intern/cycles/integrator/render_scheduler.cpp

@@ -886,7 +886,7 @@ int RenderScheduler::get_num_samples_during_navigation(int resolution_divider) c
 {
   /* Special trick for fast navigation: schedule multiple samples during fast navigation
    * (which will prefer to use lower resolution to keep up with refresh rate). This gives more
-   * usable visual feedback for artists. There are a couple of tricks though. */
+   * usable visual feedback for artists. */
 
   if (is_denoise_active_during_update()) {
     /* When denoising is used during navigation prefer using a higher resolution with less samples
@@ -896,25 +896,12 @@ int RenderScheduler::get_num_samples_during_navigation(int resolution_divider) c
     return 1;
   }
 
-  if (resolution_divider <= pixel_size_) {
-    /* When resolution divider is at or below pixel size, schedule one sample. This doesn't effect
-     * the sample count at this resolution division, but instead assists in the calculation of
-     * the resolution divider. */
-    return 1;
-  }
-
-  if (resolution_divider == pixel_size_ * 2) {
-    /* When resolution divider is the previous step to the final resolution, schedule two samples.
-     * This is so that rendering on lower resolution does not exceed time that it takes to render
-     * first sample at the full resolution. */
-    return 2;
-  }
-
-  /* Always render 4 samples, even if scene is configured for less.
-   * The idea here is to have enough information on the screen. Resolution divider of 2 allows us
-   * to have 4 time extra samples, so overall worst case timing is the same as the final resolution
-   * at one sample. */
-  return 4;
+  /* Schedule samples equal to the resolution divider up to a maximum of 4.
+   * The idea is to have enough information on the screen by increasing the sample count as the
+   * resolution is decreased. */
+  /* NOTE: Changeing this formula will change the formula in
+   * "RenderScheduler::calculate_resolution_divider_for_time()"*/
+  return min(max(1, resolution_divider / pixel_size_), 4);
 }
 
 bool RenderScheduler::work_need_adaptive_filter() const
@@ -1100,9 +1087,10 @@ void RenderScheduler::update_start_resolution_divider()
   /* TODO(sergey): Need to add hysteresis to avoid resolution divider bouncing around when actual
    * render time is somewhere on a boundary between two resolutions. */
 
-  /* Never increase resolution to higher than the pixel size (which is possible if the scene is
-   * simple and compute device is fast). */
-  start_resolution_divider_ = max(resolution_divider_for_update, pixel_size_);
+  /* Don't let resolution drop below the desired one. It's better to be slow than provide an
+   * unreadable viewport render. */
+  start_resolution_divider_ = min(resolution_divider_for_update,
+                                  default_start_resolution_divider_);
 
   VLOG_WORK << "Calculated resolution divider is " << start_resolution_divider_;
 }
@@ -1187,24 +1175,24 @@ void RenderScheduler::check_time_limit_reached()
 
 int RenderScheduler::calculate_resolution_divider_for_time(double desired_time, double actual_time)
 {
-  /* TODO(sergey): There should a non-iterative analytical formula here. */
+  const double ratio_between_times = actual_time / desired_time;
 
-  int resolution_divider = 1;
+  /* We can pass "ratio_between_times" to "get_num_samples_during_navigation()" to get our
+   * navigation samples because the equation for calculating the resolution divider is as follows:
+   * "actual_time / desired_time = sqr(resolution_divider) / sample_count".
+   * While "resolution_divider" is less than or equal to 4, "resolution_divider = sample_count"
+   * (This relationship is determined in "get_num_samples_during_navigation()"). With some
+   * substitution we end up with "actual_time / desired_time = resolution_divider" while the
+   * resolution divider is less than or equal to 4. Once the resolution divider increases above 4,
+   * the relationsip of "actual_time / desired_time = resolution_divider" is no longer true,
+   * however the sample count retrieved from "get_num_samples_during_navigation()" is still
+   * accurate if we continue using this assumption. It should be noted that the interaction between
+   * pixel_size, sample count, and resolution divider are automatically accounted for and that's
+   * why pixel_size isn't included in any of the equations. */
+  const int navigation_samples = get_num_samples_during_navigation(
+      ceil_to_int(ratio_between_times));
 
-  /* This algorithm iterates through resolution dividers until a divider is found that achieves
-   * the desired render time. A limit of default_start_resolution_divider_ is put in place as the
-   * maximum resolution divider to avoid an unreadable viewport due to a low resolution.
-   * pre_resolution_division_samples and post_resolution_division_samples are used in this
-   * calculation to better predict the performance impact of changing resolution divisions as
-   * the sample count can also change between resolution divisions. */
-  while (actual_time > desired_time && resolution_divider < default_start_resolution_divider_) {
-    int pre_resolution_division_samples = get_num_samples_during_navigation(resolution_divider);
-    resolution_divider = resolution_divider * 2;
-    int post_resolution_division_samples = get_num_samples_during_navigation(resolution_divider);
-    actual_time /= 4.0 * pre_resolution_division_samples / post_resolution_division_samples;
-  }
-
-  return resolution_divider;
+  return ceil_to_int(sqrt(navigation_samples * ratio_between_times));
 }
 
 int calculate_resolution_divider_for_resolution(int width, int height, int resolution)