Add adaptive QP for 360 degree video

Adds option --erp-aqp for enabling adaptive QP for 360 degree video with
equirectangular projection. When projected into a spherical surface,
the middle part of the video covers relatively larger area than the top
and bottom parts. Enabling --erp-aqp sets up a ROI delta QP array which
uses higher QPs for the top and bottom of the video and lower QPs for
the middle part.

README.md

@@ -100,6 +100,8 @@ Video structure:
                                    delta QP values in raster order.
                                    The delta QP map can be any size or aspect
                                    ratio, and will be mapped to LCU's.
+      --(no-)erp-aqp         : Use adaptive QP for 360 video with
+                               equirectangular projection
 
 Compression tools:
       --deblock [<beta:tc>]  : Deblocking

doc/kvazaar.1

@@ -1,4 +1,4 @@
-.TH KVAZAAR "1" "February 2017" "kvazaar v1.1.0" "User Commands"
+.TH KVAZAAR "1" "April 2017" "kvazaar v1.1.0" "User Commands"
 .SH NAME
 kvazaar \- open source HEVC encoder
 .SH SYNOPSIS
@@ -131,6 +131,10 @@ Use a delta QP map for region of interest
     delta QP values in raster order.
     The delta QP map can be any size or aspect
     ratio, and will be mapped to LCU's.
+.TP
+\fB\-\-(no\-)erp\-aqp        
+Use adaptive QP for 360 video with
+equirectangular projection
 
 .SS "Compression tools:"
 .TP

src/cfg.c

@@ -119,6 +119,8 @@ int kvz_config_init(kvz_config *cfg)
   cfg->roi.height = 0;
   cfg->roi.dqps = NULL;
 
+  cfg->erp_aqp = false;
+
   cfg->slices = KVZ_SLICES_NONE;
 
   return 1;
@@ -1045,6 +1047,8 @@ int kvz_config_parse(kvz_config *cfg, const char *name, const char *value)
 
     fclose(f);
   }
+  else if OPT("erp-aqp")
+    cfg->erp_aqp = (bool)atobool(value);
   else
     return 0;
 #undef OPT

src/cli.c

@@ -119,6 +119,8 @@ static const struct option long_options[] = {
   { "implicit-rdpcm",           no_argument, NULL, 0 },
   { "no-implicit-rdpcm",        no_argument, NULL, 0 },
   { "roi",                required_argument, NULL, 0 },
+  { "erp-aqp",                  no_argument, NULL, 0 },
+  { "no-erp-aqp",               no_argument, NULL, 0 },
   {0, 0, 0, 0}
 };
 
@@ -388,6 +390,8 @@ void print_help(void)
     "                                   delta QP values in raster order.\n"
     "                                   The delta QP map can be any size or aspect\n"
     "                                   ratio, and will be mapped to LCU's.\n"
+    "      --(no-)erp-aqp         : Use adaptive QP for 360 video with\n"
+    "                               equirectangular projection\n"
     "\n"
     /* Word wrap to this width to stay under 80 characters (including ") ************/
     "Compression tools:\n"

src/encoder.c

@@ -20,6 +20,9 @@
 
 #include "encoder.h"
 
+// This define is required for M_PI on Windows.
+#define _USE_MATH_DEFINES
+#include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -27,6 +30,14 @@
 #include "strategyselector.h"
 
 
+/**
+ * \brief Strength of QP adjustments when using adaptive QP for 360 video.
+ *
+ * Determined empirically.
+ */
+static const double ERP_AQP_STRENGTH = 3.0;
+
+
 static int encoder_control_init_gop_layer_weights(encoder_control_t * const);
 
 static int size_of_wpp_ends(int threads)
@@ -114,6 +125,82 @@ static unsigned cfg_num_threads(void)
 }
 
