Add TXTP @downmix macro and improve plugin stereo downmixing

doc/TXTP.md

@@ -364,6 +364,7 @@ Manually setting values gets old, so TXTP supports a bunch of simple macros. The
 - `remix N (channels)`: same, but mixes selected channels to N channels properly adjusting volume (for layered bgm)
 - `crosstrack N`: crossfades between Nch tracks after every loop (loop count is adjusted as needed)
 - `crosslayer-v/b/e N`: crossfades Nch layers to the main track after every loop (loop count is adjusted as needed)
+- `downmix`: downmixes up to 8 channels (7.1, 5.1, etc) to stereo, using standard downmixing formulas.
 
 `channels` can be multiple comma-separated channels or N~M ranges and may be ommited were applicable to mean "all channels" (channel order doesn't matter but it's internally fixed).
 

src/meta/txtp.c

@@ -27,6 +27,7 @@ typedef enum {
     MACRO_LAYER,
     MACRO_CROSSTRACK,
     MACRO_CROSSLAYER,
+    MACRO_DOWNMIX,
 
 } txtp_mix_t;
 
@@ -498,6 +499,7 @@ static void apply_config(VGMSTREAM *vgmstream, txtp_entry *current) {
                 case MACRO_LAYER:       mixing_macro_layer(vgmstream, mix.max, mix.mask, mix.mode); break;
                 case MACRO_CROSSTRACK:  mixing_macro_crosstrack(vgmstream, mix.max); break;
                 case MACRO_CROSSLAYER:  mixing_macro_crosslayer(vgmstream, mix.max, mix.mode); break;
+                case MACRO_DOWNMIX:     mixing_macro_downmix(vgmstream, mix.max); break;
 
                 default:
                     break;
@@ -1071,6 +1073,16 @@ static void parse_config(txtp_entry *cfg, char *config) {
 
             add_mixing(cfg, &mix, type);
         }
+        else if (strcmp(command,"@downmix") == 0) {
+            txtp_mix_data mix = {0};
+
+            mix.max = 2; /* stereo only for now */
+            //nm = get_int(config, &mix.max);
+            //config += nm;
+            //if (nm == 0) continue;
+
+            add_mixing(cfg, &mix, MACRO_DOWNMIX);
+        }
         else if (config[nc] == ' ') {
             //;VGM_LOG("TXTP:   comment\n");
             break; /* comment, ignore rest */

src/mixing.c

@@ -925,6 +925,120 @@ void mixing_macro_crosslayer(VGMSTREAM* vgmstream, int max, char mode) {
     mixing_push_killmix(vgmstream, max);
 }
 
