Diff of /xvidcore/src/plugins/plugin_2pass2.c

-revision 1.1.2.32, Sun Dec 21 12:41:48 2003 UTC
+revision 1.5, Sat Jun 12 13:53:19 2004 UTC
 Line 35
  /* forces second pass not to be bigger than first */
  #undef PASS_SMALLER
+ /* automatically alters overflow controls (strength and improvement/degradation)
+         to fight most common problems without user's knowladge */
+ #define SMART_OVERFLOW_SETTING
  #include <stdio.h>
  #include <math.h>
  #include <limits.h>
-Line 130
+Line 134
          /*----------------------------------
           * Zones statistical data
-          *
-          * ToDo: Fix zones, current
-          *       implementation is buggy
           *--------------------------------*/
-         /* Average weight of the zones */
-         double avg_weight;
          /* Total length used by XVID_ZONE_QUANT zones */
          uint64_t tot_quant;
          uint64_t tot_quant_invariant;
+         /* Holds the total amount of frame bytes, zone weighted (only scalable
+          * part of frame bytes) */
+         uint64_t tot_weighted;
          /*----------------------------------
           * Advanced settings helper ratios
           *--------------------------------*/
-Line 175
+Line 177
          twopass_stat_t * stats;
          /*----------------------------------
-          * Histerysis helpers
+          * Hysteresis helpers
           *--------------------------------*/
          /* This field holds the int2float conversion errors of each quant per
-Line 210
+Line 212
           * ToDo: description */
          double fq_error;
+         int min_quant; /* internal minimal quant, prevents wrong quants from being used */
          /*----------------------------------
           * Debug
           *--------------------------------*/
-Line 266
+Line 270
  static void first_pass_stats_prepare_data(rc_2pass2_t * rc);
  static void first_pass_scale_curve_internal(rc_2pass2_t *rc);
  static void scaled_curve_apply_advanced_parameters(rc_2pass2_t * rc);
+ static  int check_curve_for_vbv_compliancy(rc_2pass2_t * rc, const float fps);
+ static  int scale_curve_for_vbv_compliancy(rc_2pass2_t * rc, const float fps);
  #if 0
  static void stats_print(rc_2pass2_t * rc);
  #endif
-Line 284
+Line 290
          if (rc == NULL)
                  return XVID_ERR_MEMORY;
-         rc->param = *param;
+         /* v1.0.x */
+         rc->param.version = param->version;
+         rc->param.bitrate = param->bitrate;
+         rc->param.filename = param->filename;
+         rc->param.keyframe_boost = param->keyframe_boost;
+         rc->param.curve_compression_high = param->curve_compression_high;
+         rc->param.curve_compression_low = param->curve_compression_low;
+         rc->param.overflow_control_strength = param->overflow_control_strength;
+         rc->param.max_overflow_improvement = param->max_overflow_improvement;
+         rc->param.max_overflow_degradation = param->max_overflow_degradation;
+         rc->param.kfreduction = param->kfreduction;
+         rc->param.kfthreshold = param->kfthreshold;
+         rc->param.container_frame_overhead = param->container_frame_overhead;
+         if (XVID_VERSION_MINOR(param->version) >= 1) {
+                 rc->param.vbv_size = param->vbv_size;
+                 rc->param.vbv_initial = param->vbv_initial;
+                 rc->param.vbv_maxrate = param->vbv_maxrate;
+                 rc->param.vbv_peakrate = param->vbv_peakrate;
+         }else{
+                 rc->param.vbv_size =
+                 rc->param.vbv_initial =
+                 rc->param.vbv_maxrate =
+                 rc->param.vbv_peakrate = 0;
+         }
          /* Initialize all defaults */
  #define _INIT(a, b) if((a) <= 0) (a) = (b)
-Line 313
+Line 343
          for (i=0; i<3; i++) rc->last_quant[i] = 0;
          rc->fq_error = 0;
+         rc->min_quant = 1;
