diff --git a/common/frame.c b/common/frame.c
old mode 100644
new mode 100755
index 87abd03..70bcf8a
--- a/common/frame.c
+++ b/common/frame.c
@@ -844,11 +844,39 @@ void x264_frame_cond_wait( x264_frame_t *frame, int i_lines_completed )
     x264_pthread_mutex_unlock( &frame->mutex );
 }
 
+void x264_frame_size_estimated_set( x264_t *h, int bits )
+{
+    x264_pthread_mutex_lock( &h->fenc->mutex );
+    x264_ratecontrol_set_estimated_size(h, bits);
+    x264_pthread_mutex_unlock( &h->fenc->mutex );
+}
+
+int x264_frame_size_estimated_get( x264_t const *h)
+{
+    int size;
+    x264_pthread_mutex_lock( &h->fenc->mutex );
+    size = x264_ratecontrol_get_estimated_size(h);
+    x264_pthread_mutex_unlock( &h->fenc->mutex );
+    return size;
+}
+
 #else
 void x264_frame_cond_broadcast( x264_frame_t *frame, int i_lines_completed )
 {}
 void x264_frame_cond_wait( x264_frame_t *frame, int i_lines_completed )
 {}
+
+void x264_frame_size_estimated_set( x264_t *h, int bits )
+{
+    x264_ratecontrol_set_estimated_size(h, bits);
+}
+
+int x264_frame_size_estimated_get( x264_t const *h)
+{
+    int size;
+    size = x264_ratecontrol_set_estimated_size(h);
+    return size;
+}
 #endif
 
 
diff --git a/common/frame.h b/common/frame.h
old mode 100644
new mode 100755
index 1cd7b64..25a373e
--- a/common/frame.h
+++ b/common/frame.h
@@ -121,6 +121,9 @@ void          x264_deblock_init( int cpu, x264_deblock_function_t *pf );
 void          x264_frame_cond_broadcast( x264_frame_t *frame, int i_lines_completed );
 void          x264_frame_cond_wait( x264_frame_t *frame, int i_lines_completed );
 
+void          x264_frame_size_estimated_set( x264_t *h, int bits );
+int           x264_frame_size_estimated_get( x264_t const *h);
+
 void          x264_frame_push( x264_frame_t **list, x264_frame_t *frame );
 x264_frame_t *x264_frame_pop( x264_frame_t **list );
 void          x264_frame_unshift( x264_frame_t **list, x264_frame_t *frame );
diff --git a/encoder/encoder.c b/encoder/encoder.c
index 39c5d5f..93bfd70 100644
--- a/encoder/encoder.c
+++ b/encoder/encoder.c
@@ -631,6 +631,7 @@ x264_t *x264_encoder_open   ( x264_param_t *param )
           || h->param.rc.i_rc_method == X264_RC_CRF
           || h->param.b_bframe_adaptive
           || h->param.b_pre_scenecut );
+    h->frames.b_have_lowres |= (h->param.rc.b_stat_read && h->param.rc.i_vbv_buffer_size > 0);
 
     h->frames.i_last_idr = - h->param.i_keyint_max;
     h->frames.i_input    = 0;
diff --git a/encoder/ratecontrol.c b/encoder/ratecontrol.c
old mode 100644
new mode 100755
index a0e0859..7d94434
--- a/encoder/ratecontrol.c
+++ b/encoder/ratecontrol.c
@@ -43,6 +43,7 @@ typedef struct
     int p_tex_bits;
     int misc_bits;
     uint64_t expected_bits;
+    double expected_vbv;
     float new_qscale;
     int new_qp;
     int i_count;
@@ -121,6 +122,7 @@ struct x264_ratecontrol_t
     int frame_count[5];         /* number of frames of each type */
 
     /* MBRC stuff */
+    double frame_size_estimated;
     double frame_size_planned;
     predictor_t *row_pred;
     predictor_t row_preds[5];
@@ -331,7 +333,7 @@ int x264_ratecontrol_new( x264_t *h )
         rc->rate_tolerance = 0.01;
     }
 
-    h->mb.b_variable_qp = (rc->b_vbv && !rc->b_2pass) || h->param.rc.i_aq_mode;
+    h->mb.b_variable_qp = rc->b_vbv || h->param.rc.i_aq_mode;
 
     if( rc->b_abr )
     {
@@ -718,6 +720,16 @@ void x264_ratecontrol_delete( x264_t *h )
     x264_free( rc );
 }
 
