Cycles / Decoupled Ray Marching: Skip consecutive empty steps.

This merges consecutive empty steps in the decoupled record function,
which can lead to fewer iterations in the scatter functions.

Only helps slightly though (1%), but doesn't hurt to have this.

Differential Revision: https://developer.blender.org/D873

1 files changed, 17 insertions, 6 deletions

diff --git a/intern/cycles/kernel/kernel_volume.h b/intern/cycles/kernel/kernel_volume.h
index 8e67ad5464c..0300e1d4c7f 100644
--- a/intern/cycles/kernel/kernel_volume.h
+++ b/intern/cycles/kernel/kernel_volume.h
@@ -649,6 +649,7 @@ ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg, PathState *sta
 
 	segment->numsteps = 0;
 	segment->closure_flag = 0;
+	bool is_last_step_empty = false;
 
 	VolumeStep *step = segment->steps;
 
@@ -690,12 +691,24 @@ ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg, PathState *sta
 			step->closure_flag = closure_flag;
 
 			segment->closure_flag |= closure_flag;
+
+			is_last_step_empty = false;
+			segment->numsteps++;
 		}
 		else {
-			/* store empty step (todo: skip consecutive empty steps) */
-			step->sigma_t = make_float3(0.0f, 0.0f, 0.0f);
-			step->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
-			step->closure_flag = 0;
+			if(is_last_step_empty) {
+				/* consecutive empty step, merge */
+				step--;
+			}
+			else {
+				/* store empty step */
+				step->sigma_t = make_float3(0.0f, 0.0f, 0.0f);
+				step->sigma_s = make_float3(0.0f, 0.0f, 0.0f);
+				step->closure_flag = 0;
+
+				segment->numsteps++;
+				is_last_step_empty = true;
+			}
 		}
 
 		step->accum_transmittance = accum_transmittance;
@@ -703,8 +716,6 @@ ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg, PathState *sta
 		step->t = new_t;
 		step->shade_t = t + random_jitter_offset;
 
-		segment->numsteps++;
-
 		/* stop if at the end of the volume */
 		t = new_t;
 		if(t == ray->t)