Open-GOP support
authorLamont Alston <wewk584@gmail.com>
Wed, 16 Jun 2010 17:05:17 +0000 (10:05 -0700)
committerJason Garrett-Glaser <darkshikari@gmail.com>
Fri, 25 Jun 2010 07:42:52 +0000 (00:42 -0700)
Allows B-frames immediately prior to keyframes (in display order).
This helps reduce keyframe popping and improve compression with short keyframe intervals.
Due to a staggering display of braindamage in the Blu-ray spec, two open-GOP modes are available.
The two modes calculate keyframe interval differently: one based on coded distance and one based on display distance.
The latter is superior compression-wise, but for no comprehensible reason, Blu-ray requires the former if open-GOP is used.

common/common.c
common/common.h
encoder/encoder.c
encoder/ratecontrol.c
encoder/slicetype.c
x264.c
x264.h

index 2a9c76e..4612bb5 100644 (file)
@@ -676,6 +676,15 @@ int x264_param_parse( x264_param_t *p, const char *name, const char *value )
             p->i_bframe_pyramid = atoi(value);
         }
     }
+    OPT("open-gop")
+    {
+        b_error |= parse_enum( value, x264_open_gop_names, &p->i_open_gop );
+        if( b_error )
+        {
+            b_error = 0;
+            p->i_open_gop = atoi(value);
+        }
+    }
     OPT("nf")
         p->b_deblocking_filter = !atobool(value);
     OPT2("filter", "deblock")
@@ -1190,9 +1199,9 @@ char *x264_param2string( x264_param_t *p, int b_res )
     s += sprintf( s, " bframes=%d", p->i_bframe );
     if( p->i_bframe )
     {
-        s += sprintf( s, " b_pyramid=%d b_adapt=%d b_bias=%d direct=%d weightb=%d",
+        s += sprintf( s, " b_pyramid=%d b_adapt=%d b_bias=%d direct=%d weightb=%d open_gop=%d",
                       p->i_bframe_pyramid, p->i_bframe_adaptive, p->i_bframe_bias,
-                      p->analyse.i_direct_mv_pred, p->analyse.b_weighted_bipred );
+                      p->analyse.i_direct_mv_pred, p->analyse.b_weighted_bipred, p->i_open_gop );
     }
     s += sprintf( s, " weightp=%d", p->analyse.i_weighted_pred > 0 ? p->analyse.i_weighted_pred : 0 );
 
index 60899fe..dfa1121 100644 (file)
@@ -471,7 +471,11 @@ struct x264_t
         /* frames used for reference + sentinels */
         x264_frame_t *reference[16+2];
 
-        int i_last_keyframe; /* Frame number of the last keyframe */
+        int i_last_keyframe;       /* Frame number of the last keyframe */
+        int i_last_idr;            /* Frame number of the last IDR (not RP)*/
+        int i_poc_last_open_gop;   /* Poc of the I frame of the last open-gop. The value
+                                    * is only assigned during the period between that
+                                    * I frame and the next P or I frame, else -1 */
 