+void x264_ratecontrol_set_estimated_size( x264_t *h, int bits )
+{
+    h->rc->frame_size_estimated = bits;
+}
+
+int x264_ratecontrol_get_estimated_size( x264_t const *h)
+{
+    return  h->rc->frame_size_estimated;
+}
+
 static void accum_p_qp_update( x264_t *h, float qp )
 {
     x264_ratecontrol_t *rc = h->rc;
@@ -851,17 +863,25 @@ double predict_row_size( x264_t *h, int y, int qp )
     return (pred_s + pred_t) / 2;
 }
 
-double predict_row_size_sum( x264_t *h, int y, int qp )
+double row_bits_so_far( x264_t *h, int y )
 {
     int i;
     double bits = 0;
     for( i = 0; i <= y; i++ )
         bits += h->fdec->i_row_bits[i];
+    return bits;
+}
+
+double predict_row_size_sum( x264_t *h, int y, int qp )
+{
+    int i;
+    double bits = row_bits_so_far(h, y);
     for( i = y+1; i < h->sps->i_mb_height; i++ )
         bits += predict_row_size( h, i, qp );
     return bits;
 }
 
+
 void x264_ratecontrol_mb( x264_t *h, int bits )
 {
     x264_ratecontrol_t *rc = h->rc;
@@ -873,7 +893,7 @@ void x264_ratecontrol_mb( x264_t *h, int bits )
     rc->qpa_rc += rc->qpm;
     rc->qpa_aq += h->mb.i_qp;
 
-    if( h->mb.i_mb_x != h->sps->i_mb_width - 1 || !rc->b_vbv || rc->b_2pass )
+    if( h->mb.i_mb_x != h->sps->i_mb_width - 1 || !rc->b_vbv)
         return;
 
     h->fdec->i_row_qp[y] = rc->qpm;
@@ -883,9 +903,13 @@ void x264_ratecontrol_mb( x264_t *h, int bits )
         /* B-frames shouldn't use lower QP than their reference frames */
         if( y < h->sps->i_mb_height-1 )
         {
-            rc->qpm = X264_MAX( rc->qp,
-                      X264_MIN( h->fref0[0]->i_row_qp[y+1],
-                                h->fref1[0]->i_row_qp[y+1] ));
+            int i_estimated;
+            int avg_qp = X264_MAX(h->fref0[0]->i_row_qp[y+1], h->fref1[0]->i_row_qp[y+1])
+                + rc->pb_offset * ((h->fenc->i_type == X264_TYPE_BREF) ? 0.5 : 1);
+            rc->qpm = X264_MIN(X264_MAX( rc->qp, avg_qp), 51); //avg_qp could go higher than 51 due to pb_offset
+            i_estimated = row_bits_so_far(h, y); //FIXME: compute full estimated size
+            if (i_estimated > h->rc->frame_size_planned)
+                x264_frame_size_estimated_set(h, i_estimated);
         }
     }
     else
@@ -901,26 +925,49 @@ void x264_ratecontrol_mb( x264_t *h, int bits )
             int i_qp_max = X264_MIN( prev_row_qp + h->param.rc.i_qp_step, h->param.rc.i_qp_max );
             int i_qp_min = X264_MAX( prev_row_qp - h->param.rc.i_qp_step, h->param.rc.i_qp_min );
             float buffer_left_planned = rc->buffer_fill - rc->frame_size_planned;
+            float rc_tol = 1;
+            float headroom = 0;
+
+           /* Don't modify the row QPs until a sufficent amount of the bits of the frame have been processed, in case a flat */
+           /* area at the top of the frame was measured inaccurately. */
+            if(row_bits_so_far(h,y) < 0.05 * rc->frame_size_planned)
+            {
+                return;
+            }
+ 
+            headroom = buffer_left_planned/rc->buffer_size;
+            if(h->sh.i_type != SLICE_TYPE_I)
+                headroom /= 2;
+            rc_tol += headroom;
 
             if( !rc->b_vbv_min_rate )
                 i_qp_min = X264_MAX( i_qp_min, h->sh.i_qp );
 
             while( rc->qpm < i_qp_max
-                   && (b1 > rc->frame_size_planned * 1.15
+                   && (b1 > rc->frame_size_planned * rc_tol
                     || (rc->buffer_fill - b1 < buffer_left_planned * 0.5)))
             {
                 rc->qpm ++;
                 b1 = predict_row_size_sum( h, y, rc->qpm );
             }
 