          /* Count frames (and intra frames) in the stats file, store the result into
           * the rc structure */
-Line 375
+Line 406
          if(rc->param.container_frame_overhead)
                  DPRINTF(XVID_DEBUG_RC, "[xvid rc] -- New target filesize after container compensation: %lld\n", rc->target);
+         /* When bitrate is not given it means it has been scaled by an external
+          * application */
+         if (rc->param.bitrate) {
+                 /* Apply zone settings
+                  * - set rc->tot_quant which represents the total num of bytes spent in
+                  *   fixed quant zones
+                  * - set rc->tot_weighted which represents the total amount of bytes
+                  *   spent in normal or weighted zones in first pass (normal zones can
+                  *   be considered weight=1)
+                  * - set rc->tot_quant_invariant which represents the total num of bytes
+                  *   spent in fixed quant zones for headers */
+                 zone_process(rc, create);
+         } else {
+                 /* External scaling -- zones are ignored */
+                 for (i=0;i<rc->num_frames;i++) {
+                         rc->stats[i].zone_mode = XVID_ZONE_WEIGHT;
+                         rc->stats[i].weight = 1.0;
+                 }
+                 rc->tot_quant = 0;
+         }
          /* Gathers some information about first pass stats:
           *  - finds the minimum frame length for each frame type during 1st pass.
           *     rc->min_size[]
-Line 390
+Line 442
           */
          first_pass_stats_prepare_data(rc);
-         /* When bitrate is not given it means it has been scaled by an external
+         /* If we have a user bitrate, it means it's an internal curve scaling */
-          * application */
          if (rc->param.bitrate) {
-                 /* Apply zone settings
-                  * - set rc->tot_quant which represents the total num of bytes spent in
-                  *   fixed quant zones
-                  * - set rc->tot_quant_invariant which represents the total num of bytes spent
-                  *   in fixed quant zones for headers */
-                 zone_process(rc, create);
                  /* Perform internal curve scaling */
                  first_pass_scale_curve_internal(rc);
-         } else {
-                 /* External scaling -- zones are ignored */
-                 for (i=0;i<rc->num_frames;i++) {
-                         rc->stats[i].zone_mode = XVID_ZONE_WEIGHT;
-                         rc->stats[i].weight = 1.0;
-                 }
-                 rc->avg_weight = 1.0;
-                 rc->tot_quant = 0;
          }
          /* Apply advanced curve options, and compute some parameters in order to
           * shape the curve in the BEFORE/AFTER pair of functions */
          scaled_curve_apply_advanced_parameters(rc);
+         /* Check curve for VBV compliancy and rescale if necessary */
+ #ifdef VBV_FORCE
+         if (rc->param.vbv_size==0) {
+                 rc->param.vbv_size      =  3145728;
+                 rc->param.vbv_initial   =  2359296;
+                 rc->param.vbv_maxrate  =  4000000;
+                 rc->param.vbv_peakrate = 10000000;
+         }
+ #endif
+         /* vbv_size==0 switches VBV check off */
+         if (rc->param.vbv_size > 0)  {
+                 const double fps = (double)create->fbase/(double)create->fincr;
+                 int status = check_curve_for_vbv_compliancy(rc, fps);
+                 if (status) {
+                         DPRINTF(XVID_DEBUG_RC, "[xvid rc] Underflow detected - Scaling Curve for compliancy.\n");
+                 }
+                 status = scale_curve_for_vbv_compliancy(rc, fps);
+                 if (status == 0) {
+                         DPRINTF(XVID_DEBUG_RC, "[xvid rc] VBV compliant curve scaling done.\n");
