Index: encoder/encoder.c
===================================================================
--- encoder/encoder.c	(revision 667)
+++ encoder/encoder.c	(working copy)
@@ -523,6 +523,7 @@
         h->param.rc.f_qblur = 0;
     if( h->param.rc.f_complexity_blur < 0 )
         h->param.rc.f_complexity_blur = 0;
+    h->param.rc.f_aq_cssim = x264_clip3f( h->param.rc.f_aq_cssim, 0.0, 1.0 );
 
     h->param.i_sps_id &= 31;
 
Index: encoder/ratecontrol.c
===================================================================
--- encoder/ratecontrol.c	(revision 667)
+++ encoder/ratecontrol.c	(working copy)
@@ -240,8 +240,13 @@
         x264_log(h, X264_LOG_WARNING, "bitrate tolerance too small, using .01\n");
         rc->rate_tolerance = 0.01;
     }
+    if( h->param.rc.b_aq_cssim && h->param.i_bframe && h->param.analyse.b_transform_8x8 )
+    {
+        x264_log( h, X264_LOG_ERROR, "not implemented: aq-cssim + B-frames + 8x8dct\n" );
+        return -1;
+    }
 
-    h->mb.b_variable_qp = rc->b_vbv && !rc->b_2pass;
+    h->mb.b_variable_qp = (rc->b_vbv && !rc->b_2pass) || h->param.rc.b_aq_cssim;
 
     if( rc->b_abr )
     {
@@ -765,6 +770,13 @@
 
     x264_cpu_restore( h->param.cpu );
 
+    if( h->param.rc.b_aq_cssim )
+    {
+        // cssim wants to record the real qp. other types of aq might want the qp from before applying aq.
+        rc->qpa += h->mb.i_qp;
+        return;
+    }
+
     h->fdec->i_row_bits[y] += bits;
     rc->qpa += rc->qpm;
 
@@ -945,7 +957,9 @@
 
     if( h->mb.b_variable_qp )
     {
-        if( h->sh.i_type == SLICE_TYPE_B )
+        if( h->param.rc.b_aq_cssim )
+            rc->qp_constant[h->sh.i_type] = (int)( rc->qpa + .5 );
+        else if( h->sh.i_type == SLICE_TYPE_B )
         {
             rc->bframe_bits += bits;
             if( !h->frames.current[0] || !IS_X264_TYPE_B(h->frames.current[0]->i_type) )
Index: encoder/analyse.c
===================================================================
--- encoder/analyse.c	(revision 667)
+++ encoder/analyse.c	(working copy)
@@ -29,6 +29,7 @@
 #endif
 
 #include "common/common.h"
+#include "common/cpu.h"
 #include "macroblock.h"
 #include "me.h"
 #include "ratecontrol.h"
@@ -193,6 +194,9 @@
 {
     memset( a, 0, sizeof( x264_mb_analysis_t ) );
 
+    if( h->param.rc.b_aq_cssim )
+        i_qp = h->mb.i_last_qp;
+
     /* conduct the analysis using this lamda and QP */
     a->i_qp = h->mb.i_qp = i_qp;
     h->mb.i_chroma_qp = i_chroma_qp_table[x264_clip3( i_qp + h->pps->i_chroma_qp_index_offset, 0, 51 )];
@@ -2029,7 +2033,46 @@
     }
 }
 
+static void aq_constant_ssim( x264_t *h, x264_mb_analysis_t *a )
+{
+    int transform_bak = h->mb.b_transform_8x8;
+    int mbtype_bak = h->mb.i_type == P_SKIP ? P_L0
+                   : h->mb.i_type == B_SKIP ? B_DIRECT
+                   : h->mb.i_type;
+    int qp = a->i_qp, bqp = qp;
+    int prev_dir = 0;
+    float bdssim;
 
