Fix T94729: GPU subdivision does not support meshes without polygons

There are two issues revealed in the bug report:
- the GPU subdivision does not support meshes with only loose geometry
- the loose geometry is not subdivided

For the first case, checks are added to ensure we still fill the
buffers with loose geometry even if no polygons are present.

For the second case, this adds
`BKE_subdiv_mesh_interpolate_position_on_edge` which encapsulates the
loose vertex interpolation mechanism previously found in
`subdiv_mesh_vertex_of_loose_edge`.

The subdivided loose geometry is stored in a new specific data structure
`DRWSubdivLooseGeom` so as to not pollute `MeshExtractLooseGeom`. These
structures store the corresponding coarse element data, which will be
used for filling GPU buffers appropriately.

Differential Revision: https://developer.blender.org/D14171

1 files changed, 23 insertions, 2 deletions

diff --git a/source/blender/draw/intern/draw_cache_extract_mesh.cc b/source/blender/draw/intern/draw_cache_extract_mesh.cc
index 8b4962f24b0..fb791916b46 100644
--- a/source/blender/draw/intern/draw_cache_extract_mesh.cc
+++ b/source/blender/draw/intern/draw_cache_extract_mesh.cc
@@ -822,7 +822,27 @@ static void mesh_buffer_cache_create_requested_subdiv(MeshBatchCache *cache,
     extractors.append(&extract_fdots_pos);
   }
 
-  EXTRACT_ADD_REQUESTED(ibo, lines);
+  if (DRW_ibo_requested(mbuflist->ibo.lines_loose)) {
+    /* `ibo.lines_loose` require the `ibo.lines` buffer. */
+    if (mbuflist->ibo.lines == nullptr) {
+      DRW_ibo_request(nullptr, &mbuflist->ibo.lines);
+    }
+    const MeshExtract *extractor = DRW_ibo_requested(mbuflist->ibo.lines) ?
+                                       &extract_lines_with_lines_loose :
+                                       &extract_lines_loose_only;
+    extractors.append(extractor);
+  }
+  else if (DRW_ibo_requested(mbuflist->ibo.lines)) {
+    const MeshExtract *extractor;
+    if (mbuflist->ibo.lines_loose != nullptr) {
+      /* Update `ibo.lines_loose` as it depends on `ibo.lines`. */
+      extractor = &extract_lines_with_lines_loose;
+    }
+    else {
+      extractor = &extract_lines;
+    }
+    extractors.append(extractor);
+  }
   EXTRACT_ADD_REQUESTED(ibo, edituv_points);
   EXTRACT_ADD_REQUESTED(ibo, edituv_tris);
   EXTRACT_ADD_REQUESTED(ibo, edituv_lines);
@@ -851,6 +871,7 @@ static void mesh_buffer_cache_create_requested_subdiv(MeshBatchCache *cache,
 
   mesh_render_data_update_looptris(mr, MR_ITER_LOOPTRI, MR_DATA_LOOPTRI);
   mesh_render_data_update_loose_geom(mr, mbc, MR_ITER_LEDGE | MR_ITER_LVERT, MR_DATA_LOOSE_GEOM);
+  DRW_subdivide_loose_geom(subdiv_cache, mbc);
 
   void *data_stack = MEM_mallocN(extractors.data_size_total(), __func__);
   uint32_t data_offset = 0;
@@ -884,7 +905,7 @@ static void mesh_buffer_cache_create_requested_subdiv(MeshBatchCache *cache,
     }
 
     if (extractor->iter_loose_geom_subdiv) {
-      extractor->iter_loose_geom_subdiv(subdiv_cache, mr, &mbc->loose_geom, buffer, data);
+      extractor->iter_loose_geom_subdiv(subdiv_cache, mr, buffer, data);
     }
 
     if (extractor->finish_subdiv) {