lavfi/vf_libplacebo: support blending multiple inputs

Subsequent inputs require frame blending to be enabled, in order to not
overwrite the existing frame contents.

For output metadata, we implicitly copy the metadata of the *first*
available stream (falling back to the second stream if the first has
already reached EOF, and so on). This is done to resolve any conflicts
between inputs with differing metadata. So when e.g. input 1 is HDR and
output 2 is SDR, the output will be HDR, and vice versa. This logic
could probablly be improved by dynamically determining some "superior"
set of metadata, but I don't want to handle that complexity in this
series.

1 files changed, 20 insertions, 4 deletions

diff --git a/libavfilter/vf_libplacebo.c b/libavfilter/vf_libplacebo.c
index 5b45d4f8fb..5ea6fcd792 100644
--- a/libavfilter/vf_libplacebo.c
+++ b/libavfilter/vf_libplacebo.c
@@ -786,12 +786,18 @@ static int output_frame(AVFilterContext *ctx, int64_t pts)
 {
     int err = 0, ok, changed_csp;
     LibplaceboContext *s = ctx->priv;
-    LibplaceboInput *in = &s->inputs[0];
     AVFilterLink *outlink = ctx->outputs[0];
     const AVPixFmtDescriptor *outdesc = av_pix_fmt_desc_get(outlink->format);
-    const AVFrame *ref = ref_frame(&in->mix);
     struct pl_frame target;
+    const AVFrame *ref = NULL;
     AVFrame *out;
+
+    /* Use the first active input as metadata reference */
+    for (int i = 0; i < s->nb_inputs; i++) {
+        const LibplaceboInput *in = &s->inputs[i];
+        if (in->qstatus == PL_QUEUE_OK && (ref = ref_frame(&in->mix)))
+            break;
+    }
     if (!ref)
         return 0;
 
@@ -862,8 +868,18 @@ static int output_frame(AVFilterContext *ctx, int64_t pts)
         goto fail;
     }
 
-    update_crops(ctx, in, &target, out->pts * av_q2d(outlink->time_base));
-    pl_render_image_mix(in->renderer, &in->mix, &target, &s->params);
+    /* Draw first frame opaque, others with blending */
+    s->params.skip_target_clearing = false;
+    s->params.blend_params = NULL;
+    for (int i = 0; i < s->nb_inputs; i++) {
+        LibplaceboInput *in = &s->inputs[i];
+        if (in->qstatus != PL_QUEUE_OK)
+            continue;
+        update_crops(ctx, in, &target, out->pts * av_q2d(outlink->time_base));
+        pl_render_image_mix(in->renderer, &in->mix, &target, &s->params);
+        s->params.skip_target_clearing = true;
+        s->params.blend_params = &pl_alpha_overlay;
+    }
 
     if (outdesc->flags & AV_PIX_FMT_FLAG_HWACCEL) {
         pl_unmap_avframe(s->gpu, &target);