprecated
>  uint64_t av_channel_layout_extract_channel(uint64_t channel_layout, int index);
>  
>  /**
>   * Get the name of a given channel.
>   *
>   * @return channel name on success, NULL on error.
> + *
> + * @deprecated use av_channel_name()
>   */
> +attribute_deprecated
>  const char *av_get_channel_name(uint64_t channel);
>  
>  /**
> @@ -210,6 +419,7 @@ const char *av_get_channel_name(uint64_t channel);
>   * @param channel  a channel layout with a single channel
>   * @return  channel description on success, NULL on error
>   */
> +attribute_deprecated
>  const char *av_get_channel_description(uint64_t channel);
>  
>  /**
> @@ -221,9 +431,158 @@ const char *av_get_channel_description(uint64_t channel);
>   * @return  0  if the layout exists,
>   *          <0 if index is beyond the limits
>   */
> +attribute_deprecated
>  int av_get_standard_channel_layout(unsigned index, uint64_t *layout,
>                                     const char **name);
>  
> +#endif
> +
> +/**
> + * This is the inverse function of @ref av_channel_from_string().
> + *
> + * @return a string describing a given channel, or NULL if not found.
> + */
> +const char *av_channel_name(enum AVChannel channel);
> +
> +/**
> + * This is the inverse function of @ref av_channel_name().
> + *
> + * @return a channel described by the given string, or a negative AVERROR value.
> + */
> +int av_channel_from_string(const char *name);
> +
> +/**
> + * Initialize a native channel layout from a bitmask indicating which channels
> + * are present.
> + *
> + * @note channel_layout should be properly allocated as described above.
> + *
> + * @param channel_layout the layout structure to be initialized
> + * @param mask bitmask describing the channel layout
> + */
> +void av_channel_layout_from_mask(AVChannelLayout *channel_layout, uint64_t mask);
> +
> +/**
> + * Initialize a channel layout from a given string description.
> + * The input string can be represented by:
> + *  - the formal channel layout name (returned by av_channel_layout_describe())
> + *  - single or multiple channel names (returned by av_channel_name()
> + *    or concatenated with "+")
> + *  - a hexadecimal value of a channel layout (eg. "0x4")
> + *  - the number of channels with default layout (eg. "5")
> + *  - the number of unordered channels (eg. "4 channels")
> + *
> + * @note channel_layout should be properly allocated as described above.
> + *
> + * @param channel_layout input channel layout
> + * @param str string describing the channel layout
> + * @return 0 channel layout was detected, AVERROR_INVALIDATATA otherwise
> + */
> +int av_channel_layout_from_string(AVChannelLayout *channel_layout,
> +                                  const char *str);
> +
> +/**
> + * Get the default channel layout for a given number of channels.
> + *
> + * @note channel_layout should be properly allocated as described above.
> + *
> + * @param channel_layout the layout structure to be initialized
> + * @param nb_channels number of channels
> + */

> +void av_channel_layout_default(AVChannelLayout *ch_layout, int nb_channels);

unsigned nb_channels

> +
> +/**
> + * Free any allocated data in the channel layout and reset the channel
> + * count to 0.
> + *
> + * @note this only used for structure initialization and for freeing the
> + *       allocated memory for AV_CHANNEL_ORDER_CUSTOM order.
> + *
> + * @param channel_layout the layout structure to be uninitialized
> + */
> +void av_channel_layout_uninit(AVChannelLayout *channel_layout);
> +
> +/**
> + * Make a copy of a channel layout. This differs from just assigning src to dst
> + * in that it allocates and copies the map for AV_CHANNEL_ORDER_CUSTOM.
> + *
> + * @note the destination channel_layout will be always uninitialized before copy.
> + *
> + * @param dst destination channel layout
> + * @param src source channel layout
> + * @return 0 on success, a negative AVERROR on error.
> + */
> +int av_channel_layout_copy(AVChannelLayout *dst, const AVChannelLayout *src);
> +

> +/**
> + * Get a human-readable string describing the channel layout properties.
> + *
> + * @note The returned string is allocated with av_malloc(),
> + *       and must be freed by the caller with av_free().

The relation between av_channel_layout_copy() and
av_channel_layout_from_string() needs to be documented.