+                 } else {
+                         DPRINTF(XVID_DEBUG_RC, "[xvid rc] VBV compliant curve scaling impossible.\n");
+                 }
+         }
          *handle = rc;
          return(0);
  }
-Line 457
+Line 521
          if (data->quant > 0)
                  return(0);
-         /* Second case: insufficent stats data */
+         /* Second case: insufficent stats data
+          * We can't guess much what we should do, let core decide all alone */
          if (data->frame_num >= rc->num_frames) {
                  DPRINTF(XVID_DEBUG_RC,"[xvid rc] -- stats file too short (now processing frame %d)",
                          data->frame_num);
                  return(0);
          }
-         /* Third case: We are in a Quant zone */
+         /* Third case: We are in a Quant zone
+          * Quant zones must just ensure we use the same settings as first pass
+          * So set the quantizer and the type */
          if (s->zone_mode == XVID_ZONE_QUANT) {
+                 /* Quant stuff */
                  rc->fq_error += s->weight;
                  data->quant = (int)rc->fq_error;
                  rc->fq_error -= data->quant;
+                 /* The type stuff */
+                 data->type = s->type;
+                 /* The only required data for AFTER step is this one for the overflow
+                  * control */
                  s->desired_length = s->length;
                  return(0);
-Line 604
+Line 677
  #ifdef PASS_SMALLER
          if (dbytes > s->length) {
                  dbytes = s->length;
-         } else
+         }
  #endif
+         /* Prevent stupid desired sizes under logical values */
                  if (dbytes < rc->min_length[s->type-1]) {
                  dbytes = rc->min_length[s->type-1];
-         } else if (dbytes > rc->max_length) {
-                 /* ToDo: this condition is always wrong as max_length == maximum frame
-                  * length of first pass, so the first condition already caps the frame
-                  * size... */
-                 capped_to_max_framesize = 1;
-                 dbytes = rc->max_length;
-                 DPRINTF(XVID_DEBUG_RC,"[xvid rc] -- frame:%d Capped to maximum frame size\n",
-                                 data->frame_num);
          }
          /*------------------------------------------------------------------------
-Line 700
+Line 767
                  data->quant = data->max_quant[s->type-1];
          }
+         if (data->quant < rc->min_quant) data->quant = rc->min_quant;
          /* To avoid big quality jumps from frame to frame, we apply a "security"
           * rule that makes |last_quant - new_quant| <= 2. This rule only applies
           * to predicted frames (P and B) */
-Line 975
+Line 1044
  /* pre-process the statistics data
   * - for each type, count, tot_length, min_length, max_length
-  * - set keyframes_locations */
+  * - set keyframes_locations, tot_prescaled */
  static void
  first_pass_stats_prepare_data(rc_2pass2_t * rc)
  {
-Line 992
+Line 1061
          }
          rc->max_length = INT_MIN;
+         rc->tot_weighted = 0;
          /* Loop through all frames and find/compute all the stuff this function
           * is supposed to do */
-Line 1001
+Line 1071
                  rc->count[s->type-1]++;
                  rc->tot_length[s->type-1] += s->length;
                  rc->tot_invariant[s->type-1] += s->invariant;
+                 if (s->zone_mode != XVID_ZONE_QUANT)
+                         rc->tot_weighted += (int)(s->weight*(s->length - s->invariant));
                  if (s->length < rc->min_length[s->type-1]) {
                          rc->min_length[s->type-1] = s->length;
-Line 1035
+Line 1107
          int i,j;
          int n = 0;
-         rc->avg_weight = 0.0;
          rc->tot_quant = 0;
          rc->tot_quant_invariant = 0;
