sc_factor support for encoding.

Here's the description I'll add to the mplayer man page:
---
Causes frames with higher quantizers to be more likely to trigger a
scene change detection and make libavcodec use an I-frame (default: 1).
1-16 is a sane range.
Values between 2 and 6 may yield increasing PSNR (up to approximately
0.04 dB) and better placement of I-frames in high-motion scenes.
Higher values than 6 may give very slightly better PSNR (approximately
0.01 dB more than sc_factor=6), but noticably worse visual quality.
---

Original idea from Michael; patch by me.

Originally committed as revision 4883 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -1989,6 +1989,13 @@ typedef struct AVCodecContext {
      * - decoding: unused
      */
     int cutoff;
+
+    /**
+     * multiplied by qscale for each frame and added to scene_change_score
+     * - encoding: set by user.
+     * - decoding: unused
+     */
+    int scenechange_factor;
 } AVCodecContext;
 
 /**

libavcodec/motion_est.c

@@ -1179,7 +1179,7 @@ void ff_estimate_p_frame_motion(MpegEncContext * s,
             if (vard <= 64<<8 || vard < varc) { //FIXME
                 c->scene_change_score+= ff_sqrt(vard) - ff_sqrt(varc);
             }else{
-                c->scene_change_score+= s->qscale;
+                c->scene_change_score+= s->qscale * s->avctx->scenechange_factor;
             }
             return;
         }
@@ -1270,7 +1270,7 @@ void ff_estimate_p_frame_motion(MpegEncContext * s,
         if (vard <= 64<<8 || vard < varc)
             c->scene_change_score+= ff_sqrt(vard) - ff_sqrt(varc);
         else
-            c->scene_change_score+= s->qscale;
+            c->scene_change_score+= s->qscale * s->avctx->scenechange_factor;
 
         if(mb_type == CANDIDATE_MB_TYPE_INTER){
             c->sub_motion_search(s, &mx, &my, dmin, 0, 0, 0, 16);
@@ -1291,7 +1291,7 @@ void ff_estimate_p_frame_motion(MpegEncContext * s,
         if (vard <= 64<<8 || vard < varc)
             c->scene_change_score+= ff_sqrt(vard) - ff_sqrt(varc);
         else
-            c->scene_change_score+= s->qscale;
+            c->scene_change_score+= s->qscale * s->avctx->scenechange_factor;
 
         if (vard*2 + 200*256 > varc)
             mb_type|= CANDIDATE_MB_TYPE_INTRA;
@@ -1397,7 +1397,7 @@ void ff_estimate_p_frame_motion(MpegEncContext * s,
         if (vard <= 64<<8 || vard < varc) { //FIXME
             c->scene_change_score+= ff_sqrt(vard) - ff_sqrt(varc);
         }else{
-            c->scene_change_score+= s->qscale;
+            c->scene_change_score+= s->qscale * s->avctx->scenechange_factor;
         }
     }
 
@@ -1843,7 +1843,7 @@ void ff_estimate_b_frame_motion(MpegEncContext * s,
 /*            if (vard <= 64<<8 || vard < varc) {
                 c->scene_change_score+= ff_sqrt(vard) - ff_sqrt(varc);
             }else{
-                c->scene_change_score+= s->qscale;
+                c->scene_change_score+= s->qscale * s->avctx->scenechange_factor;
             }*/
             return;
         }

libavcodec/utils.c

@@ -734,6 +734,7 @@ static AVOption options[]={
 {"partp4x4", NULL, 0, FF_OPT_TYPE_CONST, X264_PART_P4X4, INT_MIN, INT_MAX, V|E, "partitions"},
 {"partp8x8", NULL, 0, FF_OPT_TYPE_CONST, X264_PART_P8X8, INT_MIN, INT_MAX, V|E, "partitions"},
 {"partb8x8", NULL, 0, FF_OPT_TYPE_CONST, X264_PART_B8X8, INT_MIN, INT_MAX, V|E, "partitions"},
+{"sc_factor", NULL, OFFSET(scenechange_factor), FF_OPT_TYPE_INT, 6, 0, INT_MAX, V|E},
 {NULL},
 };