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, 71 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_subdiv_modifier.h b/source/blender/blenkernel/BKE_subdiv_modifier.h
new file mode 100644
index 00000000000..94068613101
--- /dev/null
+++ b/source/blender/blenkernel/BKE_subdiv_modifier.h
@@ -0,0 +1,71 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2021 by Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup bke
+ */
+
+#pragma once
+
+#include "BLI_sys_types.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct Mesh;
+struct Object;
+struct Scene;
+struct Subdiv;
+struct SubdivSettings;
+struct SubsurfModifierData;
+
+void BKE_subsurf_modifier_subdiv_settings_init(struct SubdivSettings *settings,
+                                               const struct SubsurfModifierData *smd,
+                                               const bool use_render_params);
+
+/* If skip_check_is_last is true, we assume that the modifier passed is the last enabled modifier
+ * in the stack. */
+bool BKE_subsurf_modifier_can_do_gpu_subdiv_ex(const struct Scene *scene,
+                                               const struct Object *ob,
+                                               const struct SubsurfModifierData *smd,
+                                               int required_mode,
+                                               bool skip_check_is_last);
+
+bool BKE_subsurf_modifier_can_do_gpu_subdiv(const struct Scene *scene,
+                                            const struct Object *ob,
+                                            const int required_mode);
+
+extern void (*BKE_subsurf_modifier_free_gpu_cache_cb)(struct Subdiv *subdiv);
+
+struct Subdiv *BKE_subsurf_modifier_subdiv_descriptor_ensure(
+    const struct SubsurfModifierData *smd,
+    const struct SubdivSettings *subdiv_settings,
+    const struct Mesh *mesh,
+    const bool for_draw_code);
+
+struct SubsurfRuntimeData *BKE_subsurf_modifier_ensure_runtime(struct SubsurfModifierData *smd);
+
+/* Return the #ModifierMode required for the evaluation of the subsurf modifier, which should be
+ * used to check if the modifier is enabled. */
+int BKE_subsurf_modifier_eval_required_mode(bool is_final_render, bool is_edit_mode);
+
+#ifdef __cplusplus
+}
+#endif