DrawManager: Use Compute Shader to Update Hair.

This patch will use compute shaders to create the VBO for hair.
The previous implementation uses tranform feedback.

Timings master (transform feedback with GPU_USAGE_STATIC between 0.000069s and 0.000362s
Timings transform feedback with GPU_USAGE_DEVICE_ONLY. between 0.000057s and 0.000122s
Timings compute shader between 0.000032 and 0.000092s

Future improvements:
* Generate hair Index buffer using compute shaders: currently done single threaded on CPU, easy to add as compute shader.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D11057

3 files changed, 100 insertions, 47 deletions

diff --git a/source/blender/draw/intern/shaders/common_hair_lib.glsl b/source/blender/draw/intern/shaders/common_hair_lib.glsl
index 8684d82f228..02c335ddae2 100644
--- a/source/blender/draw/intern/shaders/common_hair_lib.glsl
+++ b/source/blender/draw/intern/shaders/common_hair_lib.glsl
@@ -28,6 +28,9 @@ uniform bool hairCloseTip = true;
 
 uniform vec4 hairDupliMatrix[4];
 
+/* Strand batch offset when used in compute shaders. */
+uniform int hairStrandOffset = 0;
+
 /* -- Per control points -- */
 uniform samplerBuffer hairPointBuffer; /* RGBA32F */
 #define point_position xyz
@@ -43,13 +46,37 @@ uniform usamplerBuffer hairStrandSegBuffer; /* R16UI */
 
 /* -- Subdivision stage -- */
 /**
- * We use a transform feedback to preprocess the strands and add more subdivision to it.
- * For the moment these are simple smooth interpolation but one could hope to see the full
+ * We use a transform feedback or compute shader to preprocess the strands and add more subdivision
+ * to it. For the moment these are simple smooth interpolation but one could hope to see the full
  * children particle modifiers being evaluated at this stage.
  *
  * If no more subdivision is needed, we can skip this step.
  */
 
+#ifdef GPU_VERTEX_SHADER
+float hair_get_local_time()
+{
+  return float(gl_VertexID % hairStrandsRes) / float(hairStrandsRes - 1);
+}
+
+int hair_get_id()
+{
+  return gl_VertexID / hairStrandsRes;
+}
+#endif
+
+#ifdef GPU_COMPUTE_SHADER
+float hair_get_local_time()
+{
+  return float(gl_GlobalInvocationID.y) / float(hairStrandsRes - 1);
+}
+
+int hair_get_id()
+{
+  return int(gl_GlobalInvocationID.x) + hairStrandOffset;
+}
+#endif
+
 #ifdef HAIR_PHASE_SUBDIV
 int hair_get_base_id(float local_time, int strand_segments, out float interp_time)
 {
@@ -64,9 +91,9 @@ int hair_get_base_id(float local_time, int strand_segments, out float interp_tim
 void hair_get_interp_attrs(
     out vec4 data0, out vec4 data1, out vec4 data2, out vec4 data3, out float interp_time)
 {
-  float local_time = float(gl_VertexID % hairStrandsRes) / float(hairStrandsRes - 1);
+  float local_time = hair_get_local_time();
 
-  int hair_id = gl_VertexID / hairStrandsRes;
+  int hair_id = hair_get_id();
   int strand_offset = int(texelFetch(hairStrandBuffer, hair_id).x);
   int strand_segments = int(texelFetch(hairStrandSegBuffer, hair_id).x);
 
