lavfi/color: cache and reuse colored picture in context

Avoid to fill the same picture again and again with the same content.

Optimize computation, and provides an example for the use of the
AV_PERM_REUSE permission flag.

libavfilter/vsrc_color.c

@@ -45,6 +45,7 @@ typedef struct {
     uint64_t pts;
     FFDrawContext draw;
     FFDrawColor color;
+    AVFilterBufferRef *picref;  ///< cached reference containing the painted picture
 } ColorContext;
 
 #define OFFSET(x) offsetof(ColorContext, x)
@@ -92,6 +93,12 @@ end:
     return ret;
 }
 
+static av_cold void color_uninit(AVFilterContext *ctx)
+{
+    ColorContext *color = ctx->priv;
+    avfilter_unref_bufferp(&color->picref);
+}
+
 static int query_formats(AVFilterContext *ctx)
 {
     ff_set_common_formats(ctx, ff_draw_supported_pixel_formats(0));
@@ -124,18 +131,24 @@ static int color_config_props(AVFilterLink *inlink)
 static int color_request_frame(AVFilterLink *link)
 {
     ColorContext *color = link->src->priv;
-    AVFilterBufferRef *picref = ff_get_video_buffer(link, AV_PERM_WRITE, color->w, color->h);
     AVFilterBufferRef *buf_out;
     int ret;
 
-    if (!picref)
-        return AVERROR(ENOMEM);
-
-    picref->video->sample_aspect_ratio = (AVRational) {1, 1};
-    picref->pts = color->pts++;
-    picref->pos = -1;
+    if (!color->picref) {
+        color->picref =
+            ff_get_video_buffer(link, AV_PERM_WRITE|AV_PERM_PRESERVE|AV_PERM_REUSE,
+                                color->w, color->h);
+        if (!color->picref)
+            return AVERROR(ENOMEM);
+        ff_fill_rectangle(&color->draw, &color->color,
+                          color->picref->data, color->picref->linesize,
+                          0, 0, color->w, color->h);
+        color->picref->video->sample_aspect_ratio = (AVRational) {1, 1};
+        color->picref->pos = -1;
+    }
 
-    buf_out = avfilter_ref_buffer(picref, ~0);
+    color->picref->pts = color->pts++;
+    buf_out = avfilter_ref_buffer(color->picref, ~AV_PERM_WRITE);
     if (!buf_out) {
         ret = AVERROR(ENOMEM);
         goto fail;
@@ -145,8 +158,6 @@ static int color_request_frame(AVFilterLink *link)
     if (ret < 0)
         goto fail;
 
-    ff_fill_rectangle(&color->draw, &color->color, picref->data, picref->linesize,
-                      0, 0, color->w, color->h);
     ret = ff_draw_slice(link, 0, color->h, 1);
     if (ret < 0)
         goto fail;
@@ -154,8 +165,6 @@ static int color_request_frame(AVFilterLink *link)
     ret = ff_end_frame(link);
 
 fail:
-    avfilter_unref_buffer(picref);
-
     return ret;
 }
 
@@ -165,6 +174,7 @@ AVFilter avfilter_vsrc_color = {
 
     .priv_size = sizeof(ColorContext),
     .init      = color_init,
+    .uninit    = color_uninit,
 
     .query_formats = query_formats,