         int i_input;    /* Number of input frames already accepted */
 
index 283783d..aa84192 100644 (file)
@@ -573,12 +573,10 @@ static int x264_validate_parameters( x264_t *h )
         x264_log( h, X264_LOG_WARNING, "subme=0 + direct=temporal is not supported\n" );
         h->param.analyse.i_direct_mv_pred = X264_DIRECT_PRED_SPATIAL;
     }
-    h->param.i_bframe = x264_clip3( h->param.i_bframe, 0, X264_BFRAME_MAX );
+    h->param.i_bframe = x264_clip3( h->param.i_bframe, 0, X264_MIN( X264_BFRAME_MAX, h->param.i_keyint_max-1 ) );
+    h->param.i_open_gop = x264_clip3( h->param.i_open_gop, X264_OPEN_GOP_NONE, X264_OPEN_GOP_CODED_ORDER );
     if( h->param.i_keyint_max == 1 )
-    {
-        h->param.i_bframe = 0;
         h->param.b_intra_refresh = 0;
-    }
     h->param.i_bframe_bias = x264_clip3( h->param.i_bframe_bias, -90, 100 );
     if( h->param.i_bframe <= 1 )
         h->param.i_bframe_pyramid = X264_B_PYRAMID_NONE;
@@ -588,6 +586,7 @@ static int x264_validate_parameters( x264_t *h )
         h->param.i_bframe_adaptive = X264_B_ADAPT_NONE;
         h->param.analyse.i_direct_mv_pred = 0;
         h->param.analyse.b_weighted_bipred = 0;
+        h->param.i_open_gop = X264_OPEN_GOP_NONE;
     }
     if( h->param.b_intra_refresh && h->param.i_bframe_pyramid == X264_B_PYRAMID_NORMAL )
     {
@@ -599,6 +598,11 @@ static int x264_validate_parameters( x264_t *h )
         x264_log( h, X264_LOG_WARNING, "ref > 1 + intra-refresh is not supported\n" );
         h->param.i_frame_reference = 1;
     }
+    if( h->param.b_intra_refresh && h->param.i_open_gop )
+    {
+        x264_log( h, X264_LOG_WARNING, "intra-refresh is not compatible with open-gop\n" );
+        h->param.i_open_gop = X264_OPEN_GOP_NONE;
+    }
     if( h->param.i_keyint_min == X264_KEYINT_MIN_AUTO )
         h->param.i_keyint_min = h->param.i_keyint_max / 10;
     h->param.i_keyint_min = x264_clip3( h->param.i_keyint_min, 1, h->param.i_keyint_max/2+1 );
@@ -978,9 +982,11 @@ x264_t *x264_encoder_open( x264_param_t *param )
     h->frames.b_have_lowres |= h->param.rc.b_stat_read && h->param.rc.i_vbv_buffer_size > 0;
     h->frames.b_have_sub8x8_esa = !!(h->param.analyse.inter & X264_ANALYSE_PSUB8x8);
 
+    h->frames.i_last_idr =
     h->frames.i_last_keyframe = - h->param.i_keyint_max;
     h->frames.i_input    = 0;
     h->frames.i_largest_pts = h->frames.i_second_largest_pts = -1;
+    h->frames.i_poc_last_open_gop = -1;
 
     CHECKED_MALLOCZERO( h->frames.unused[0], (h->frames.i_delay + 3) * sizeof(x264_frame_t *) );
     /* Allocate room for max refs plus a few extra just in case. */
@@ -1688,35 +1694,37 @@ static inline void x264_reference_hierarchy_reset( x264_t *h )
 {
     int ref;
     int b_hasdelayframe = 0;
-    if( !h->param.i_bframe_pyramid )
-        return;
 
     /* look for delay frames -- chain must only contain frames that are disposable */
     for( int i = 0; h->frames.current[i] && IS_DISPOSABLE( h->frames.current[i]->i_type ); i++ )
         b_hasdelayframe |= h->frames.current[i]->i_coded
                         != h->frames.current[i]->i_frame + h->sps->vui.i_num_reorder_frames;
 
-    if( h->param.i_bframe_pyramid != X264_B_PYRAMID_STRICT && !b_hasdelayframe )
+    /* This function must handle b-pyramid and clear frames for open-gop */
+    if( h->param.i_bframe_pyramid != X264_B_PYRAMID_STRICT && !b_hasdelayframe && h->frames.i_poc_last_open_gop == -1 )
         return;
 
     /* Remove last BREF. There will never be old BREFs in the
      * dpb during a BREF decode when pyramid == STRICT */
     for( ref = 0; h->frames.reference[ref]; ref++ )
     {
-        if( h->param.i_bframe_pyramid == X264_B_PYRAMID_STRICT
+        if( h->param.i_bframe_pyramid == X264_B_PYRAMID_STRICT
             && h->frames.reference[ref]->i_type == X264_TYPE_BREF )
+            || ( h->frames.reference[ref]->i_poc < h->frames.i_poc_last_open_gop
+            && h->sh.i_type != SLICE_TYPE_B ) )
         {
             int diff = h->i_frame_num - h->frames.reference[ref]->i_frame_num;
             h->sh.mmco[h->sh.i_mmco_command_count].i_difference_of_pic_nums = diff;
             h->sh.mmco[h->sh.i_mmco_command_count++].i_poc = h->frames.reference[ref]->i_poc;
-            x264_frame_push_unused( h, x264_frame_pop( h->frames.reference ) );
+            x264_frame_push_unused( h, x264_frame_shift( &h->frames.reference[ref] ) );
             h->b_ref_reorder[0] = 1;
-            break;
+            ref--;
         }
     }
 
