ffmpeg: only copy bits_per_sample from decoder when it remains valid

I.e. when the only filters that are applied do not modify the frame
data.

fftools/ffmpeg.c

@@ -3335,7 +3335,7 @@ static int init_output_stream_encode(OutputStream *ost, AVFrame *frame)
 
         if (ost->bits_per_raw_sample)
             enc_ctx->bits_per_raw_sample = ost->bits_per_raw_sample;
-        else if (dec_ctx)
+        else if (dec_ctx && ost->filter->graph->is_meta)
             enc_ctx->bits_per_raw_sample = FFMIN(dec_ctx->bits_per_raw_sample,
                                                  av_get_bytes_per_sample(enc_ctx->sample_fmt) << 3);
 
@@ -3361,7 +3361,10 @@ static int init_output_stream_encode(OutputStream *ost, AVFrame *frame)
             av_buffersink_get_sample_aspect_ratio(ost->filter->filter);
 
         enc_ctx->pix_fmt = av_buffersink_get_format(ost->filter->filter);
-        if (dec_ctx)
+
+        if (ost->bits_per_raw_sample)
+            enc_ctx->bits_per_raw_sample = ost->bits_per_raw_sample;
+        else if (dec_ctx && ost->filter->graph->is_meta)
             enc_ctx->bits_per_raw_sample = FFMIN(dec_ctx->bits_per_raw_sample,
                                                  av_pix_fmt_desc_get(enc_ctx->pix_fmt)->comp[0].depth);
 
@@ -3377,13 +3380,6 @@ static int init_output_stream_encode(OutputStream *ost, AVFrame *frame)
 
         ost->st->avg_frame_rate = ost->frame_rate;
 
-        if (!dec_ctx ||
-            enc_ctx->width   != dec_ctx->width  ||
-            enc_ctx->height  != dec_ctx->height ||
-            enc_ctx->pix_fmt != dec_ctx->pix_fmt) {
-            enc_ctx->bits_per_raw_sample = ost->bits_per_raw_sample;
-        }
-
         // Field order: autodetection
         if (frame) {
             if (enc_ctx->flags & (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME) &&

fftools/ffmpeg.h

@@ -287,6 +287,9 @@ typedef struct FilterGraph {
 
     AVFilterGraph *graph;
     int reconfiguration;
+    // true when the filtergraph contains only meta filters
+    // that do not modify the frame data
+    int is_meta;
 
     InputFilter   **inputs;
     int          nb_inputs;

fftools/ffmpeg_filter.c

@@ -960,6 +960,30 @@ static void cleanup_filtergraph(FilterGraph *fg)
     avfilter_graph_free(&fg->graph);
 }
 
+static int filter_is_buffersrc(const AVFilterContext *f)
+{
+    return f->nb_inputs == 0 &&
+           (!strcmp(f->filter->name, "buffersrc") ||
+            !strcmp(f->filter->name, "abuffersrc"));
+}
+
+static int graph_is_meta(AVFilterGraph *graph)
+{
+    for (unsigned i = 0; i < graph->nb_filters; i++) {
+        const AVFilterContext *f = graph->filters[i];
+
+        /* in addition to filters flagged as meta, also
+         * disregard sinks and buffersources (but not other sources,
+         * since they introduce data we are not aware of)
+         */
+        if (!((f->filter->flags & AVFILTER_FLAG_METADATA_ONLY) ||
+              f->nb_outputs == 0                               ||
+              filter_is_buffersrc(f)))
+            return 0;
+    }
+    return 1;
+}
+
 int configure_filtergraph(FilterGraph *fg)
 {
     AVFilterInOut *inputs, *outputs, *cur;
@@ -1060,6 +1084,8 @@ int configure_filtergraph(FilterGraph *fg)
     if ((ret = avfilter_graph_config(fg->graph, NULL)) < 0)
         goto fail;
 
+    fg->is_meta = graph_is_meta(fg->graph);
+
     /* limit the lists of allowed formats to the ones selected, to
      * make sure they stay the same if the filtergraph is reconfigured later */
     for (i = 0; i < fg->nb_outputs; i++) {