+            /* avoid VBV underflow */
+            while( (rc->qpm < h->param.rc.i_qp_max)
+                   && (rc->buffer_fill - b1 < rc->buffer_size * 0.005))
+            {
+                rc->qpm ++;
+                b1 = predict_row_size_sum( h, y, rc->qpm );
+            }
+
             while( rc->qpm > i_qp_min
-                   && buffer_left_planned > rc->buffer_size * 0.4
+                   && ((buffer_left_planned > rc->buffer_size * 0.4) || rc->qpm > h->fdec->i_row_qp[0])
                    && ((b1 < rc->frame_size_planned * 0.8 && rc->qpm <= prev_row_qp)
                      || b1 < (rc->buffer_fill - rc->buffer_size + rc->buffer_rate) * 1.1) )
             {
                 rc->qpm --;
                 b1 = predict_row_size_sum( h, y, rc->qpm );
             }
+            x264_frame_size_estimated_set(h, b1);
         }
     }
 }
@@ -1249,7 +1296,7 @@ static void update_vbv( x264_t *h, int bits )
         return;
 
     rct->buffer_fill_final += rct->buffer_rate - bits;
-    if( rct->buffer_fill_final < 0 && !rct->b_2pass )
+    if( rct->buffer_fill_final < 0 )
         x264_log( h, X264_LOG_WARNING, "VBV underflow (%.0f bits)\n", rct->buffer_fill_final );
     rct->buffer_fill_final = x264_clip3f( rct->buffer_fill_final, 0, rct->buffer_size );
 }
@@ -1269,6 +1316,7 @@ static void update_vbv_plan( x264_t *h )
             double bits = t->rc->frame_size_planned;
             if( !t->b_thread_active )
                 continue;
+            bits  = X264_MAX(bits, x264_frame_size_estimated_get(t));
             rcc->buffer_fill += rcc->buffer_rate - bits;
             rcc->buffer_fill = x264_clip3( rcc->buffer_fill, 0, rcc->buffer_size );
         }
@@ -1405,6 +1453,7 @@ static float rate_estimate_qscale( x264_t *h )
             q += rcc->pb_offset;
 
         rcc->frame_size_planned = predict_size( rcc->pred_b_from_p, q, h->fref1[h->i_ref1-1]->i_satd );
+        x264_frame_size_estimated_set(h, rcc->frame_size_planned);
         rcc->last_satd = 0;
         return qp2qscale(q);
     }
@@ -1425,6 +1474,24 @@ static float rate_estimate_qscale( x264_t *h )
                 double w = x264_clip3f( time*100, 0.0, 1.0 );
                 q *= pow( (double)total_bits / rcc->expected_bits_sum, w );
             }
+            if( rcc->b_vbv )
+            {
+                double expected_size = qscale2bits(&rce, q);
+                double expected_vbv = rcc->buffer_fill + rcc->buffer_rate - expected_size;
+                double expected_fullness =  rce.expected_vbv / rcc->buffer_size;
+                double qmax = q*(2 - expected_fullness);
+                double size_constraint = 1 + expected_fullness;
+                if (expected_fullness < .05)
+                    qmax = lmax;
+                qmax = X264_MIN(qmax, lmax);
+                while( (expected_vbv < rce.expected_vbv/size_constraint) && (q < qmax) )
+                {
+                    q *= 1.05;
+                    expected_size = qscale2bits(&rce, q);
+                    expected_vbv = rcc->buffer_fill + rcc->buffer_rate - expected_size;
+                }
+                rcc->last_satd = x264_rc_analyse_slice( h );
+            }
             q = x264_clip3f( q, lmin, lmax );
         }
         else /* 1pass ABR */
@@ -1509,10 +1576,14 @@ static float rate_estimate_qscale( x264_t *h )
         rcc->last_qscale_for[pict_type] =
         rcc->last_qscale = q;
 
-        if( !rcc->b_2pass && h->fenc->i_frame == 0 )
+        if( !(rcc->b_2pass && !rcc->b_vbv) && h->fenc->i_frame == 0 )
             rcc->last_qscale_for[SLICE_TYPE_P] = q;
 
-        rcc->frame_size_planned = predict_size( &rcc->pred[h->sh.i_type], q, rcc->last_satd );
+        if( rcc->b_2pass && rcc->b_vbv)
+            rcc->frame_size_planned = qscale2bits(&rce, q);
+        else
+            rcc->frame_size_planned = predict_size( &rcc->pred[h->sh.i_type], q, rcc->last_satd );
+        x264_frame_size_estimated_set(h, rcc->frame_size_planned);
         return q;
     }
 }
@@ -1555,6 +1626,134 @@ void x264_thread_sync_ratecontrol( x264_t *cur, x264_t *prev, x264_t *next )
     /* the rest of the variables are either constant or thread-local */
 }
 