> + *
> + * @param channel_layout channel layout to be described
> + * @return a string describing the structure or NULL on failure in the same
> + *         format that is accepted by @ref av_channel_layout_from_string().
> + */
> +char *av_channel_layout_describe(const AVChannelLayout *channel_layout);
> +
> +/**
> + * Get the channel with the given index in a channel layout.
> + *
> + * @param channel_layout input channel layout
> + * @return channel with the index idx in channel_layout on success or a negative
> + *                 AVERROR on failure (if idx is not valid or the channel order
> + *                 is unspecified)
> + */

> +int av_channel_layout_get_channel(const AVChannelLayout *channel_layout, int idx);

unsigned idx

But do we really need this as part of the API?

> +
> +/**
> + * Get the index of a given channel in a channel layout. In case multiple
> + * channels are found, only the first match will be returned.
> + *
> + * @param channel_layout input channel layout
> + * @return index of channel in channel_layout on success or a negative number if
> + *         channel is not present in channel_layout.
> + */

> +int av_channel_layout_channel_index(const AVChannelLayout *channel_layout,
> +                                    enum AVChannel channel);

Same: do we reall yneed this?

> +
> +/**
> + * Find out what channels from a given set are present in a channel layout,
> + * without regard for their positions.
> + *
> + * @param channel_layout input channel layout
> + * @param mask a combination of AV_CH_* representing a set of channels
> + * @return a bitfield representing all the channels from mask that are present
> + *         in channel_layout
> + */
> +uint64_t av_channel_layout_subset(const AVChannelLayout *channel_layout,
> +                                  uint64_t mask);

Same.

> +
> +/**
> + * Check whether a channel layout is valid, i.e. can possibly describe audio
> + * data.
> + *
> + * @param channel_layout input channel layout
> + * @return 1 if channel_layout is valid, 0 otherwise.
> + */
> +int av_channel_layout_check(const AVChannelLayout *channel_layout);
> +
> +/**
> + * Check whether two channel layouts are semantically the same, i.e. the same
> + * channels are present on the same positions in both.
> + *
> + * If one of the channel layouts is AV_CHANNEL_ORDER_UNSPEC, while the other is
> + * not, they are considered to be unequal. If both are AV_CHANNEL_ORDER_UNSPEC,
> + * they are considered equal iff the channel counts are the same in both.
> + *
> + * @param chl input channel layout
> + * @param chl1 input channel layout
> + * @return 0 if chl and chl1 are equal, 1 or -1 if they are not equal but are
> + *         same order. -2 code if they do not match some other way.
> + */
> +int av_channel_layout_compare(const AVChannelLayout *chl, const AVChannelLayout *chl1);
> +
>  /**
>   * @}
>   * @}
> diff --git a/libavutil/version.h b/libavutil/version.h
> index 27d663baf1..5e025c463f 100644
> --- a/libavutil/version.h
> +++ b/libavutil/version.h
> @@ -129,6 +129,9 @@
>  #ifndef FF_API_PSEUDOPAL
>  #define FF_API_PSEUDOPAL                (LIBAVUTIL_VERSION_MAJOR < 57)
>  #endif
> +#ifndef FF_API_OLD_CHANNEL_LAYOUT
> +#define FF_API_OLD_CHANNEL_LAYOUT       (LIBAVUTIL_VERSION_MAJOR < 58)
> +#endif
>  
>  
>  /**

Regards,

Paul B Mahol (12019-10-28):
> +char *av_channel_layout_describe(const AVChannelLayout *channel_layout)
> +{
> +    int i;
> +
> +    switch (channel_layout->order) {
> +    case AV_CHANNEL_ORDER_NATIVE:
> +        for (i = 0; channel_layout_map[i].name; i++)
> +            if (channel_layout->u.mask == channel_layout_map[i].layout.u.mask)
> +                return av_strdup(channel_layout_map[i].name);
> +        // fall-through
> +    case AV_CHANNEL_ORDER_CUSTOM: {

> +        // max 4 bytes for channel name + a separator
> +        int size = 5 * channel_layout->nb_channels + 1;
> +        char *ret;
> +
> +        ret = av_mallocz(size);
> +        if (!ret)
> +            return NULL;
> +
> +        for (i = 0; i < channel_layout->nb_channels; i++) {
> +            enum AVChannel ch = av_channel_layout_get_channel(channel_layout, i);
> +            const char *ch_name = av_channel_name(ch);
> +
> +            if (i)
> +                av_strlcat(ret, "+", size);
> +            av_strlcat(ret, ch_name, size);
> +        }
> +        return ret;
> +        }

We can do that with AVBPrint.

