lavfi/pan: allow negative gain parameters also for other inputs than the first named

Expands the parser to also accept the separator '-' in addition to
'+', and take the negative sign into consideration.

The optional sign for the first factor in the expression is already
covered by parsing for an integer.
Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -3026,7 +3026,7 @@ output channel layout or number of channels
 
 @item outdef
 output channel specification, of the form:
-"@var{out_name}=[@var{gain}*]@var{in_name}[+[@var{gain}*]@var{in_name}...]"
+"@var{out_name}=[@var{gain}*]@var{in_name}[(+-)[@var{gain}*]@var{in_name}...]"
 
 @item out_name
 output channel to define, either a channel name (FL, FR, etc.) or a channel

libavfilter/af_pan.c

@@ -102,7 +102,7 @@ static av_cold int init(AVFilterContext *ctx)
 {
     PanContext *const pan = ctx->priv;
     char *arg, *arg0, *tokenizer, *args = av_strdup(pan->args);
-    int out_ch_id, in_ch_id, len, named, ret;
+    int out_ch_id, in_ch_id, len, named, ret, sign = 1;
     int nb_in_channels[2] = { 0, 0 }; // number of unnamed and named input channels
     double gain;
 
@@ -178,14 +178,18 @@ static av_cold int init(AVFilterContext *ctx)
                 ret = AVERROR(EINVAL);
                 goto fail;
             }
-            pan->gain[out_ch_id][in_ch_id] = gain;
+            pan->gain[out_ch_id][in_ch_id] = sign * gain;
             skip_spaces(&arg);
             if (!*arg)
                 break;
-            if (*arg != '+') {
+            if (*arg == '-') {
+                sign = -1;
+            } else if (*arg != '+') {
                 av_log(ctx, AV_LOG_ERROR, "Syntax error near \"%.8s\"\n", arg);
                 ret = AVERROR(EINVAL);
                 goto fail;
+            } else {
+                sign = 1;
             }
             arg++;
         }