-Line 1044
+Line 1115
                          rc->stats[j].zone_mode = XVID_ZONE_WEIGHT;
                          rc->stats[j].weight = 1.0;
                  }
-                 rc->avg_weight += rc->num_frames * 1.0;
                  n += rc->num_frames;
          }
-Line 1053
+Line 1123
                  int next = (i+1<create->num_zones) ? create->zones[i+1].frame : rc->num_frames;
+                 /* Zero weight make no sense */
+                 if (create->zones[i].increment == 0) create->zones[i].increment = 1;
+                 /* And obviously an undetermined infinite makes even less sense */
+                 if (create->zones[i].base == 0) create->zones[i].base = 1;
                  if (i==0 && create->zones[i].frame > 0) {
                          for (j = 0; j < create->zones[i].frame && j < rc->num_frames; j++) {
                                  rc->stats[j].zone_mode = XVID_ZONE_WEIGHT;
                                  rc->stats[j].weight = 1.0;
                          }
-                         rc->avg_weight += create->zones[i].frame * 1.0;
                          n += create->zones[i].frame;
                  }
-Line 1068
+Line 1142
                                  rc->stats[j].weight = (double)create->zones[i].increment / (double)create->zones[i].base;
                          }
                          next -= create->zones[i].frame;
-                         rc->avg_weight += (double)(next * create->zones[i].increment) / (double)create->zones[i].base;
                          n += next;
                  } else{  /* XVID_ZONE_QUANT */
                          for (j = create->zones[i].frame; j < next && j < rc->num_frames; j++ ) {
-Line 1079
+Line 1152
                          }
                  }
          }
-         rc->avg_weight = n>0 ? rc->avg_weight/n : 1.0;
-         DPRINTF(XVID_DEBUG_RC, "[xvid rc] -- center_weight:%f (for %d frames)  fixed_bytes:%d\n", rc->avg_weight, n, rc->tot_quant);
  }
-Line 1090
+Line 1160
  first_pass_scale_curve_internal(rc_2pass2_t *rc)
  {
          int64_t target;
-         int64_t pass1_length;
          int64_t total_invariant;
          double scaler;
          int i, num_MBs;
-Line 1109
+Line 1178
          target  = rc->target;
          target -= rc->tot_quant;
-         /* Do the same for the first pass data */
-         pass1_length  = rc->tot_length[XVID_TYPE_IVOP-1];
-         pass1_length += rc->tot_length[XVID_TYPE_PVOP-1];
-         pass1_length += rc->tot_length[XVID_TYPE_BVOP-1];
-         pass1_length -= rc->tot_quant;
          /* Let's compute a linear scaler in order to perform curve scaling */
-         scaler = (double)(target - total_invariant) / (double)(pass1_length - total_invariant);
+         scaler = (double)(target - total_invariant) / (double)(rc->tot_weighted);
- #ifdef PASS_SMALLER
+ #ifdef SMART_OVERFLOW_SETTING
-         if ((target - total_invariant) <= 0 ||
+         if (scaler > 0.9) {
-                 (pass1_length - total_invariant) <= 0 ||
+                 rc->param.max_overflow_degradation *= 5;
-                 target >= pass1_length) {
+                 rc->param.max_overflow_improvement *= 5;
-                 DPRINTF(XVID_DEBUG_RC, "[xvid rc] -- WARNING: Undersize detected before correction\n");
+                 rc->param.overflow_control_strength *= 3;
-                 scaler = 1.0;
+         } else if (scaler > 0.6) {
+                 rc->param.max_overflow_degradation *= 2;
+                 rc->param.max_overflow_improvement *= 2;
+                 rc->param.overflow_control_strength *= 2;
+         } else {
+                 rc->min_quant = 2;
          }
  #endif
          /* Compute min frame lengths (for each frame type) according to the number
           * of MBs. We sum all block type counters of frame 0, this gives us the
           * number of MBs.