> +    case AV_CHANNEL_ORDER_UNSPEC: {
> +        char buf[64];
> +        snprintf(buf, sizeof(buf), "%d channels", channel_layout->nb_channels);
> +        return av_strdup(buf);
> +        }
> +    default:
> +        return NULL;
> +    }
> +}

What is the user interface to designate a specific channel in a channel
layout with duplicates?

For example, I could imagine an equalizer that splits each channel into
three (low, mid, high). Then, with pan :

	pan=stereo|FL=0.5*FL0+0.8*FL1+1*FL2|FR=0.5*FR0+0.8*FR1+1*FR2

Unfortunately:

- The API has nothing to specify why there are three FL channels and
  thee FR channels. The possibility to attach an arbitrary label for
  custom layouts should be considered.

- We need something like
  idx = av_channel_layout_get_channel_by_descr(cl, "FL1");


Note: Paul, please do not take these comments as attacks against your
work. You took work that was developed in libav and ported it here, that
is a worthy achievement. But at the time it was designed, libav had
about 1.5 active developers, and they have about a quarter of our
features, and therefore of our needs. With these circumstances, it is
normal they forgot a lot of things. Our goal should be to design an API
that will be convenient to use in the long run, not to integrate their
imperfect API as fast as possible.

Regards,

On Mon, Oct 28, 2019 at 10:48 PM Paul B Mahol <onemda@gmail.com> wrote:

> The new API is more extensible and allows for custom layouts.
> More accurate information is exported, eg for decoders that do not
> set a channel layout, lavc will not make one up for them.
>
> Deprecate the old API working with just uint64_t bitmasks.
>
> Original commit by Anton Khirnov <anton@khirnov.net>.
> Expanded and completed by Vittorio Giovara <vittorio.giovara@gmail.com>.
> Adapted for FFmpeg by Paul B Mahol <onemda@gmail.com>.
>
> Signed-off-by: Anton Khirnov <anton@khirnov.net>
> Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
> Signed-off-by: Paul B Mahol <onemda@gmail.com>
> ---
>

Hello,
this seems to coming from an outdated branch.

Please use https://git.khirnov.net/libav.git/log/?h=ambisonic2 as
reference: it contains several improvements from the last iteration, such
as a fully functional backward compatibility layer, and a reworked
implementation of ambisonics, and contains a fully rebased tree with all
ffmpeg formats and codecs.

Really looking forward to getting this merged, thanks for your help in the
process.

On 11/10/19, Vittorio Giovara <vittorio.giovara@gmail.com> wrote:
> On Mon, Oct 28, 2019 at 10:48 PM Paul B Mahol <onemda@gmail.com> wrote:
>
>> The new API is more extensible and allows for custom layouts.
>> More accurate information is exported, eg for decoders that do not
>> set a channel layout, lavc will not make one up for them.
>>
>> Deprecate the old API working with just uint64_t bitmasks.
>>
>> Original commit by Anton Khirnov <anton@khirnov.net>.
>> Expanded and completed by Vittorio Giovara <vittorio.giovara@gmail.com>.
>> Adapted for FFmpeg by Paul B Mahol <onemda@gmail.com>.
>>
>> Signed-off-by: Anton Khirnov <anton@khirnov.net>
>> Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
>> Signed-off-by: Paul B Mahol <onemda@gmail.com>
>> ---
>>
>
> Hello,
> this seems to coming from an outdated branch.
>
> Please use https://git.khirnov.net/libav.git/log/?h=ambisonic2 as
> reference: it contains several improvements from the last iteration, such
> as a fully functional backward compatibility layer, and a reworked
> implementation of ambisonics, and contains a fully rebased tree with all
> ffmpeg formats and codecs.
>
> Really looking forward to getting this merged, thanks for your help in the
> process.

Only way to get this approved is sending it to this mailing list.

> --
> Vittorio
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