shithub: libvpx

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Optimize vp9_get_sub_block_energy.

Because energy scaling is non-decreasing, we can work on the variance
and scale after the loop. This avoids costly computations (in
particular, log()) within the loop.
We've measured that we spend 0.8% of our total time computing the log.

Change-Id: I302fc0ecd9fd8cf96ee9f31b8673e82de1b2b3e2

--- a/vp9/encoder/vp9_aq_variance.c

+++ b/vp9/encoder/vp9_aq_variance.c

@@ -186,6 +186,17 @@

   return log(var + 1.0);

 }

+#define DEFAULT_E_MIDPOINT 10.0

+static int scale_block_energy(VP9_COMP *cpi, unsigned int block_var) {

+  double energy;

+  double energy_midpoint;

+  energy_midpoint =

+      (cpi->oxcf.pass == 2) ? cpi->twopass.mb_av_energy : DEFAULT_E_MIDPOINT;

+  energy = log(block_var + 1.0) - energy_midpoint;

+  return clamp((int)round(energy), ENERGY_MIN, ENERGY_MAX);

+}

+#undef DEFAULT_E_MIDPOINT

+

 // Get the range of sub block energy values;

 void vp9_get_sub_block_energy(VP9_COMP *cpi, MACROBLOCK *mb, int mi_row,

                               int mi_col, BLOCK_SIZE bsize, int *min_e,

@@ -202,18 +213,25 @@

     *min_e = vp9_block_energy(cpi, mb, bsize);

     *max_e = *min_e;

   } else {

-    int energy;

-    *min_e = ENERGY_MAX;

-    *max_e = ENERGY_MIN;

+    unsigned int var;

+    // Because scale_block_energy is non-decreasing, we can find the min/max

+    // block variance and scale afterwards. This avoids a costly scaling at

+    // every iteration.

+    unsigned int min_var = UINT_MAX;

+    unsigned int max_var = 0;

     for (y = 0; y < ymis; ++y) {

       for (x = 0; x < xmis; ++x) {

         vp9_setup_src_planes(mb, cpi->Source, mi_row + y, mi_col + x);

-        energy = vp9_block_energy(cpi, mb, BLOCK_8X8);

-        *min_e = VPXMIN(*min_e, energy);

-        *max_e = VPXMAX(*max_e, energy);

+        vpx_clear_system_state();

+        var = block_variance(cpi, mb, BLOCK_8X8);

+        vpx_clear_system_state();

+        min_var = VPXMIN(min_var, var);

+        max_var = VPXMAX(max_var, var);

       }

     }

+    *min_e = scale_block_energy(cpi, min_var);

+    *max_e = scale_block_energy(cpi, max_var);

   }

   // Re-instate source pointers back to what they should have been on entry.

@@ -220,13 +238,10 @@

   vp9_setup_src_planes(mb, cpi->Source, mi_row, mi_col);

 }

-#define DEFAULT_E_MIDPOINT 10.0

 int vp9_block_energy(VP9_COMP *cpi, MACROBLOCK *x, BLOCK_SIZE bs) {

-  double energy;

-  double energy_midpoint;

+  unsigned int var;

   vpx_clear_system_state();

-  energy_midpoint =

-      (cpi->oxcf.pass == 2) ? cpi->twopass.mb_av_energy : DEFAULT_E_MIDPOINT;

-  energy = vp9_log_block_var(cpi, x, bs) - energy_midpoint;

-  return clamp((int)round(energy), ENERGY_MIN, ENERGY_MAX);

+  var = block_variance(cpi, x, bs);

+  vpx_clear_system_state();

+  return scale_block_energy(cpi, var);

 }