@@ -96,6 +123,7 @@ void hair_get_interp_attrs(
  */
 
 #if !defined(HAIR_PHASE_SUBDIV) && defined(GPU_VERTEX_SHADER)
+
 int hair_get_strand_id(void)
 {
   return gl_VertexID / (hairStrandsRes * hairThicknessRes);
@@ -227,3 +255,45 @@ vec2 hair_resolve_barycentric(vec2 vert_barycentric)
     return vec2(1.0 - vert_barycentric.x, 0.0);
   }
 }
+
+/* Hair interpolation functions. */
+vec4 hair_get_weights_cardinal(float t)
+{
+  float t2 = t * t;
+  float t3 = t2 * t;
+#if defined(CARDINAL)
+  float fc = 0.71;
+#else /* defined(CATMULL_ROM) */
+  float fc = 0.5;
+#endif
+
+  vec4 weights;
+  /* GLSL Optimized version of key_curve_position_weights() */
+  float fct = t * fc;
+  float fct2 = t2 * fc;
+  float fct3 = t3 * fc;
+  weights.x = (fct2 * 2.0 - fct3) - fct;
+  weights.y = (t3 * 2.0 - fct3) + (-t2 * 3.0 + fct2) + 1.0;
+  weights.z = (-t3 * 2.0 + fct3) + (t2 * 3.0 - (2.0 * fct2)) + fct;
+  weights.w = fct3 - fct2;
+  return weights;
+}
+
+/* TODO(fclem): This one is buggy, find why. (it's not the optimization!!) */
+vec4 hair_get_weights_bspline(float t)
+{
+  float t2 = t * t;
+  float t3 = t2 * t;
+
+  vec4 weights;
+  /* GLSL Optimized version of key_curve_position_weights() */
+  weights.xz = vec2(-0.16666666, -0.5) * t3 + (0.5 * t2 + 0.5 * vec2(-t, t) + 0.16666666);
+  weights.y = (0.5 * t3 - t2 + 0.66666666);
+  weights.w = (0.16666666 * t3);
+  return weights;
+}
+
+vec4 hair_interp_data(vec4 v0, vec4 v1, vec4 v2, vec4 v3, vec4 w)
+{
+  return v0 * w.x + v1 * w.y + v2 * w.z + v3 * w.w;
+}
diff --git a/source/blender/draw/intern/shaders/common_hair_refine_comp.glsl b/source/blender/draw/intern/shaders/common_hair_refine_comp.glsl
new file mode 100644
index 00000000000..4dcde4b0245
--- /dev/null
+++ b/source/blender/draw/intern/shaders/common_hair_refine_comp.glsl
@@ -0,0 +1,24 @@
+
+/*
+ * To be compiled with common_hair_lib.glsl.
+ */
+
+layout(local_size_x = 1, local_size_y = 1) in;
+layout(std430, binding = 0) writeonly buffer hairPointOutputBuffer
+{
+  vec4 posTime[];
+}
+out_vertbuf;
+
+void main(void)
+{
+  float interp_time;
+  vec4 data0, data1, data2, data3;
+  hair_get_interp_attrs(data0, data1, data2, data3, interp_time);
+
+  vec4 weights = hair_get_weights_cardinal(interp_time);
+  vec4 result = hair_interp_data(data0, data1, data2, data3, weights);
+
+  uint index = uint(hair_get_id() * hairStrandsRes) + gl_GlobalInvocationID.y;
+  out_vertbuf.posTime[index] = result;
+}
diff --git a/source/blender/draw/intern/shaders/common_hair_refine_vert.glsl b/source/blender/draw/intern/shaders/common_hair_refine_vert.glsl
index 3f5e3f8226f..371d43827b9 100644
--- a/source/blender/draw/intern/shaders/common_hair_refine_vert.glsl
+++ b/source/blender/draw/intern/shaders/common_hair_refine_vert.glsl
@@ -3,47 +3,6 @@
 
 out vec4 finalColor;
 
-vec4 get_weights_cardinal(float t)
-{
-  float t2 = t * t;
-  float t3 = t2 * t;
-#if defined(CARDINAL)
-  float fc = 0.71;
-#else /* defined(CATMULL_ROM) */
-  float fc = 0.5;
-#endif
-
-  vec4 weights;
-  /* GLSL Optimized version of key_curve_position_weights() */
-  float fct = t * fc;
-  float fct2 = t2 * fc;
-  float fct3 = t3 * fc;
-  weights.x = (fct2 * 2.0 - fct3) - fct;
-  weights.y = (t3 * 2.0 - fct3) + (-t2 * 3.0 + fct2) + 1.0;
-  weights.z = (-t3 * 2.0 + fct3) + (t2 * 3.0 - (2.0 * fct2)) + fct;
-  weights.w = fct3 - fct2;
-  return weights;
-}
-
-/* TODO(fclem): This one is buggy, find why. (it's not the optimization!!) */
-vec4 get_weights_bspline(float t)
-{
-  float t2 = t * t;
-  float t3 = t2 * t;
-
-  vec4 weights;
-  /* GLSL Optimized version of key_curve_position_weights() */
-  weights.xz = vec2(-0.16666666, -0.5) * t3 + (0.5 * t2 + 0.5 * vec2(-t, t) + 0.16666666);
-  weights.y = (0.5 * t3 - t2 + 0.66666666);
-  weights.w = (0.16666666 * t3);
-  return weights;
-}
-
-vec4 interp_data(vec4 v0, vec4 v1, vec4 v2, vec4 v3, vec4 w)
-{
-  return v0 * w.x + v1 * w.y + v2 * w.z + v3 * w.w;
-}
-
 #ifdef TF_WORKAROUND
 uniform int targetWidth;
 uniform int targetHeight;
@@ -56,8 +15,8 @@ void main(void)
   vec4 data0, data1, data2, data3;
   hair_get_interp_attrs(data0, data1, data2, data3, interp_time);
 
-  vec4 weights = get_weights_cardinal(interp_time);
-  finalColor = interp_data(data0, data1, data2, data3, weights);
+  vec4 weights = hair_get_weights_cardinal(interp_time);
+  finalColor = hair_interp_data(data0, data1, data2, data3, weights);
 
 #ifdef TF_WORKAROUND
   int id = gl_VertexID - idOffset;