-    /* Prepare to room in the dpb for the delayed display time of the later b-frame's */
-    h->sh.i_mmco_remove_from_end = X264_MAX( ref + 2 - h->frames.i_max_dpb, 0 );
+    /* Prepare room in the dpb for the delayed display time of the later b-frame's */
+    if( h->param.i_bframe_pyramid )
+        h->sh.i_mmco_remove_from_end = X264_MAX( ref + 2 - h->frames.i_max_dpb, 0 );
 }
 
 static inline void x264_slice_init( x264_t *h, int i_nal_type, int i_global_qp )
@@ -2321,12 +2329,17 @@ int     x264_encoder_encode( x264_t *h,
     {
         h->frames.i_last_keyframe = h->fenc->i_frame;
         if( h->fenc->i_type == X264_TYPE_IDR )
+        {
             h->i_frame_num = 0;
+            h->frames.i_last_idr = h->fenc->i_frame;
+        }
     }
     h->sh.i_mmco_command_count =
     h->sh.i_mmco_remove_from_end = 0;
     h->b_ref_reorder[0] =
     h->b_ref_reorder[1] = 0;
+    h->fdec->i_poc =
+    h->fenc->i_poc = 2 * ( h->fenc->i_frame - X264_MAX( h->frames.i_last_idr, 0 ) );
 
     /* ------------------- Setup frame context ----------------------------- */
     /* 5: Init data dependent of frame type */
@@ -2337,6 +2350,7 @@ int     x264_encoder_encode( x264_t *h,
         i_nal_ref_idc = NAL_PRIORITY_HIGHEST;
         h->sh.i_type = SLICE_TYPE_I;
         x264_reference_reset( h );
+        h->frames.i_poc_last_open_gop = -1;
     }
     else if( h->fenc->i_type == X264_TYPE_I )
     {
@@ -2344,6 +2358,8 @@ int     x264_encoder_encode( x264_t *h,
         i_nal_ref_idc = NAL_PRIORITY_HIGH; /* Not completely true but for now it is (as all I/P are kept as ref)*/
         h->sh.i_type = SLICE_TYPE_I;
         x264_reference_hierarchy_reset( h );
+        if( h->param.i_open_gop )
+            h->frames.i_poc_last_open_gop = h->fenc->b_keyframe ? h->fenc->i_poc : -1;
     }
     else if( h->fenc->i_type == X264_TYPE_P )
     {
@@ -2351,6 +2367,7 @@ int     x264_encoder_encode( x264_t *h,
         i_nal_ref_idc = NAL_PRIORITY_HIGH; /* Not completely true but for now it is (as all I/P are kept as ref)*/
         h->sh.i_type = SLICE_TYPE_P;
         x264_reference_hierarchy_reset( h );
+        h->frames.i_poc_last_open_gop = -1;
     }
     else if( h->fenc->i_type == X264_TYPE_BREF )
     {
@@ -2366,8 +2383,6 @@ int     x264_encoder_encode( x264_t *h,
         h->sh.i_type = SLICE_TYPE_B;
     }
 
-    h->fdec->i_poc =
-    h->fenc->i_poc = 2 * (h->fenc->i_frame - h->frames.i_last_keyframe);
     h->fdec->i_type = h->fenc->i_type;
     h->fdec->i_frame = h->fenc->i_frame;
     h->fenc->b_kept_as_ref =
@@ -2484,7 +2499,7 @@ int     x264_encoder_encode( x264_t *h,
 
         if( h->fenc->i_type != X264_TYPE_IDR )
         {
-            int time_to_recovery = X264_MIN( h->mb.i_mb_width - 1, h->param.i_keyint_max ) + h->param.i_bframe;
+            int time_to_recovery = h->param.i_open_gop ? 0 : X264_MIN( h->mb.i_mb_width - 1, h->param.i_keyint_max ) + h->param.i_bframe;
             x264_nal_start( h, NAL_SEI, NAL_PRIORITY_DISPOSABLE );
             x264_sei_recovery_point_write( h, &h->out.bs, time_to_recovery );
             x264_nal_end( h );
index 16afbf0..1030ef2 100644 (file)
@@ -724,6 +724,7 @@ int x264_ratecontrol_new( x264_t *h )
             CMP_OPT_FIRST_PASS( "b_pyramid", h->param.i_bframe_pyramid );
             CMP_OPT_FIRST_PASS( "intra_refresh", h->param.b_intra_refresh );
             CMP_OPT_FIRST_PASS( "keyint", h->param.i_keyint_max );
+            CMP_OPT_FIRST_PASS( "open_gop", h->param.i_open_gop );
 
