libavfilter/vf_scale: implement process_command

Signed-off-by: Bernd Bleßmann <bb@it-entwicklung.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -8906,6 +8906,19 @@ scale=w='min(500\, iw*3/2):h=-1'
 @end example
 @end itemize
 
+@subsection Commands
+
+This filter supports the following commands:
+@table @option
+@item width, w
+@item height, h
+Set the output video dimension expression.
+The command accepts the same syntax of the corresponding option.
+
+If the specified expression is not valid, it is kept at its current
+value.
+@end table
+
 @section separatefields
 
 The @code{separatefields} takes a frame-based video input and splits

libavfilter/vf_scale.c

@@ -544,6 +544,30 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
     return ff_filter_frame(outlink, out);
 }
 
+static int process_command(AVFilterContext *ctx, const char *cmd, const char *args,
+                           char *res, int res_len, int flags)
+{
+    ScaleContext *scale = ctx->priv;
+    int ret;
+
+    if (   !strcmp(cmd, "width")  || !strcmp(cmd, "w")
+        || !strcmp(cmd, "height") || !strcmp(cmd, "h")) {
+
+        int old_w = scale->w;
+        int old_h = scale->h;
+        AVFilterLink *outlink = ctx->outputs[0];
+
+        av_opt_set(scale, cmd, args, 0);
+        if ((ret = config_props(outlink)) < 0) {
+            scale->w = old_w;
+            scale->h = old_h;
+        }
+    } else
+        ret = AVERROR(ENOSYS);
+
+    return ret;
+}
+
 static const AVClass *child_class_next(const AVClass *prev)
 {
     return prev ? NULL : sws_get_class();
@@ -619,4 +643,5 @@ AVFilter ff_vf_scale = {
     .priv_class      = &scale_class,
     .inputs          = avfilter_vf_scale_inputs,
     .outputs         = avfilter_vf_scale_outputs,
+    .process_command = process_command,
 };