lavfi/vsrc_ddagrab: add an option to avoid duplicating frames

Tested-by: Jiří Eliášek, Misha Aizatulin

doc/filters.texi

@@ -28713,10 +28713,10 @@ it'll always be captured.
 @item framerate
 Maximum framerate at which the desktop will be captured - the interval between
 successive frames will not be smaller than the inverse of the framerate. When
-the desktop is not being updated often enough, the filter will duplicate
-a previous frame. Note that there is no background buffering going on, so when
-the filter is not polled often enough then the actual inter-frame interval may
-be significantly larger.
+@var{dup_frames} is true (the default) and the desktop is not being updated
+often enough, the filter will duplicate a previous frame. Note that there is no
+background buffering going on, so when the filter is not polled often enough
+then the actual inter-frame interval may be significantly larger.
 
 Defaults to 30 FPS.
 
@@ -28749,6 +28749,13 @@ Passes all supported output formats to DDA and returns what DDA decides to use.
 Filter initialization will fail if 10 bit format is requested but unavailable.
 @end table
 
+@item dup_frames
+When this option is set to true (the default), the filter will duplicate frames
+when the desktop has not been updated in order to maintain approximately
+constant target framerate. When this option is set to false, the filter will
+wait for the desktop to be updated (inter-frame intervals may vary significantly
+in this case).
+
 @end table
 
 @subsection Examples

libavfilter/vsrc_ddagrab.c

@@ -101,6 +101,7 @@ typedef struct DdagrabContext {
     int        out_fmt;
     int        allow_fallback;
     int        force_fmt;
+    int        dup_frames;
 } DdagrabContext;
 
 #define OFFSET(x) offsetof(DdagrabContext, x)
@@ -124,6 +125,8 @@ static const AVOption ddagrab_options[] = {
                                                    OFFSET(allow_fallback), AV_OPT_TYPE_BOOL,   { .i64 = 0    },       0,       1, FLAGS },
     { "force_fmt",  "exclude BGRA from format list (experimental, discouraged by Microsoft)",
                                                    OFFSET(force_fmt),  AV_OPT_TYPE_BOOL,       { .i64 = 0    },       0,       1, FLAGS },
+    { "dup_frames", "duplicate frames to maintain framerate",
+                                                   OFFSET(dup_frames), AV_OPT_TYPE_BOOL,       { .i64 = 1    },       0,       1, FLAGS },
     { NULL }
 };
 
@@ -1067,7 +1070,9 @@ static int ddagrab_request_frame(AVFilterLink *outlink)
     now -= dda->first_pts;
 
     if (!dda->probed_texture) {
-        ret = next_frame_internal(avctx, &cur_texture, 0);
+        do {
+            ret = next_frame_internal(avctx, &cur_texture, 0);
+        } while (ret == AVERROR(EAGAIN) && !dda->dup_frames);
     } else {
         cur_texture = dda->probed_texture;
         dda->probed_texture = NULL;