             if( strstr( opts, "qp=0" ) && h->param.rc.i_rc_method == X264_RC_ABR )
                 x264_log( h, X264_LOG_WARNING, "1st pass was lossless, bitrate prediction will be inaccurate\n" );
index 60f3a24..abd4979 100644 (file)
@@ -1058,7 +1058,7 @@ void x264_slicetype_analyse( x264_t *h, int keyframe )
 {
     x264_mb_analysis_t a;
     x264_frame_t *frames[X264_LOOKAHEAD_MAX+3] = { NULL, };
-    int num_frames, orig_num_frames, keyint_limit, idr_frame_type, framecnt;
+    int num_frames, orig_num_frames, keyint_limit, framecnt;
     int i_mb_count = NUM_MBS;
     int cost1p0, cost2p0, cost1b1, cost2p1;
     int i_max_search = X264_MIN( h->lookahead->next.i_size, X264_LOOKAHEAD_MAX );
@@ -1080,7 +1080,6 @@ void x264_slicetype_analyse( x264_t *h, int keyframe )
     orig_num_frames = num_frames = h->param.b_intra_refresh ? framecnt : X264_MIN( framecnt, keyint_limit );
 
     x264_lowres_context_init( h, &a );
-    idr_frame_type = frames[1]->i_frame - h->lookahead->i_last_keyframe >= h->param.i_keyint_min ? X264_TYPE_IDR : X264_TYPE_I;
 
     /* This is important psy-wise: if we have a non-scenecut keyframe,
      * there will be significant visual artifacts if the frames just before
@@ -1092,12 +1091,12 @@ void x264_slicetype_analyse( x264_t *h, int keyframe )
     {
         frames[1]->i_type = X264_TYPE_P;
         if( h->param.i_scenecut_threshold && scenecut( h, &a, frames, 0, 1, 1, orig_num_frames ) )
-            frames[1]->i_type = idr_frame_type;
+            frames[1]->i_type = X264_TYPE_I;
         return;
     }
     else if( num_frames == 0 )
     {
-        frames[1]->i_type = idr_frame_type;
+        frames[1]->i_type = X264_TYPE_I;
         return;
     }
 
@@ -1106,7 +1105,7 @@ void x264_slicetype_analyse( x264_t *h, int keyframe )
     int reset_start;
     if( h->param.i_scenecut_threshold && scenecut( h, &a, frames, 0, 1, 1, orig_num_frames ) )
     {
-        frames[1]->i_type = idr_frame_type;
+        frames[1]->i_type = X264_TYPE_I;
         return;
     }
 
@@ -1210,15 +1209,19 @@ void x264_slicetype_analyse( x264_t *h, int keyframe )
 
     /* Enforce keyframe limit. */
     if( !h->param.b_intra_refresh )
-        for( int j = 0; j < num_frames; j++ )
+        for( int i = keyint_limit+1; i <= num_frames; i += h->param.i_keyint_max )
         {
-            if( ((j-keyint_limit) % h->param.i_keyint_max) == 0 )
+            int j = i;
+            if( h->param.i_open_gop == X264_OPEN_GOP_CODED_ORDER )
             {
-                if( j && h->param.i_keyint_max > 1 )
-                    frames[j]->i_type = X264_TYPE_P;
-                frames[j+1]->i_type = X264_TYPE_IDR;
-                reset_start = X264_MIN( reset_start, j+2 );
+                while( IS_X264_TYPE_B( frames[i]->i_type ) )
+                    i++;
+                while( IS_X264_TYPE_B( frames[j-1]->i_type ) )
+                    j--;
             }
+            frames[i]->i_type = X264_TYPE_I;
+            reset_start = X264_MIN( reset_start, i+1 );
+            i = j;
         }
 
     if( h->param.rc.i_vbv_buffer_size )
@@ -1303,13 +1306,39 @@ void x264_slicetype_decide( x264_t *h )
                       frm->i_frame, x264_b_pyramid_names[h->param.i_bframe_pyramid], h->param.i_frame_reference );
         }
 
