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swresample: pass input layout in mpv's order via custom channel order #18034

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Merged
kasper93 merged 2 commits into from
Jun 1, 2026
Merged

swresample: pass input layout in mpv's order via custom channel order #18034

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621f4c2
swresample: remove dead fudge_layout_conversion
kasper93 May 26, 2026
9d532ec
swresample: pass input layout in mpv's order via custom channel order
kasper93 May 26, 2026
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43 changes: 43 additions & 0 deletions audio/chmap_avchannel.c
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,8 @@
*/

#include <libavutil/channel_layout.h>
#include <libavutil/error.h>
#include <libavutil/mem.h>

#include "chmap.h"
#include "chmap_avchannel.h"
Expand Down Expand Up @@ -49,3 +51,44 @@ void mp_chmap_to_av_layout(AVChannelLayout *dst, const struct mp_chmap *src)
av_channel_layout_from_mask(dst, mp_chmap_to_lavc(src));
}
}

static int custom_init(AVChannelLayout *dst, int nb_channels)
{
#if LIBAVUTIL_VERSION_INT >= AV_VERSION_INT(58, 37, 100)
return av_channel_layout_custom_init(dst, nb_channels);
#else
AVChannelCustom *map = av_calloc(nb_channels, sizeof(*map));
if (!map)
return AVERROR(ENOMEM);
dst->order = AV_CHANNEL_ORDER_CUSTOM;
dst->nb_channels = nb_channels;
dst->u.map = map;
return 0;
#endif
}

void mp_chmap_to_av_layout_custom(AVChannelLayout *dst,
const struct mp_chmap *src)
{
*dst = (AVChannelLayout){0};

if (mp_chmap_is_unknown(src) || src->num <= 0 ||
custom_init(dst, src->num) < 0)
{
dst->order = AV_CHANNEL_ORDER_UNSPEC;
dst->nb_channels = src->num;
return;
}

// mp_speaker_id values < 64 match the AVChannel enum directly. NA maps to
// AV_CHAN_UNUSED. Anything else is unknown.
for (int n = 0; n < src->num; n++) {
if (src->speaker[n] == MP_SPEAKER_ID_NA) {
dst->u.map[n].id = AV_CHAN_UNUSED;
} else if (src->speaker[n] < 64) {
dst->u.map[n].id = src->speaker[n];
} else {
dst->u.map[n].id = AV_CHAN_UNKNOWN;
}
}
}
5 changes: 5 additions & 0 deletions audio/chmap_avchannel.h
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Original file line number Diff line number Diff line change
Expand Up @@ -26,3 +26,8 @@
bool mp_chmap_from_av_layout(struct mp_chmap *dst, const AVChannelLayout *src);

void mp_chmap_to_av_layout(AVChannelLayout *dst, const struct mp_chmap *src);

// Like mp_chmap_to_av_layout(), but always emit AV_CHANNEL_ORDER_CUSTOM for
// known layouts so that the original channel order is preserved.
void mp_chmap_to_av_layout_custom(AVChannelLayout *dst,
const struct mp_chmap *src);
65 changes: 4 additions & 61 deletions filters/f_swresample.c
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Expand Up @@ -44,8 +44,6 @@ struct priv {
struct mp_aframe *pre_out_fmt; // format before final conversion
struct SwrContext *avrctx_out; // for output channel reordering
struct mp_resample_opts *opts; // opts requested by the user
// At least libswresample keeps a pointer around for this:
int reorder_in[MP_NUM_CHANNELS];
int reorder_out[MP_NUM_CHANNELS];
struct mp_aframe_pool *reorder_buffer;
struct mp_aframe_pool *out_pool;
Expand Down Expand Up @@ -109,53 +107,6 @@ static int rate_from_speed(int rate, double speed)
return lrint(rate * speed);
}

static const struct mp_chmap fudge_pairs[][2] = {
{MP_CHMAP2(BL, BR), MP_CHMAP2(SL, SR)},
{MP_CHMAP2(SL, SR), MP_CHMAP2(BL, BR)},
{MP_CHMAP2(SDL, SDR), MP_CHMAP2(SL, SR)},
{MP_CHMAP2(SL, SR), MP_CHMAP2(SDL, SDR)},
};

