lavfi/curves: make possible to customize presets.

doc/filters.texi

@@ -2371,8 +2371,9 @@ The filter accepts the following options:
 
 @table @option
 @item preset
-Select one of the available color presets. This option can not be used in
-addition to the @option{r}, @option{g}, @option{b} parameters.
+Select one of the available color presets. This option can be used in addition
+to the @option{r}, @option{g}, @option{b} parameters; in this case, the later
+options takes priority on the preset values.
 Available presets are:
 @table @samp
 @item none

libavfilter/vf_curves.c

@@ -333,14 +333,9 @@ static av_cold int init(AVFilterContext *ctx, const char *args)
     }
 
     if (curves->preset != PRESET_NONE) {
-        if (pts[0] || pts[1] || pts[2]) {
-            av_log(ctx, AV_LOG_ERROR, "It is not possible to mix a preset "
-                   "with explicit points placements\n");
-            return AVERROR(EINVAL);
-        }
-        pts[0] = av_strdup(curves_presets[curves->preset].r);
-        pts[1] = av_strdup(curves_presets[curves->preset].g);
-        pts[2] = av_strdup(curves_presets[curves->preset].b);
+        if (!pts[0]) pts[0] = av_strdup(curves_presets[curves->preset].r);
+        if (!pts[1]) pts[1] = av_strdup(curves_presets[curves->preset].g);
+        if (!pts[2]) pts[2] = av_strdup(curves_presets[curves->preset].b);
         if (!pts[0] || !pts[1] || !pts[2])
             return AVERROR(ENOMEM);
     }