+        if( frm->i_type == X264_TYPE_KEYFRAME )
+            frm->i_type = h->param.i_open_gop ? X264_TYPE_I : X264_TYPE_IDR;
+
         /* Limit GOP size */
         if( (!h->param.b_intra_refresh || frm->i_frame == 0) && frm->i_frame - h->lookahead->i_last_keyframe >= h->param.i_keyint_max )
         {
-            if( frm->i_type == X264_TYPE_AUTO )
+            if( frm->i_type == X264_TYPE_AUTO || frm->i_type == X264_TYPE_I )
+                frm->i_type = h->param.i_open_gop && h->lookahead->i_last_keyframe >= 0 ? X264_TYPE_I : X264_TYPE_IDR;
+            int warn = frm->i_type != X264_TYPE_IDR;
+            if( warn && h->param.i_open_gop == X264_OPEN_GOP_DISPLAY_ORDER )
+                warn &= frm->i_type != X264_TYPE_I && frm->i_type != X264_TYPE_KEYFRAME;
+            if( warn && h->param.i_open_gop == X264_OPEN_GOP_CODED_ORDER )
+            {
+                /* if this minigop ends with i, it's not a violation */
+                int j = bframes;
+                while( IS_X264_TYPE_B( h->lookahead->next.list[j]->i_type ) )
+                    j++;
+                warn = h->lookahead->next.list[j]->i_type != X264_TYPE_I && h->lookahead->next.list[j]->i_type != X264_TYPE_KEYFRAME;
+            }
+            if( warn )
+                x264_log( h, X264_LOG_WARNING, "specified frame type (%d) at %d is not compatible with keyframe interval\n", frm->i_type, frm->i_frame );
+        }
+        if( frm->i_type == X264_TYPE_I && frm->i_frame - h->lookahead->i_last_keyframe >= h->param.i_keyint_min )
+        {
+            if( h->param.i_open_gop )
+            {
+                h->lookahead->i_last_keyframe = frm->i_frame; // Use display order
+                if( h->param.i_open_gop == X264_OPEN_GOP_CODED_ORDER )
+                    h->lookahead->i_last_keyframe -= bframes; // Use coded order
+                frm->b_keyframe = 1;
+            }
+            else
                 frm->i_type = X264_TYPE_IDR;
-            if( frm->i_type != X264_TYPE_IDR )
-                x264_log( h, X264_LOG_WARNING, "specified frame type (%d) is not compatible with keyframe interval\n", frm->i_type );
         }
         if( frm->i_type == X264_TYPE_IDR )
         {
diff --git a/x264.c b/x264.c
index 09bad61..8722565 100644 (file)
--- a/x264.c
+++ b/x264.c
@@ -380,6 +380,12 @@ static void Help( x264_param_t *defaults, int longhelp )
         "                                  - strict: Strictly hierarchical pyramid\n"
         "                                  - normal: Non-strict (not Blu-ray compatible)\n",
         strtable_lookup( x264_b_pyramid_names, defaults->i_bframe_pyramid ) );
+    H1( "      --open-gop <string>     Use recovery points to close GOPs [none]\n"
+        "                                  - none: Use standard closed GOPs\n"
+        "                                  - display: Base GOP length on display order\n"
+        "                                             (not Blu-ray compatible)\n"
+        "                                  - coded: Base GOP length on coded order\n"
+        "                              Only available with b-frames\n" );
     H1( "      --no-cabac              Disable CABAC\n" );
     H1( "  -r, --ref <integer>         Number of reference frames [%d]\n", defaults->i_frame_reference );
     H1( "      --no-deblock            Disable loop filter\n" );
@@ -441,7 +447,8 @@ static void Help( x264_param_t *defaults, int longhelp )
         "                                  or  b=<float> (bitrate multiplier)\n" );
     H2( "      --qpfile <string>       Force frametypes and QPs for some or all frames\n"
         "                              Format of each line: framenumber frametype QP\n"
-        "                              QP of -1 lets x264 choose. Frametypes: I,i,P,B,b.\n"
+        "                              QP of -1 lets x264 choose. Frametypes: I,i,K,P,B,b.\n"
+        "                                  K=<I or i> depending on open-gop setting\n"
         "                              QPs are restricted by qpmin/qpmax.\n" );
     H1( "\n" );
     H1( "Analysis:\n" );
@@ -627,6 +634,7 @@ static struct option long_options[] =
     { "no-b-adapt",        no_argument, NULL, 0 },
     { "b-bias",      required_argument, NULL, 0 },
     { "b-pyramid",   required_argument, NULL, 0 },
+    { "open-gop",    required_argument, NULL, 0 },
     { "min-keyint",  required_argument, NULL, 'i' },
     { "keyint",      required_argument, NULL, 'I' },
     { "intra-refresh",     no_argument, NULL, 0 },
@@ -1305,6 +1313,7 @@ static void parse_qpfile( cli_opt_t *opt, x264_picture_t *pic, int i_frame )
         pic->i_qpplus1 = qp+1;
         if     ( type == 'I' ) pic->i_type = X264_TYPE_IDR;
         else if( type == 'i' ) pic->i_type = X264_TYPE_I;
+        else if( type == 'K' ) pic->i_type = X264_TYPE_KEYFRAME;
         else if( type == 'P' ) pic->i_type = X264_TYPE_P;
         else if( type == 'B' ) pic->i_type = X264_TYPE_BREF;
         else if( type == 'b' ) pic->i_type = X264_TYPE_B;
diff --git a/x264.h b/x264.h
index 9cd4600..09183fd 100644 (file)
--- a/x264.h
+++ b/x264.h
@@ -35,7 +35,7 @@
 