+
+typedef enum {
+    pos_FL  = 0,
+    pos_FR  = 1,
+    pos_FC  = 2,
+    pos_LFE = 3,
+    pos_BL  = 4,
+    pos_BR  = 5,
+    pos_FLC = 6,
+    pos_FRC = 7,
+    pos_BC  = 8,
+    pos_SL  = 9,
+    pos_SR  = 10,
+} mixing_position_t;
+
+void mixing_macro_downmix(VGMSTREAM* vgmstream, int max /*, mapping_t output_mapping*/) {
+    mixing_data *data = vgmstream->mixing_data;
+    int ch, output_channels, mp_in, mp_out, ch_in, ch_out;
+    mapping_t input_mapping, output_mapping;
+    const double vol_max = 1.0;
+    const double vol_sqrt = 1 / sqrt(2);
+    const double vol_half = 1 / 2;
+    double matrix[16][16] = {{0}};
+
+
+    if (!data)
+        return;
+    if (max <= 1 || data->output_channels <= max || max >= 8)
+        return;
+
+    /* assume WAV defaults if not set */
+    input_mapping = vgmstream->channel_layout;
+    if (input_mapping == 0) {
+        switch(data->output_channels) {
+            case 1: input_mapping = mapping_MONO; break;
+            case 2: input_mapping = mapping_STEREO; break;
+            case 3: input_mapping = mapping_2POINT1; break;
+            case 4: input_mapping = mapping_QUAD; break;
+            case 5: input_mapping = mapping_5POINT0; break;
+            case 6: input_mapping = mapping_5POINT1; break;
+            case 7: input_mapping = mapping_7POINT0; break;
+            case 8: input_mapping = mapping_7POINT1; break;
+            default: return;
+        }
+    }
+
+    /* build mapping matrix[input channel][output channel] = volume,
+     * using standard WAV/AC3 downmix formulas
+     * - https://www.audiokinetic.com/library/edge/?source=Help&id=downmix_tables
+     * - https://www.audiokinetic.com/library/edge/?source=Help&id=standard_configurations
+     */
+    switch(max) {
+        case 1:
+            output_mapping = mapping_MONO;
+            matrix[pos_FL][pos_FC] = vol_sqrt;
+            matrix[pos_FR][pos_FC] = vol_sqrt;
+            matrix[pos_FC][pos_FC] = vol_max;
+            matrix[pos_SL][pos_FC] = vol_half;
+            matrix[pos_SR][pos_FC] = vol_half;
+            matrix[pos_BL][pos_FC] = vol_half;
+            matrix[pos_BR][pos_FC] = vol_half;
+            break;
+        case 2:
+            output_mapping = mapping_STEREO;
+            matrix[pos_FL][pos_FL] = vol_max;
+            matrix[pos_FR][pos_FR] = vol_max;
+            matrix[pos_FC][pos_FL] = vol_sqrt;
+            matrix[pos_FC][pos_FR] = vol_sqrt;
+            matrix[pos_SL][pos_FL] = vol_sqrt;
+            matrix[pos_SR][pos_FR] = vol_sqrt;
+            matrix[pos_BL][pos_FL] = vol_sqrt;
+            matrix[pos_BR][pos_FR] = vol_sqrt;
+            break;
+        default:
+            /* not sure if +3ch would use FC/LFE, SL/BR and whatnot without passing extra config, so ignore for now */
+            return;
+    }
+
+    /* save and make N fake channels at the beginning for easier calcs */
+    output_channels = data->output_channels;
+    for (ch = 0; ch < max; ch++) {
+        mixing_push_upmix(vgmstream, 0);
+    }
+
+    /* downmix */
+    ch_in = 0;
+    for (mp_in = 0; mp_in < 16; mp_in++) {
+        /* read input mapping (ex. 5.1) and find channel */
+        if (!(input_mapping & (1<<mp_in)))
+            continue;
+
+        ch_out = 0;
+        for (mp_out = 0; mp_out < 16; mp_out++) {
+            /* read output mapping (ex. 2.0) and find channel */
+            if (!(output_mapping & (1<<mp_out)))
+                continue;
+
+            VGM_LOG("i=%i,j=%i, ch_out=%i, ch_in=%i + %i, %f\n", mp_in,mp_out,ch_out, max, ch_in, matrix[mp_out][mp_in]);
+            mixing_push_add(vgmstream, ch_out, max + ch_in, matrix[mp_in][mp_out]);
+
+            ch_out++;
+            if (ch_out > max)
+                break;
+        }
+
+        ch_in++;
+        if (ch_in >= output_channels)
+            break;
+    }
+
+    /* remove unneeded channels */
+    mixing_push_killmix(vgmstream, max);
+}
+
 /* ******************************************************************* */
 
 void mixing_setup(VGMSTREAM * vgmstream, int32_t max_sample_count) {

src/mixing.h

@@ -36,6 +36,7 @@ void mixing_macro_track(VGMSTREAM* vgmstream, uint32_t mask);
 void mixing_macro_layer(VGMSTREAM* vgmstream, int max, uint32_t mask, char mode);
 void mixing_macro_crosstrack(VGMSTREAM* vgmstream, int max);
 void mixing_macro_crosslayer(VGMSTREAM* vgmstream, int max, char mode);
+void mixing_macro_downmix(VGMSTREAM* vgmstream, int max /*, mapping_t output_mapping*/);
 
 
 #endif /* _MIXING_H_ */

src/plugins.c

@@ -234,12 +234,14 @@ void vgmstream_mixing_autodownmix(VGMSTREAM *vgmstream, int max_channels) {
     if (max_channels <= 0)
         return;
 
-    /* guess mixing the best we can */
-    //todo: could use standard downmixing for known max_channels <> vgmstream->channels combos:
-    //  https://www.audiokinetic.com/library/edge/?source=Help&id=downmix_tables#tbl_mono
-    //  https://www.audiokinetic.com/library/edge/?source=Help&id=standard_configurations
-
-    mixing_macro_layer(vgmstream, max_channels, 0, 'e');
+    /* guess mixing the best we can, using standard downmixing if possible
+     * (without mapping we can't be sure if format is using a standard layout) */
+    if (vgmstream->channel_layout && max_channels <= 2) {
+        mixing_macro_downmix(vgmstream, max_channels);
+    }
+    else {
+        mixing_macro_layer(vgmstream, max_channels, 0, 'e');
+    }
 
     return;
 }