OpenSubDiv: add support for an OpenGL evaluator

This evaluator is used in order to evaluate subdivision at render time, allowing for
faster renders of meshes with a subdivision surface modifier placed at the last
position in the modifier list.

When evaluating the subsurf modifier, we detect whether we can delegate evaluation
to the draw code. If so, the subdivision is first evaluated on the GPU using our own
custom evaluator (only the coarse data needs to be initially sent to the GPU), then,
buffers for the final `MeshBufferCache` are filled on the GPU using a set of
compute shaders. However, some buffers are still filled on the CPU side, if doing so
on the GPU is impractical (e.g. the line adjacency buffer used for x-ray, whose
logic is hardly GPU compatible).

This is done at the mesh buffer extraction level so that the result can be readily used
in the various OpenGL engines, without having to write custom geometry or tesselation
shaders.

We use our own subdivision evaluation shaders, instead of OpenSubDiv's vanilla one, in
order to control the data layout, and interpolation. For example, we store vertex colors
as compressed 16-bit integers, while OpenSubDiv's default evaluator only work for float
types.

In order to still access the modified geometry on the CPU side, for use in modifiers
or transform operators, a dedicated wrapper type is added `MESH_WRAPPER_TYPE_SUBD`.
Subdivision will be lazily evaluated via `BKE_object_get_evaluated_mesh` which will
create such a wrapper if possible. If the final subdivision surface is not needed on
the CPU side, `BKE_object_get_evaluated_mesh_no_subsurf` should be used.

Enabling or disabling GPU subdivision can be done through the user preferences (under
Viewport -> Subdivision).

See patch description for benchmarks.

Reviewed By: campbellbarton, jbakker, fclem, brecht, #eevee_viewport

Differential Revision: https://developer.blender.org/D12406

1 files changed, 56 insertions, 0 deletions

diff --git a/source/blender/draw/intern/shaders/common_subdiv_normals_accumulate_comp.glsl b/source/blender/draw/intern/shaders/common_subdiv_normals_accumulate_comp.glsl
new file mode 100644
index 00000000000..575090472b1
--- /dev/null
+++ b/source/blender/draw/intern/shaders/common_subdiv_normals_accumulate_comp.glsl
@@ -0,0 +1,56 @@
+
+/* To be compile with common_subdiv_lib.glsl */
+
+layout(std430, binding = 0) readonly buffer inputVertexData
+{
+  PosNorLoop pos_nor[];
+};
+
+layout(std430, binding = 1) readonly buffer faceAdjacencyOffsets
+{
+  uint face_adjacency_offsets[];
+};
+
+layout(std430, binding = 2) readonly buffer faceAdjacencyLists
+{
+  uint face_adjacency_lists[];
+};
+
+layout(std430, binding = 3) writeonly buffer vertexNormals
+{
+  vec3 normals[];
+};
+
+void main()
+{
+  uint vertex_index = get_global_invocation_index();
+  if (vertex_index >= total_dispatch_size) {
+    return;
+  }
+
+  uint first_adjacent_face_offset = face_adjacency_offsets[vertex_index];
+  uint number_of_adjacent_faces = face_adjacency_offsets[vertex_index + 1] -
+                                  first_adjacent_face_offset;
+
+  vec3 accumulated_normal = vec3(0.0);
+
+  /* For each adjacent face. */
+  for (uint i = 0; i < number_of_adjacent_faces; i++) {
+    uint adjacent_face = face_adjacency_lists[first_adjacent_face_offset + i];
+    uint start_loop_index = adjacent_face * 4;
+
+    /* Compute face normal. */
+    vec3 adjacent_verts[3];
+    for (uint j = 0; j < 3; j++) {
+      adjacent_verts[j] = get_vertex_pos(pos_nor[start_loop_index + j]);
+    }
+
+    vec3 face_normal = normalize(
+        cross(adjacent_verts[1] - adjacent_verts[0], adjacent_verts[2] - adjacent_verts[0]));
+    accumulated_normal += face_normal;
+  }
+
+  float weight = 1.0 / float(number_of_adjacent_faces);
+  vec3 normal = normalize(accumulated_normal);
+  normals[vertex_index] = normal;
+}