Add --owf=auto option.

- The optimal value for Overlapping Wave Front (OWF) depends on a bunch of
  variables. Attempt to set the optimal owf value, at least for all intra.

src/config.c

@@ -433,6 +433,8 @@ static int config_parse(config *cfg, const char *name, const char *value)
     if (cfg->owf < 0) {
       fprintf(stderr, "--owf parameter smaller than 0, set to 0\n");
       cfg->owf = 0;
+    } else if (cfg->owf == 0 && !strcmp(value, "auto")) {
+      cfg->owf = -1;
     }
   }
   else if OPT("slice-addresses")

src/encmain.c

@@ -159,7 +159,7 @@ int main(int argc, char *argv[])
             "\n"
             "  Wpp:\n"
             "          --wpp                  : Enable wavefront parallel processing\n"
-            "          --owf <integer>        : Enable parallel processing of multiple frames\n"
+            "          --owf <integer>|auto   : Number of parallel frames to process. 0 to disable.\n"
             "\n"
             "  Slices:\n"
             "          --slice-addresses <string>|u<int>: \n"
@@ -191,6 +191,52 @@ int main(int argc, char *argv[])
     strcpy(cfg->debug + left_len, dim_str);
   }
 
+  if (cfg->owf == -1) {
+    if (cfg->wpp) {
+      // If --owf=auto and wpp is on, select owf according to the lesser dimension.
+      // An ok rule for all intra seems to be to always have at least 4 wpp
+      // streams per thread. For a single frame that would mean that all threads
+      // are working for at least half of the frame.
+      int lcu_width = (cfg->width + LCU_WIDTH - 1) / LCU_WIDTH;
+      int lcu_height = (cfg->height + LCU_WIDTH - 1) / LCU_WIDTH;
+      int min_dimension = MIN(lcu_width, lcu_height);
+      int threads = (cfg->threads > 1 ? cfg->threads : 1);
+
+      // Find the largest number of threads per frame that satifies the
+      // the condition that there are 4 wpp streams per thread.
+      int threads_per_frame = 1;
+      while (min_dimension / (threads_per_frame + 1) >= 4) {
+        ++threads_per_frame;
+      }
+
+      // Get ceil(threads / threads_per_frame).
+      int frames = (threads + threads_per_frame - 1) / threads_per_frame;
+      cfg->owf = CLIP(0, threads - 1, frames - 1);
+    } else {
+      // If --owf=auto and wpp is not on, select owf such that there is enough
+      // tiles for twice the number of threads. That should make sure there are
+      // always some tiles to work on.
+
+      int tiles_per_frame = 1;
+      if (cfg->tiles_width_split != NULL) {
+        tiles_per_frame *= cfg->tiles_width_count;
+      }
+      if (cfg->tiles_height_split != NULL) {
+        tiles_per_frame *= cfg->tiles_height_count;
+      }
+      int threads = (cfg->threads > 1 ? cfg->threads : 1);
+      // Get ceil(threads * 2 / tiles_per_frame).
+      int frames = (threads * 2 + tiles_per_frame - 1) / tiles_per_frame;
+
+      // Limit number of frames to 1.25x the number of threads for the case
+      // where there is only 1 tile per frame.
+      frames = CLIP(1, threads * 4 / 3, frames);
+      cfg->owf = frames - 1;
+    }
+
+    fprintf(stderr, "--owf=auto value set to %d.\n", cfg->owf);
+  }
+
   // Do more validation to make sure the parameters we have make sense.
   if (!config_validate(cfg)) {
     goto exit_failure;