@@ -4279,7 +4279,8 @@ Set when the expressions for @option{x}, @option{y}, and

 It accepts the following values:

 @table @samp

 @item init

-only evaluate expressions once during the filter initialization

+only evaluate expressions once during the filter initialization or

+when a command is processed

 @item frame

 evaluate expressions for each incoming frame

@@ -4360,6 +4361,20 @@ the @var{movie} filter.

 You can chain together more overlays but you should test the

 efficiency of such approach.

+@subsection Commands

+

+This filter supports the following command:

+@table @option

+@item x

+Set the @option{x} option expression.

+

+@item y

+Set the @option{y} option expression.

+

+@item enable

+Set the @option{enable} option expression.

+@end table

+

 @subsection Examples

 @itemize

@@ -30,7 +30,7 @@

 #define LIBAVFILTER_VERSION_MAJOR  3

 #define LIBAVFILTER_VERSION_MINOR  50

-#define LIBAVFILTER_VERSION_MICRO 102

+#define LIBAVFILTER_VERSION_MICRO 103

 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \

                                                LIBAVFILTER_VERSION_MINOR, \

@@ -164,6 +164,74 @@ static av_cold void uninit(AVFilterContext *ctx)

     av_expr_free(over->enable_pexpr); over->enable_pexpr = NULL;

 }

+static inline int normalize_xy(double d, int chroma_sub)

+{

+    if (isnan(d))

+        return INT_MAX;

+    return (int)d & ~((1 << chroma_sub) - 1);

+}

+

+enum EvalTarget { EVAL_XY, EVAL_ENABLE, EVAL_ALL };

+

+static void eval_expr(AVFilterContext *ctx, enum EvalTarget eval_tgt)

+{

+    OverlayContext  *over = ctx->priv;

+

+    if (eval_tgt == EVAL_XY || eval_tgt == EVAL_ALL) {

+        over->var_values[VAR_X] = av_expr_eval(over->x_pexpr, over->var_values, NULL);

+        over->var_values[VAR_Y] = av_expr_eval(over->y_pexpr, over->var_values, NULL);

+        over->var_values[VAR_X] = av_expr_eval(over->x_pexpr, over->var_values, NULL);

+        over->x = normalize_xy(over->var_values[VAR_X], over->hsub);

+        over->y = normalize_xy(over->var_values[VAR_Y], over->vsub);

+    }

+    if (eval_tgt == EVAL_ENABLE || eval_tgt == EVAL_ALL) {

+        over->enable = av_expr_eval(over->enable_pexpr, over->var_values, NULL);

+    }

+}

+

+static int set_expr(AVExpr **pexpr, const char *expr, void *log_ctx)

+{

+    int ret;

+

+    if (*pexpr)

+        av_expr_free(*pexpr);

+    *pexpr = NULL;

+    ret = av_expr_parse(pexpr, expr, var_names,

+                        NULL, NULL, NULL, NULL, 0, log_ctx);

+    if (ret < 0)

+        av_log(log_ctx, AV_LOG_ERROR,

+               "Error when evaluating the expression '%s'\n", expr);

+    return ret;

+}

+

+static int process_command(AVFilterContext *ctx, const char *cmd, const char *args,

+                           char *res, int res_len, int flags)

+{

+    OverlayContext *over = ctx->priv;

+    int ret;

+

+    if      (!strcmp(cmd, "x"))

+        ret = set_expr(&over->x_pexpr, args, ctx);

+    else if (!strcmp(cmd, "y"))

+        ret = set_expr(&over->y_pexpr, args, ctx);

+    else if (!strcmp(cmd, "enable"))

+        ret = set_expr(&over->enable_pexpr, args, ctx);

+    else

+        ret = AVERROR(ENOSYS);

+

+    if (ret < 0)

+        return ret;

+

+    if (over->eval_mode == EVAL_MODE_INIT) {

+        eval_expr(ctx, EVAL_ALL);

+        av_log(ctx, AV_LOG_VERBOSE, "x:%f xi:%d y:%f yi:%d enable:%f\n",

+               over->var_values[VAR_X], over->x,

+               over->var_values[VAR_Y], over->y,

+               over->enable);

+    }

+    return ret;

+}

+

 static int query_formats(AVFilterContext *ctx)

 {

     OverlayContext *over = ctx->priv;

@@ -244,30 +312,10 @@ static int config_input_main(AVFilterLink *inlink)

     return 0;

 }

-static inline int normalize_xy(double d, int chroma_sub)

