lavfi/overlay: implement shortest option

Force termination when the overlay stream ends. Simplify scripting logic,
for example when an infinite source is used to generate a background for
a composite video.

doc/filters.texi

@@ -3881,6 +3881,10 @@ same as @var{overlay_w} and @var{overlay_h}
 @item rgb
 If set to 1, force the filter to accept inputs in the RGB
 color space. Default value is 0.
+
+@item shortest
+If set to 1, force the output to terminate when the shortest input
+terminates. Default value is 0.
 @end table
 
 Be aware that frames are taken from each input video in timestamp
@@ -3940,6 +3944,18 @@ The above command is the same as:
 ffplay input.avi -vf 'split[b], pad=iw*2[src], [b]deshake, [src]overlay=w'
 @end example
 
+@item
+Compose output by putting two input videos side to side:
+@example
+ffmpeg -i left.avi -i right.avi -filter_complex "
+nullsrc=size=200x100 [background];
+[0:v] setpts=PTS-STARTPTS, scale=100x100 [left];
+[1:v] setpts=PTS-STARTPTS, scale=100x100 [right];
+[background][left]       overlay=shortest=1       [background+left];
+[background+left][right] overlay=shortest=1:x=100 [left+right]
+"
+@end example
+
 @item
 Chain several overlays in cascade:
 @example

libavfilter/version.h

@@ -30,7 +30,7 @@
 
 #define LIBAVFILTER_VERSION_MAJOR  3
 #define LIBAVFILTER_VERSION_MINOR  38
-#define LIBAVFILTER_VERSION_MICRO 104
+#define LIBAVFILTER_VERSION_MICRO 105
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_overlay.c

@@ -91,6 +91,7 @@ typedef struct {
     int main_pix_step[4];       ///< steps per pixel for each plane of the main output
     int overlay_pix_step[4];    ///< steps per pixel for each plane of the overlay
     int hsub, vsub;             ///< chroma subsampling values
+    int shortest;               ///< terminate stream when the shortest input terminates
 
     char *x_expr, *y_expr;
 } OverlayContext;
@@ -102,6 +103,7 @@ static const AVOption overlay_options[] = {
     { "x", "set the x expression", OFFSET(x_expr), AV_OPT_TYPE_STRING, {.str = "0"}, CHAR_MIN, CHAR_MAX, FLAGS },
     { "y", "set the y expression", OFFSET(y_expr), AV_OPT_TYPE_STRING, {.str = "0"}, CHAR_MIN, CHAR_MAX, FLAGS },
     {"rgb", "force packed RGB in input and output", OFFSET(allow_packed_rgb), AV_OPT_TYPE_INT, {.i64=0}, 0, 1, FLAGS },
+    { "shortest", "force termination when the shortest input terminates", OFFSET(shortest), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, 1, FLAGS },
     {NULL},
 };
 
@@ -560,6 +562,8 @@ static int request_frame(AVFilterLink *outlink)
         /* EOF on main is reported immediately */
         if (ret == AVERROR_EOF && input == OVERLAY) {
             over->overlay_eof = 1;
+            if (over->shortest)
+                return ret;
             if ((ret = try_filter_next_frame(ctx)) != AVERROR(EAGAIN))
                 return ret;
             ret = 0; /* continue requesting frames on main */