DrawManager: Engine Instance Data.

In the original design draw engines had to copy with a limitation that
they were not allowed to reuse complex data structures between drawing
calls. Data that could be reused were limited to:
- GPUFramebuffers
- GPUTextures
- Memory that could be removed calling MEM_freeN (storage list)
- DRWPass

This is fine when the storage list contains arrays or structs but when
more complex data types (vectors, maps) etc wasn't possible.

This patch adds instance_data that can be reused between drawing calls.
The instance_data is controlled by the draw engine and doesn't need to
be limited as described above.

When an engines stores instance_data it must implement the
`DrawEngineType.instance_free` callback to free the data.

The patch originates from eevee rewrite. But was added to master as the
image engine rewrite also has a need for it.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D13425

3 files changed, 14 insertions, 0 deletions

diff --git a/source/blender/draw/intern/DRW_render.h b/source/blender/draw/intern/DRW_render.h
index c1cab931f6a..6491f5ca411 100644
--- a/source/blender/draw/intern/DRW_render.h
+++ b/source/blender/draw/intern/DRW_render.h
@@ -121,6 +121,8 @@ typedef struct DrawEngineType {
   void (*engine_init)(void *vedata);
   void (*engine_free)(void);
 
+  void (*instance_free)(void *instance_data);
+
   void (*cache_init)(void *vedata);
   void (*cache_populate)(void *vedata, struct Object *ob);
   void (*cache_finish)(void *vedata);
diff --git a/source/blender/draw/intern/draw_view_data.cc b/source/blender/draw/intern/draw_view_data.cc
index 55ebbf82c29..af85584a054 100644
--- a/source/blender/draw/intern/draw_view_data.cc
+++ b/source/blender/draw/intern/draw_view_data.cc
@@ -120,6 +120,12 @@ static void draw_viewport_engines_data_clear(ViewportEngineData *data)
     MEM_SAFE_FREE(data->stl->storage[i]);
   }
 
+  if (data->instance_data) {
+    BLI_assert(engine_type->instance_free != nullptr);
+    engine_type->instance_free(data->instance_data);
+    data->instance_data = nullptr;
+  }
+
   MEM_SAFE_FREE(data->fbl);
   MEM_SAFE_FREE(data->txl);
   MEM_SAFE_FREE(data->psl);
diff --git a/source/blender/draw/intern/draw_view_data.h b/source/blender/draw/intern/draw_view_data.h
index c8176170a61..01b614fb021 100644
--- a/source/blender/draw/intern/draw_view_data.h
+++ b/source/blender/draw/intern/draw_view_data.h
@@ -63,6 +63,12 @@ typedef struct ViewportEngineData {
   TextureList *txl;
   PassList *psl;
   StorageList *stl;
+  /**
+   * \brief Memory block that can be freely used by the draw engine.
+   * When used the draw engine must implement #DrawEngineType.instance_free callback.
+   */
+  void *instance_data;
+
   char info[GPU_INFO_SIZE];
 
   /* we may want to put this elsewhere */