-Line 1167
+Line 1236
                  }
                  /* Compute the scaled length -- only non invariant data length is scaled */
-                 len = s->invariant + (int)((double)(s->length-s->invariant) * scaler * s->weight / rc->avg_weight);
+                 len = s->invariant + (int)((double)(s->length-s->invariant) * scaler * s->weight);
                  /* Compare with the computed minimum */
                  if (len < rc->min_length[s->type-1]) {
-Line 1179
+Line 1248
                           * total counters, as we prepare a second pass for 'regular'
                           * frames */
                          target -= s->scaled_length;
-                         pass1_length -= s->length;
                  } else {
                          /* Do nothing for now, we'll scale this later */
                          s->scaled_length = 0;
-Line 1190
+Line 1258
           * total counters. Now, it's possible to scale the 'regular' frames. */
          /* Scaling factor for 'regular' frames */
-         scaler = (double)(target - total_invariant) / (double)(pass1_length - total_invariant);
+         scaler = (double)(target - total_invariant) / (double)(rc->tot_weighted);
- #ifdef PASS_SMALLER
-         /* Detect undersizing */
-         if (target <= 0 || pass1_length <= 0 || target >= pass1_length) {
-                 DPRINTF(XVID_DEBUG_RC, "[xvid rc] -- WARNING: Undersize detected after correction\n");
-                 scaler = 1.0;
-         }
- #endif
          /* Do another pass with the new scaler */
          for (i=0; i<rc->num_frames; i++) {
-Line 1206
+Line 1266
                  /* Ignore frame with forced frame sizes */
                  if (s->scaled_length == 0)
-                         s->scaled_length = s->invariant + (int)((double)(s->length-s->invariant) * scaler * s->weight / rc->avg_weight);
+                         s->scaled_length = s->invariant + (int)((double)(s->length-s->invariant) * scaler * s->weight);
          }
          /* Job done */
-Line 1374
+Line 1434
  }
  /*****************************************************************************
+  * VBV compliancy check and scale
+  * MPEG-4 standard specifies certain restrictions for bitrate/framesize in VBR
+  * to enable playback on devices with limited readspeed and memory (and which
+  * aren't...)
+  *
+  * DivX profiles have 2 criteria: VBV as in MPEG standard
+  *                                a limit on peak bitrate for any 3 seconds
+  *
+  * But if VBV is fulfilled, peakrate is automatically fulfilled in any profile
+  * define so far, so we check for it (for completeness) but correct only VBV
+  *
+  *****************************************************************************/
+ #define VBV_COMPLIANT 0
+ #define VBV_UNDERFLOW 1 /* video buffer runs empty */
+ #define VBV_OVERFLOW 2  /* doesn't exist for VBR encoding */
+ #define VBV_PEAKRATE 4  /* peak bitrate (within 3s) violated */
+ static int
+ check_curve_for_vbv_compliancy(rc_2pass2_t * rc, const float fps)
+ {
+         /* We do all calculations in float, for higher accuracy,
+          * and in bytes for convenience.
+          *
+          * typical values from DivX Home Theater profile:
+          *  vbv_size= 384*1024 (384kB)
+          *  vbv_initial= 288*1024 (75% fill)
+          *  maxrate= 4000000 (4MBps)
+          *  peakrate= 10000000 (10MBps)
+          *
+          *  PAL: offset3s = 75 (3 seconds of 25fps)
+          *  NTSC: offset3s = 90 (3 seconds of 29.97fps) or 72 (3 seconds of 23.976fps)
+          */
+         const float vbv_size = (float)rc->param.vbv_size/8.f;
+         float vbvfill = (float)rc->param.vbv_initial/8.f;
+         float vbvmin;
+         const float maxrate = (float)rc->param.vbv_maxrate;
+         const float peakrate = (float)rc->param.vbv_peakrate;
+         const float r0 = (int)(maxrate/fps+0.5)/8.f;
+         int bytes3s = 0;
+         int offset3s = (int)(3.f*fps+0.5);
+         int i;
+         /* 1Gbit should be enough to inuitialize the vbvmin
+          *      an arbitrary high value */
+         vbvmin = 1000*1000*1000;
+         for (i=0; i<rc->num_frames; i++) {
+                 /* DivX 3s peak bitrate check  */
+                 bytes3s += rc->stats[i].scaled_length;
+                 if (i>=offset3s)
+                         bytes3s -= rc->stats[i-offset3s].scaled_length;
+                 if (8.f*bytes3s > 3*peakrate)
+                         return(VBV_PEAKRATE);
+                 /* update vbv fill level */
+                 vbvfill += r0 - rc->stats[i].scaled_length;
+                 /* this check is _NOT_ an "overflow"! only reading from disk stops then */
+                 if (vbvfill > vbv_size)
+                         vbvfill = vbv_size;
+                 /* but THIS would be an underflow. report it! */
+                 if (vbvfill < 0)
+                         return(VBV_UNDERFLOW);
+                 /* Store the minimum buffer filling */
+                 if (vbvfill < vbvmin)
+                         vbvmin = vbvfill;
+         }
+         DPRINTF(XVID_DEBUG_RC, "[xvid rc] Minimum buffer fill: %f bytes\n", vbvmin);
+         return(VBV_COMPLIANT);
+ }
+ static int
+ scale_curve_for_vbv_compliancy(rc_2pass2_t * rc, const float fps)
+ {
+         /* correct any VBV violations. Peak bitrate violations disappears
+          * by this automatically
+          *
+          * This implementation follows
+          *
+          * Westerink, Rajagopalan, Gonzales "Two-pass MPEG-2 variable-bitrate encoding"
+          * IBM J. RES. DEVELOP. VOL 43, No. 4, July 1999, p.471--488
+          *
+          * Thanks, guys! This paper rocks!!! */
+         /* For each scene of len N, we have to check up to N^2 possible buffer fills.
