avfilter/libplacebo: add option for outputting separate fields

cf. the other deinterlacing-aware filters.

doc/filters.texi

@@ -16632,6 +16632,11 @@ Yet another deinterlacing filter. See the @ref{yadif} filter for more details.
 
 @item skip_spatial_check
 Skip the spatial deinterlacing check when using @code{yadif} deinterlacing.
+
+@item send_fields
+Output a frame for each field, rather than for each frame. Note that this will
+always double the tagged output frame rate, even if the input does not contain
+any interlaced frames. Disabled by default.
 @end table
 
 @subsubsection Debanding

libavfilter/vf_libplacebo.c

@@ -215,6 +215,7 @@ typedef struct LibplaceboContext {
     /* pl_deinterlace_params */
     int deinterlace;
     int skip_spatial_check;
+    int send_fields;
 
     /* pl_deband_params */
     int deband;
@@ -967,8 +968,7 @@ static int handle_input(AVFilterContext *ctx, LibplaceboInput *input)
     int64_t pts;
 
     while ((ret = ff_inlink_consume_frame(inlink, &in)) > 0) {
-        in->opaque = s;
+        struct pl_source_frame src = {
-        pl_queue_push(input->queue, &(struct pl_source_frame) {
             .pts         = in->pts * av_q2d(inlink->time_base),
             .duration    = in->duration * av_q2d(inlink->time_base),
             .first_field = s->deinterlace ? pl_field_from_avframe(in) : PL_FIELD_NONE,
@@ -976,12 +976,21 @@ static int handle_input(AVFilterContext *ctx, LibplaceboInput *input)
             .map         = map_frame,
             .unmap       = unmap_frame,
             .discard     = discard_frame,
-        });
+        };
+
+        in->opaque = s;
+        pl_queue_push(input->queue, &src);
 
         if (!s->fps.num) {
             /* Internally queue an output frame for the same PTS */
             pts = av_rescale_q(in->pts, inlink->time_base, outlink->time_base);
             av_fifo_write(input->out_pts, &pts, 1);
+
+            if (s->send_fields && src.first_field != PL_FIELD_NONE) {
+                /* Queue the second field for interlaced content */
+                pts += av_rescale_q(in->duration, inlink->time_base, outlink->time_base) / 2;
+                av_fifo_write(input->out_pts, &pts, 1);
+            }
         }
     }
 
@@ -1239,6 +1248,13 @@ static int libplacebo_config_output(AVFilterLink *outlink)
                                           avctx->inputs[i]->time_base,
                                           AV_TIME_BASE / 2, AV_TIME_BASE_Q);
         }
+
+        if (s->deinterlace && s->send_fields) {
+            const AVRational q2 = { 2, 1 };
+            ol->frame_rate = av_mul_q(ol->frame_rate, q2);
+            /* Ensure output frame timestamps are divisible by two */
+            outlink->time_base = av_div_q(outlink->time_base, q2);
+        }
     }
 
     /* Static variables */
@@ -1373,6 +1389,7 @@ static const AVOption libplacebo_options[] = {
         { "bob",   "Naive bob deinterlacing",          0, AV_OPT_TYPE_CONST, {.i64 = PL_DEINTERLACE_BOB},   0, 0, STATIC, .unit = "deinterlace" },
         { "yadif", "Yet another deinterlacing filter", 0, AV_OPT_TYPE_CONST, {.i64 = PL_DEINTERLACE_YADIF}, 0, 0, STATIC, .unit = "deinterlace" },
     { "skip_spatial_check", "Skip yadif spatial check", OFFSET(skip_spatial_check), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, DYNAMIC },
+    { "send_fields", "Output a frame for each field", OFFSET(send_fields), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, DYNAMIC },
 
     { "deband", "Enable debanding", OFFSET(deband), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, DYNAMIC },
     { "deband_iterations", "Deband iterations", OFFSET(deband_iterations), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 16, DYNAMIC },