+static int find_underflow( x264_t *h, double *fills, int *t0, int *t1, int over )
+{
+    /* find an interval ending on an overflow or underflow (depending on whether
+     * we're adding or removing bits), and starting on the earliest frame that
+     * can influence the buffer fill of that end frame. */
+    x264_ratecontrol_t *rcc = h->rc;
+    const double buffer_min = (over ? .1 : .1) * rcc->buffer_size;
+    const double buffer_max = .9 * rcc->buffer_size;
+    double fill = fills[*t0-1];
+    double parity = over ? 1. : -1.;
+    int i, start=-1, end=-1;
+    for(i=*t0; i<rcc->num_entries; i++)
+    {
+        fill += (rcc->buffer_rate - qscale2bits(&rcc->entry[i], rcc->entry[i].new_qscale)) * parity;
+        fill = x264_clip3f(fill, 0, rcc->buffer_size);
+        fills[i] = fill;
+        if(fill <= buffer_min || i == 0)
+        {
+            if(end >= 0)
+                break;
+            start = i;
+        }
+        else if(fill >= buffer_max && start >= 0)
+            end = i;
+    }
+    *t0 = start;
+    *t1 = end;
+    return start>=0 && end>=0;
+}
+
+static int fix_underflow( x264_t *h, int t0, int t1, double adjustment, double qscale_min, double qscale_max)
+{
+    x264_ratecontrol_t *rcc = h->rc;
+    double qscale_orig, qscale_new;
+    int i;
+    int adjusted = 0;
+    if(t0 > 0)
+        t0++;
+    for(i=t0; i<=t1; i++) {
+        qscale_orig = rcc->entry[i].new_qscale;
+        qscale_orig = x264_clip3f(qscale_orig, qscale_min, qscale_max);
+        qscale_new =  qscale_orig * adjustment;
+        qscale_new = x264_clip3f(qscale_new, qscale_min, qscale_max);
+        rcc->entry[i].new_qscale  = qscale_new;
+        adjusted = adjusted || (qscale_new != qscale_orig);
+    }
+    return adjusted;
+}
+
+static double count_expected_bits( x264_t *h )
+{
+    x264_ratecontrol_t *rcc = h->rc;
+    double expected_bits = 0;
+    int i;
+    for(i=0; i<rcc->num_entries; i++)
+    {
+        ratecontrol_entry_t *rce = &rcc->entry[i];
+        rce->expected_bits = expected_bits;
+        expected_bits += qscale2bits(rce, rce->new_qscale);
+    }
+    return expected_bits;
+}
+
+static void vbv_pass2( x264_t *h )
+{
+    /* foreach interval of buffer_full .. underflow
+     *   uniformly increase the qp of all frames in the interval until either
+     *     buffer is full at some intermediate frame
+     *     or the last frame in the interval no longer underflows
+     * recompute intervals and repeat
+     * then do the converse to put bits back into overflow areas until target size is met */
+
+    x264_ratecontrol_t *rcc = h->rc;
+    double *fills = x264_malloc((rcc->num_entries+1)*sizeof(double));
+    double all_available_bits = h->param.rc.i_bitrate * 1000. * rcc->num_entries / rcc->fps;
+    double expected_bits = 0;
+    double adjustment;
+    double prev_bits = 0;
+    int i, t0, t1;
+    double qscale_min = qp2qscale(h->param.rc.i_qp_min);
+    double qscale_max = qp2qscale(h->param.rc.i_qp_max);
+    int iterations = 0;
+    int adj_min, adj_max;
+
+    fills++;
+
+    //adjust overall stream size
+    do {
+        iterations++;
+        prev_bits = expected_bits;
+
+        if (expected_bits != 0) { //not first iteration
+            adjustment = X264_MAX(X264_MIN(expected_bits / all_available_bits, 0.999), 0.9);
+            fills[-1] = rcc->buffer_size * h->param.rc.f_vbv_buffer_init;
+            t0 = 0;
+            //fix overflows
+            adj_min = 1;
+            while(adj_min && find_underflow(h, fills, &t0, &t1, 1))
+            {
+                adj_min = fix_underflow(h, t0, t1, adjustment, qscale_min, qscale_max);
+                t0 = t1;
+            }
+        }
+
+        fills[-1] = rcc->buffer_size * (1. - h->param.rc.f_vbv_buffer_init);
+        t0 = 0;
+        //fix underflows - should be done after overflow, as we'd better undersize target than underflowing VBV
+        adj_max = 1;
+        while(adj_max && find_underflow(h, fills, &t0, &t1, 0))
+        {
+            adj_max = fix_underflow(h, t0, t1, 1.001, qscale_min, qscale_max);
+        }
+
+        expected_bits = count_expected_bits(h);
+    } while(expected_bits < .995*all_available_bits && expected_bits > prev_bits);
+
+    if (!adj_max)
+        x264_log( h, X264_LOG_WARNING, "vbv-maxrate issue, qpmax or vbv-maxrate too low\n");
+
+    //store expected vbv filling values for tracking when encoding
+    for(i=0; i<rcc->num_entries; i++)
+        rcc->entry[i].expected_vbv = rcc->buffer_size - fills[i];
+
+//    x264_log( h, X264_LOG_INFO, "VBV RC initial iterations: %d \n", iterations);
+
+    x264_free(fills-1);
+}
+
 static int init_pass2( x264_t *h )
 {
     x264_ratecontrol_t *rcc = h->rc;
@@ -1643,7 +1842,6 @@ static int init_pass2( x264_t *h )
         rcc->last_non_b_pict_type = -1;
         rcc->last_accum_p_norm = 1;
         rcc->accum_p_norm = 0;
-        rcc->buffer_fill = rcc->buffer_size * h->param.rc.f_vbv_buffer_init;
 
