ratecontrol: Dynamically choose a default for rc_max_available_vbv_use

This improves handling of small buffer sizes
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/options_table.h

@@ -376,7 +376,7 @@ static const AVOption options[]={
 {"bits_per_raw_sample", NULL, OFFSET(bits_per_raw_sample), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
 {"channel_layout", NULL, OFFSET(channel_layout), AV_OPT_TYPE_INT64, {.i64 = DEFAULT }, 0, INT64_MAX, A|E|D, "channel_layout"},
 {"request_channel_layout", NULL, OFFSET(request_channel_layout), AV_OPT_TYPE_INT64, {.i64 = DEFAULT }, 0, INT64_MAX, A|D, "request_channel_layout"},
-{"rc_max_vbv_use", NULL, OFFSET(rc_max_available_vbv_use), AV_OPT_TYPE_FLOAT, {.dbl = 1.0/3 }, 0.0, FLT_MAX, V|E},
+{"rc_max_vbv_use", NULL, OFFSET(rc_max_available_vbv_use), AV_OPT_TYPE_FLOAT, {.dbl = 0 }, 0.0, FLT_MAX, V|E},
 {"rc_min_vbv_use", NULL, OFFSET(rc_min_vbv_overflow_use),  AV_OPT_TYPE_FLOAT, {.dbl = 3 },     0.0, FLT_MAX, V|E},
 {"ticks_per_frame", NULL, OFFSET(ticks_per_frame), AV_OPT_TYPE_INT, {.i64 = 1 }, 1, INT_MAX, A|V|E|D},
 {"color_primaries", NULL, OFFSET(color_primaries), AV_OPT_TYPE_INT, {.i64 = AVCOL_PRI_UNSPECIFIED }, 1, AVCOL_PRI_NB-1, V|E|D},

libavcodec/ratecontrol.c

@@ -110,6 +110,13 @@ int ff_rate_control_init(MpegEncContext *s)
     };
     emms_c();
 
+    if (!s->avctx->rc_max_available_vbv_use && s->avctx->rc_buffer_size) {
+        if (s->avctx->rc_max_rate) {
+            s->avctx->rc_max_available_vbv_use = av_clipf(s->avctx->rc_max_rate/(s->avctx->rc_buffer_size*get_fps(s->avctx)), 1.0/0.3, 1.0);
+        } else
+            s->avctx->rc_max_available_vbv_use = 1.0;
+    }
+
     res = av_expr_parse(&rcc->rc_eq_eval, s->avctx->rc_eq ? s->avctx->rc_eq : "tex^qComp", const_names, func1_names, func1, NULL, NULL, 0, s->avctx);
     if (res < 0) {
         av_log(s->avctx, AV_LOG_ERROR, "Error parsing rc_eq \"%s\"\n", s->avctx->rc_eq);