// Modify out_layout and return the new value. The intention is reducing the
// loss libswresample's rematrixing will cause by exchanging similar, but
// strictly speaking incompatible channel pairs. For example, 7.1 should be
// changed to 7.1(wide) without dropping the SL/SR channels. (We still leave
// it to libswresample to create the remix matrix.)
static uint64_t fudge_layout_conversion(struct priv *p,
uint64_t in, uint64_t out)
{
for (int n = 0; n < MP_ARRAY_SIZE(fudge_pairs); n++) {
uint64_t a = mp_chmap_to_lavc(&fudge_pairs[n][0]);
uint64_t b = mp_chmap_to_lavc(&fudge_pairs[n][1]);
if ((in & a) == a && (in & b) == 0 &&
(out & a) == 0 && (out & b) == b)
{
out = (out & ~b) | a;

MP_VERBOSE(p, "Fudge: %s -> %s\n",
mp_chmap_to_str(&fudge_pairs[n][0]),
mp_chmap_to_str(&fudge_pairs[n][1]));
}
}
return out;
}

// mp_chmap_get_reorder() performs:
// to->speaker[n] = from->speaker[src[n]]
// but libavresample does:
// to->speaker[dst[n]] = from->speaker[n]
static void transpose_order(int *map, int num)
{
int nmap[MP_NUM_CHANNELS] = {0};
for (int n = 0; n < num; n++) {
for (int i = 0; i < num; i++) {
if (map[n] == i)
nmap[i] = n;
}
}
memcpy(map, nmap, sizeof(nmap));
}

static bool configure_lavrr(struct priv *p, bool verbose)
{
close_lavrr(p);
Expand Down Expand Up @@ -234,9 +185,6 @@ static bool configure_lavrr(struct priv *p, bool verbose)
goto error;
}

mp_chmap_get_reorder(p->reorder_in, &map_in, &in_lavc);
transpose_order(p->reorder_in, map_in.num);

if (mp_chmap_equals(&out_lavc, &map_out)) {
// No intermediate step required - output new format directly.
out_samplefmtp = out_samplefmt;
Expand Down Expand Up @@ -268,14 +216,13 @@ static bool configure_lavrr(struct priv *p, bool verbose)
if (map_out.num > out_lavc.num)
mp_aframe_set_chmap(p->pool_fmt, &map_out);

out_ch_layout = fudge_layout_conversion(p, in_ch_layout, out_ch_layout);

// Real conversion; output is input to avrctx_out.
AVChannelLayout in_layout, out_layout;
mp_chmap_to_av_layout(&in_layout, &in_lavc);
mp_chmap_to_av_layout_custom(&in_layout, &map_in);
mp_chmap_to_av_layout(&out_layout, &out_lavc);
av_opt_set_chlayout(p->avrctx, "in_chlayout", &in_layout, 0);
av_opt_set_chlayout(p->avrctx, "out_chlayout", &out_layout, 0);
av_channel_layout_uninit(&in_layout);
av_channel_layout_uninit(&out_layout);
av_opt_set_int(p->avrctx, "in_sample_rate", p->in_rate, 0);
av_opt_set_int(p->avrctx, "out_sample_rate", p->out_rate, 0);
av_opt_set_int(p->avrctx, "in_sample_fmt", in_samplefmt, 0);
Expand All @@ -285,16 +232,12 @@ static bool configure_lavrr(struct priv *p, bool verbose)
av_channel_layout_default(&fake_layout, map_out.num);
av_opt_set_chlayout(p->avrctx_out, "in_chlayout", &fake_layout, 0);
av_opt_set_chlayout(p->avrctx_out, "out_chlayout", &fake_layout, 0);
av_channel_layout_uninit(&fake_layout);
av_opt_set_int(p->avrctx_out, "in_sample_fmt", out_samplefmtp, 0);
av_opt_set_int(p->avrctx_out, "out_sample_fmt", out_samplefmt, 0);
av_opt_set_int(p->avrctx_out, "in_sample_rate", p->out_rate, 0);
av_opt_set_int(p->avrctx_out, "out_sample_rate", p->out_rate, 0);

// API has weird requirements, quoting avresample.h:
// * This function can only be called when the allocated context is not open.
// * Also, the input channel layout must have already been set.
swr_set_channel_mapping(p->avrctx, p->reorder_in);

p->is_resampling = false;

if (swr_init(p->avrctx) < 0 || swr_init(p->avrctx_out) < 0) {
Expand Down
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