+          * This works well with MPEG-2 where N==12 or so, but for MPEG-4 it's a
+          * little slow...
+          *
+          * TODO: Better control on VBVfill between scenes */
+         const float vbv_size = (float)rc->param.vbv_size/8.f;
+         const float vbv_initial = (float)rc->param.vbv_initial/8.f;
+         const float maxrate = 0.9*rc->param.vbv_maxrate;
+         const float vbv_low = 0.10f*vbv_size;
+         const float r0 = (int)(maxrate/fps+0.5)/8.f;
+         int i,k,l,n,violation = 0;
+         float *scenefactor;
+         int *scenestart;
+         int *scenelength;
+         /* first step: determine how many "scenes" there are and store their
+          * boundaries we could get all this from existing keyframe_positions,
+          * somehow, but there we don't have a min_scenelength, and it's no big
+          * deal to get it again. */
+         const int min_scenelength = (int)(fps+0.5);
+         int num_scenes = 0;
+         int last_scene = -999;
+         for (i=0; i<rc->num_frames; i++) {
+                 if ((rc->stats[i].type == XVID_TYPE_IVOP) && (i-last_scene>min_scenelength)) {
+                         last_scene = i;
+                         num_scenes++;
+                 }
+         }
+         scenefactor = (float*)malloc(num_scenes*sizeof(float));
+         scenestart = (int*)malloc(num_scenes*sizeof(int));
+         scenelength = (int*)malloc(num_scenes*sizeof(int));
+         if ((!scenefactor) || (!scenestart) || (!scenelength) ) {
+                 free(scenefactor);
+                 free(scenestart);
+                 free(scenelength);
+                 /* remember: free(0) is valid and does exactly nothing. */
+                 return(-1);
+         }
+         /* count again and safe the length/position */
+         num_scenes = 0;
+         last_scene = -999;
+         for (i=0; i<rc->num_frames; i++) {
+                 if ((rc->stats[i].type == XVID_TYPE_IVOP) && (i-last_scene>min_scenelength)) {
+                         if (num_scenes>0) {
+                                 scenelength[num_scenes-1]=i-last_scene;
+                         }
+                         scenestart[num_scenes]=i;
+                         num_scenes++;
+                         last_scene = i;
+                 }
+         }
+         scenelength[num_scenes-1]=i-last_scene;
+         /* second step: check for each scene, how much we can scale its frames up or
+          * down such that the VBV restriction is just fulfilled */
+ #define R(k,n) (((n)+1-(k))*r0)     /* how much enters the buffer between frame k and n */
+         for (l=0; l<num_scenes;l++) {
+                 const int start = scenestart[l];
+                 const int length = scenelength[l];
+                 twopass_stat_t * frames = &rc->stats[start];
+                 float S0n,Skn;
+                 float f,minf = 99999.f;
+                 S0n=0.;
+                 for (n=0;n<=length-1;n++) {
+                         S0n += frames[n].scaled_length;
+                         k = 0;
+                         Skn = S0n;
+                         f = (R(k,n-1) + (vbv_initial - vbv_low)) / Skn;
+                         if (f < minf)
+                                 minf = f;
+                         for (k=1;k<=n;k++) {
+                                 Skn -= frames[k].scaled_length;
+                                 f = (R(k,n-1) + (vbv_size - vbv_low)) / Skn;
+                                 if (f < minf)
+                                         minf = f;
+                         }
+                 }
+                 /* special case: at the end, fill buffer up to vbv_initial again
+                  *
+                  * TODO: Allow other values for buffer fill between scenes
+                  * e.g. if n=N is smallest f-value, then check for better value */
+                 n=length;
+                 k=0;
+                 Skn = S0n;
+                 f = R(k,n-1)/Skn;
+                 if (f < minf)
+                         minf = f;
+                 for (k=1;k<=n-1;k++) {
+                         Skn -= frames[k].scaled_length;