 #include <stdarg.h>
 
-#define X264_BUILD 98
+#define X264_BUILD 99
 
 /* x264_t:
  *      opaque handler for encoder */
@@ -104,6 +104,9 @@ typedef struct x264_t x264_t;
 #define X264_B_PYRAMID_STRICT        1
 #define X264_B_PYRAMID_NORMAL        2
 #define X264_KEYINT_MIN_AUTO         0
+#define X264_OPEN_GOP_NONE           0
+#define X264_OPEN_GOP_DISPLAY_ORDER  1
+#define X264_OPEN_GOP_CODED_ORDER    2
 
 static const char * const x264_direct_pred_names[] = { "none", "spatial", "temporal", "auto", 0 };
 static const char * const x264_motion_est_names[] = { "dia", "hex", "umh", "esa", "tesa", 0 };
@@ -115,6 +118,7 @@ static const char * const x264_colorprim_names[] = { "", "bt709", "undef", "", "
 static const char * const x264_transfer_names[] = { "", "bt709", "undef", "", "bt470m", "bt470bg", "smpte170m", "smpte240m", "linear", "log100", "log316", 0 };
 static const char * const x264_colmatrix_names[] = { "GBR", "bt709", "undef", "", "fcc", "bt470bg", "smpte170m", "smpte240m", "YCgCo", 0 };
 static const char * const x264_nal_hrd_names[] = { "none", "vbr", "cbr", 0 };
+static const char * const x264_open_gop_names[] = { "none", "display", "coded", 0 };
 
 /* Colorspace type
  * legacy only; nothing other than I420 is really supported. */
@@ -138,6 +142,7 @@ static const char * const x264_nal_hrd_names[] = { "none", "vbr", "cbr", 0 };
 #define X264_TYPE_P             0x0003
 #define X264_TYPE_BREF          0x0004  /* Non-disposable B-frame */
 #define X264_TYPE_B             0x0005
+#define X264_TYPE_KEYFRAME      0x0006  /* IDR or I depending on b_open_gop option */
 #define IS_X264_TYPE_I(x) ((x)==X264_TYPE_I || (x)==X264_TYPE_IDR)
 #define IS_X264_TYPE_B(x) ((x)==X264_TYPE_B || (x)==X264_TYPE_BREF)
 
@@ -221,6 +226,7 @@ typedef struct x264_param_t
     int         i_bframe_adaptive;
     int         i_bframe_bias;
     int         i_bframe_pyramid;   /* Keep some B-frames as references: 0=off, 1=strict hierarchical, 2=normal */
+    int         i_open_gop;         /* Open gop: 1=display order, 2=coded order to determine gop size */
 
     int         b_deblocking_filter;
     int         i_deblocking_filter_alphac0;    /* [-6, 6] -6 light filter, 6 strong */