 
+/**
+ * \brief Return weight for 360 degree ERP video
+ *
+ * Returns the scaling factor of area from equirectangular projection to
+ * spherical surface.
+ *
+ * \param y   y-coordinate of the pixel
+ * \param h   height of the picture
+ */
+static double ws_weight(int y, int h)
+{
+  return cos((y - 0.5 * h + 0.5) * (M_PI / h));
+}
+
+
+
+/**
+ * \brief Update ROI QPs for 360 video with equirectangular projection.
+ *
+ * Writes updated ROI parameters to encoder->cfg.roi.
+ *
+ * \param encoder       encoder control
+ * \param orig_roi      original delta QPs or NULL
+ * \param orig_width    width of orig_roi
+ * \param orig_height   height of orig_roi
+ */
+static void init_erp_aqp_roi(encoder_control_t* encoder,
+                             int8_t *orig_roi,
+                             int32_t orig_width,
+                             int32_t orig_height)
+{
+  // Update ROI with WS-PSNR delta QPs.
+  int height = encoder->in.height_in_lcu;
+  int width  = orig_roi ? orig_width : 1;
+
+  int frame_height = encoder->in.real_height;
+
+  encoder->cfg.roi.width  = width;
+  encoder->cfg.roi.height = height;
+  encoder->cfg.roi.dqps   = calloc(width * height, sizeof(orig_roi[0]));
+
+  double total_weight = 0.0;
+  for (int y = 0; y < frame_height; y++) {
+    total_weight += ws_weight(y, frame_height);
+  }
+
+  for (int y_lcu = 0; y_lcu < height; y_lcu++) {
+    int y_orig = LCU_WIDTH * y_lcu;
+    int lcu_height = MIN(LCU_WIDTH, frame_height - y_orig);
+
+    double lcu_weight = 0.0;
+    for (int y = y_orig; y < y_orig + lcu_height; y++) {
+      lcu_weight += ws_weight(y, frame_height);
+    }
+    // Normalize.
+    lcu_weight = (lcu_weight * frame_height) / (total_weight * lcu_height);
+
+    int8_t qp_delta = round(-ERP_AQP_STRENGTH * log2(lcu_weight));
+
+    if (orig_roi) {
+      // If a ROI array already exists, we copy the existing values to the
+      // new array while adding qp_delta to each.
+      int y_roi = y_lcu * orig_height / height;
+      for (int x = 0; x < width; x++) {
+        encoder->cfg.roi.dqps[x + y_lcu * width] =
+          CLIP(-51, 51, orig_roi[x + y_roi * width] + qp_delta);
+      }
+
+    } else {
+      // Otherwise, simply write qp_delta to the ROI array.
+      encoder->cfg.roi.dqps[y_lcu] = qp_delta;
+    }
+  }
+}
+
+
 /**
  * \brief Allocate and initialize an encoder control structure.
  *
@@ -219,16 +306,23 @@ encoder_control_t* kvz_encoder_control_init(const kvz_config *const cfg)
     goto init_failed;
   }
 
+  if (cfg->erp_aqp) {
+    init_erp_aqp_roi(encoder,
+                     cfg->roi.dqps,
+                     cfg->roi.width,
+                     cfg->roi.height);
+
+  } else if (cfg->roi.dqps) {
     // Copy delta QP array for ROI coding.
-  if (cfg->roi.dqps) {
     const size_t roi_size = encoder->cfg.roi.width * encoder->cfg.roi.height;
     encoder->cfg.roi.dqps = calloc(roi_size, sizeof(cfg->roi.dqps[0]));
     memcpy(encoder->cfg.roi.dqps,
            cfg->roi.dqps,
            roi_size * sizeof(*cfg->roi.dqps));
+
   }
 
-  encoder->lcu_dqp_enabled = cfg->target_bitrate > 0 || cfg->roi.dqps;
+  encoder->lcu_dqp_enabled = cfg->target_bitrate > 0 || encoder->cfg.roi.dqps;
 
   //Tiles
   encoder->tiles_enable = encoder->cfg.tiles_width_count > 1 ||

src/kvazaar.h

@@ -339,6 +339,11 @@ typedef struct kvz_config
   } roi; /*!< \since 3.14.0 \brief Map of delta QPs for region of interest coding. */
 
   unsigned slices; /*!< \since 3.15.0 \brief How to map slices to frame. */
+
+  /**
+   * \brief Use adaptive QP for 360 video with equirectangular projection.
+   */
+  int32_t erp_aqp;
 } kvz_config;
 
 /**