+                         f = (R(k,n-1) + (vbv_initial - vbv_low)) / Skn;
+                         if (f < minf)
+                                 minf = f;
+                 }
+                 DPRINTF(XVID_DEBUG_RC, "[xvid rc] Scene %d (Frames %d-%d): VBVfactor %f\n",
+                                 l, start, start+length-1 , minf);
+                 scenefactor[l] = minf;
+         }
+ #undef R
+         /* last step: now we know of any scene how much it can be scaled up or down
+          * without violating VBV. Next, distribute bits from the evil scenes to the
+          * good ones */
+         do {
+                 float S_red = 0.f;    /* how much to redistribute */
+                 float S_elig = 0.f;   /* sum of bit for those scenes you can still swallow something*/
+                 float f_red;
+                 int l;
+                 /* check how much is wrong */
+                 for (l=0;l<num_scenes;l++) {
+                         const int start = scenestart[l];
+                         const int length = scenelength[l];
+                         twopass_stat_t * frames = &rc->stats[start];
+                         /* exactly 1 means "don't touch this anymore!" */
+                         if (scenefactor[l] == 1.)
+                                 continue;
+                         /* within limits */
+                         if (scenefactor[l] > 1.) {
+                                 for (n= 0; n < length; n++)
+                                         S_elig += frames[n].scaled_length;
+                         } else {
+                                 /* underflowing segment */
+                                 for (n= 0; n < length; n++) {
+                                         float newbytes = (float)frames[n].scaled_length * scenefactor[l];
+                                         S_red += (float)frames[n].scaled_length - (float)newbytes;
+                                         frames[n].scaled_length =(int)newbytes;
+                                 }
+                                 scenefactor[l] = 1.f;
+                         }
+                 }
+                 /* no more underflows */
+                 if (S_red < 1.f)
+                         break;
+                 if (S_elig < 1.f) {
+                         DPRINTF(XVID_DEBUG_RC, "[xvid rc] Everything underflowing.\n");
+                         free(scenefactor);
+                         free(scenestart);
+                         free(scenelength);
+                         return(-2);
+                 }
+                 f_red = (1.f + S_red/S_elig);
+                 DPRINTF(XVID_DEBUG_RC, "[xvid rc] Moving %.0f kB to avoid buffer underflow, correction factor: %.5f\n",
+                                 S_red/1024.f, f_red);
+                 violation=0;
+                 /* scale remaining scenes up to meet total size */
+                 for (l=0; l<num_scenes; l++) {
+                         const int start = scenestart[l];
+                         const int length = scenelength[l];
+                         twopass_stat_t * frames = &rc->stats[start];
+                         if (scenefactor[l] == 1.)
+                                 continue;
+                         /* there shouldn't be any segments with factor<1 left, so all the rest is >1 */
+                         for (n= 0; n < length; n++) {
+                                 frames[n].scaled_length = (int)(frames[n].scaled_length * f_red + 0.5);
+                         }
+                         scenefactor[l] /= f_red;
+                         if (scenefactor[l] < 1.f)
+                                 violation=1;
+                 }
+         } while (violation);
+         free(scenefactor);
+         free(scenestart);
+         free(scenelength);
+         return(0);
+ }
+ /*****************************************************************************
   * Still more low level stuff (nothing to do with stats treatment)
   ****************************************************************************/

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