         /* find qscale */
         for(i=0; i<rcc->num_entries; i++){
@@ -1680,18 +1878,11 @@ static int init_pass2( x264_t *h )
         /* find expected bits */
         for(i=0; i<rcc->num_entries; i++){
             ratecontrol_entry_t *rce = &rcc->entry[i];
-            double bits;
             rce->new_qscale = clip_qscale(h, rce->pict_type, blurred_qscale[i]);
             assert(rce->new_qscale >= 0);
-            bits = qscale2bits(rce, rce->new_qscale);
-
-            rce->expected_bits = expected_bits;
-            expected_bits += bits;
-            update_vbv(h, bits);
-            rcc->buffer_fill = rcc->buffer_fill_final;
+            expected_bits += qscale2bits(rce, rce->new_qscale);
         }
 
-//printf("expected:%llu available:%llu factor:%lf avgQ:%lf\n", (uint64_t)expected_bits, all_available_bits, rate_factor);
         if(expected_bits > all_available_bits) rate_factor -= step;
     }
 
@@ -1699,6 +1890,10 @@ static int init_pass2( x264_t *h )
     if(filter_size > 1)
         x264_free(blurred_qscale);
 
+    if(rcc->b_vbv)
+        vbv_pass2(h);
+    expected_bits = count_expected_bits(h);
+
     if(fabs(expected_bits/all_available_bits - 1.0) > 0.01)
     {
         double avgq = 0;
@@ -1706,7 +1901,8 @@ static int init_pass2( x264_t *h )
             avgq += rcc->entry[i].new_qscale;
         avgq = qscale2qp(avgq / rcc->num_entries);
 
-        x264_log(h, X264_LOG_WARNING, "Error: 2pass curve failed to converge\n");
+        if ((expected_bits > all_available_bits) || (!rcc->b_vbv))
+            x264_log(h, X264_LOG_WARNING, "Error: 2pass curve failed to converge\n");
         x264_log(h, X264_LOG_WARNING, "target: %.2f kbit/s, expected: %.2f kbit/s, avg QP: %.4f\n",
                  (float)h->param.rc.i_bitrate,
                  expected_bits * rcc->fps / (rcc->num_entries * 1000.),
@@ -1725,7 +1921,7 @@ static int init_pass2( x264_t *h )
             else
                 x264_log(h, X264_LOG_WARNING, "try increasing target bitrate\n");
         }
-        else
+        else if(!(rcc->b_2pass && rcc->b_vbv))
             x264_log(h, X264_LOG_WARNING, "internal error\n");
     }
 
diff --git a/encoder/ratecontrol.h b/encoder/ratecontrol.h
old mode 100644
new mode 100755
index c55fd54..38f5f23
--- a/encoder/ratecontrol.h
+++ b/encoder/ratecontrol.h
@@ -35,6 +35,8 @@ int  x264_ratecontrol_qp( x264_t * );
 void x264_ratecontrol_end( x264_t *, int bits );
 void x264_ratecontrol_summary( x264_t * );
 void x264_adaptive_quant( x264_t * );
+void x264_ratecontrol_set_estimated_size( x264_t *, int bits );
+int  x264_ratecontrol_get_estimated_size( x264_t const *);
 
 #endif