-{

-    if (isnan(d))

-        return INT_MAX;

-    return (int)d & ~((1 << chroma_sub) - 1);

-}

-

-static void eval_expr(AVFilterContext *ctx)

-{

-    OverlayContext  *over = ctx->priv;

-

-    over->var_values[VAR_X] = av_expr_eval(over->x_pexpr, over->var_values, NULL);

-    over->var_values[VAR_Y] = av_expr_eval(over->y_pexpr, over->var_values, NULL);

-    over->var_values[VAR_X] = av_expr_eval(over->x_pexpr, over->var_values, NULL);

-    over->x = normalize_xy(over->var_values[VAR_X], over->hsub);

-    over->y = normalize_xy(over->var_values[VAR_Y], over->vsub);

-    over->enable = av_expr_eval(over->enable_pexpr, over->var_values, NULL);

-}

-

 static int config_input_overlay(AVFilterLink *inlink)

 {

     AVFilterContext *ctx  = inlink->dst;

     OverlayContext  *over = inlink->dst->priv;

-    char *expr;

     int ret;

     const AVPixFmtDescriptor *pix_desc = av_pix_fmt_desc_get(inlink->format);

@@ -287,25 +335,17 @@ static int config_input_overlay(AVFilterLink *inlink)

     over->var_values[VAR_T]     = NAN;

     over->var_values[VAR_POS]   = NAN;

-    expr = over->x_expr;

-    if ((ret = av_expr_parse(&over->x_pexpr, expr, var_names,

-                             NULL, NULL, NULL, NULL, 0, ctx)) < 0)

-        goto fail;

-    expr = over->y_expr;

-    if ((ret = av_expr_parse(&over->y_pexpr, expr, var_names,

-                             NULL, NULL, NULL, NULL, 0, ctx)) < 0)

-        goto fail;

-    expr = over->enable_expr;

-    if ((ret = av_expr_parse(&over->enable_pexpr, expr, var_names,

-                             NULL, NULL, NULL, NULL, 0, ctx)) < 0)

-        goto fail;

+    if ((ret = set_expr(&over->x_pexpr,      over->x_expr,      ctx)) < 0 ||

+        (ret = set_expr(&over->y_pexpr,      over->y_expr,      ctx)) < 0 ||

+        (ret = set_expr(&over->enable_pexpr, over->enable_expr, ctx)) < 0)

+        return ret;

     over->overlay_is_packed_rgb =

         ff_fill_rgba_map(over->overlay_rgba_map, inlink->format) >= 0;

     over->overlay_has_alpha = ff_fmt_is_in(inlink->format, alpha_pix_fmts);

     if (over->eval_mode == EVAL_MODE_INIT) {

-        eval_expr(ctx);

+        eval_expr(ctx, EVAL_ALL);

         av_log(ctx, AV_LOG_VERBOSE, "x:%f xi:%d y:%f yi:%d enable:%f\n",

                over->var_values[VAR_X], over->x,

                over->var_values[VAR_Y], over->y,

@@ -319,11 +359,6 @@ static int config_input_overlay(AVFilterLink *inlink)

            ctx->inputs[OVERLAY]->w, ctx->inputs[OVERLAY]->h,

            av_get_pix_fmt_name(ctx->inputs[OVERLAY]->format));

     return 0;

-

-fail:

-    av_log(NULL, AV_LOG_ERROR,

-           "Error when parsing the expression '%s'\n", expr);

-    return ret;

 }

 static int config_output(AVFilterLink *outlink)

@@ -581,7 +616,7 @@ static int try_filter_frame(AVFilterContext *ctx, AVFrame *mainpic)

                 NAN : mainpic->pts * av_q2d(inlink->time_base);

             over->var_values[VAR_POS] = pos == -1 ? NAN : pos;

-            eval_expr(ctx);

+            eval_expr(ctx, EVAL_ALL);

             av_log(ctx, AV_LOG_DEBUG, "n:%f t:%f pos:%f x:%f xi:%d y:%f yi:%d enable:%f\n",

                    over->var_values[VAR_N], over->var_values[VAR_T], over->var_values[VAR_POS],

                    over->var_values[VAR_X], over->x,

@@ -725,6 +760,7 @@ AVFilter avfilter_vf_overlay = {

     .priv_size = sizeof(OverlayContext),

     .query_formats = query_formats,

+    .process_command = process_command,

     .inputs    = avfilter_vf_overlay_inputs,

     .outputs   = avfilter_vf_overlay_outputs,