+    x264_cpu_restore( h->param.cpu );
+    bdssim = 2.0;
+
+    while( qp >= 0 && qp <= 51 )
+    {
+        float ssim, dssim;
+        int dir;
+        h->mb.i_type = mbtype_bak;
+        h->mb.b_transform_8x8 = transform_bak;
+        h->mb.i_qp = qp;
+        h->mb.i_chroma_qp = i_chroma_qp_table[x264_clip3( qp + h->pps->i_chroma_qp_index_offset, 0, 51 )];
+        x264_macroblock_encode( h );
+        x264_cpu_restore( h->param.cpu );
+        ssim = x264_pixel_ssim_wxh( &h->pixf, h->mb.pic.p_fenc[0], FENC_STRIDE, h->mb.pic.p_fdec[0], FDEC_STRIDE, 16, 16 );
+        dssim = fabs(ssim - h->param.rc.f_aq_cssim);
+        COPY2_IF_LT( bdssim, dssim, bqp, qp );
+//      printf( "[%d] qp:%d ssim:%f\n", h->mb.i_mb_xy, qp, ssim );
+
+        dir = ssim < h->param.rc.f_aq_cssim ? -1 : 1;
+        if( dir * prev_dir < 0 )
+            break;
+        if( dir == 1 && h->mb.i_cbp_luma == 0 )
+            break;
+        qp += dir;
+        prev_dir = dir;
+    }
+
+    h->mb.i_qp = bqp;
+}
+
 /*****************************************************************************
  * x264_macroblock_analyse:
  *****************************************************************************/
@@ -2561,6 +2604,9 @@
     if( !analysis.b_mbrd )
         x264_mb_analyse_transform( h );
 
+    if( h->param.rc.b_aq_cssim )
+        aq_constant_ssim( h, &analysis );
+
     h->mb.b_trellis = h->param.analyse.i_trellis;
     h->mb.b_noise_reduction = h->param.analyse.i_noise_reduction;
 }
Index: x264.c
===================================================================
--- x264.c	(revision 667)
+++ x264.c	(working copy)
@@ -204,6 +204,7 @@
         "                                      q=<integer> (force QP)\n"
         "                                  or  b=<float> (bitrate multiplier)\n" );
     H1( "      --qpfile <string>       Force frametypes and QPs\n" );
+    H1( "      --aq-cssim <float>      Force all macroblocks to this SSIM\n" );
     H0( "\n" );
     H0( "Analysis:\n" );
     H0( "\n" );
@@ -424,6 +425,7 @@
             { "cplxblur",required_argument, NULL, 0 },
             { "zones",   required_argument, NULL, 0 },
             { "qpfile",  required_argument, NULL, OPT_QPFILE },
+            { "aq-cssim",required_argument, NULL, 0 },
             { "threads", required_argument, NULL, 0 },
             { "thread-input", no_argument,  NULL, OPT_THREAD_INPUT },
             { "non-deterministic", no_argument, NULL, 0 },
Index: common/common.c
===================================================================
--- common/common.c	(revision 667)
+++ common/common.c	(working copy)
@@ -515,6 +515,13 @@
         p->rc.f_complexity_blur = atof(value);
     OPT("zones")
         p->rc.psz_zones = strdup(value);
+    OPT("aq-cssim")
+    {
+        p->rc.b_aq_cssim = 1;
+        p->rc.f_aq_cssim = atof(value);
+        p->rc.i_rc_method = X264_RC_CQP; // because aq-cssim replaces any other rc
+        p->rc.i_qp_constant = 26;
+    }
     OPT("psnr")
         p->analyse.b_psnr = atobool(value);
     OPT("ssim")
@@ -906,10 +913,13 @@
                   p->i_keyint_max, p->i_keyint_min, p->i_scenecut_threshold,
                   p->b_pre_scenecut ? "(pre)" : "" );
 
-    s += sprintf( s, " rc=%s", p->rc.i_rc_method == X264_RC_ABR ?
+    s += sprintf( s, " rc=%s", p->rc.b_aq_cssim ? "cssim"
+                               : p->rc.i_rc_method == X264_RC_ABR ?
                                ( p->rc.b_stat_read ? "2pass" : p->rc.i_vbv_buffer_size ? "cbr" : "abr" )
                                : p->rc.i_rc_method == X264_RC_CRF ? "crf" : "cqp" );
-    if( p->rc.i_rc_method == X264_RC_ABR || p->rc.i_rc_method == X264_RC_CRF )
+    if( p->rc.b_aq_cssim )
+        s += sprintf( s, " ssim=%.4f", p->rc.f_aq_cssim );
+    else if( p->rc.i_rc_method == X264_RC_ABR || p->rc.i_rc_method == X264_RC_CRF )
     {
         if( p->rc.i_rc_method == X264_RC_CRF )
             s += sprintf( s, " crf=%.1f", p->rc.f_rf_constant );
Index: x264.h
===================================================================
--- x264.h	(revision 667)
+++ x264.h	(working copy)
@@ -269,6 +269,9 @@
         x264_zone_t *zones;         /* ratecontrol overrides */
         int         i_zones;        /* number of zone_t's */
         char        *psz_zones;     /* alternate method of specifying zones */
+
+        int         b_aq_cssim;
+        float       f_aq_cssim;
     } rc;
 
     /* Muxing parameters */