Fix T98091: EEVEE: Volume: Crash caused by non-present grid

This was caused by the `copy_m4_m4` trying to copy the `object_to_texture`
from `drw_grid` which was `nullptr`.

Fixing this also exposed that rendering such volumes (without any valid
grid attributes) is not supported and we should follow what Cycles does.

Differential Revision: https://developer.blender.org/D15147

2 files changed, 11 insertions, 3 deletions

diff --git a/source/blender/draw/intern/draw_common.h b/source/blender/draw/intern/draw_common.h
index 779ac43178c..c078a393b35 100644
--- a/source/blender/draw/intern/draw_common.h
+++ b/source/blender/draw/intern/draw_common.h
@@ -91,7 +91,7 @@ void DRW_curves_free(void);
 /**
  * Add attributes bindings of volume grids to an existing shading group.
  * No draw call is added so the caller can decide how to use the data.
- * \return nullptr if there is something to draw.
+ * \return nullptr if there is nothing to draw.
  */
 struct DRWShadingGroup *DRW_shgroup_volume_create_sub(struct Scene *scene,
                                                       struct Object *ob,
diff --git a/source/blender/draw/intern/draw_volume.cc b/source/blender/draw/intern/draw_volume.cc
index 8d9a6f486e2..c4e58ab24cb 100644
--- a/source/blender/draw/intern/draw_volume.cc
+++ b/source/blender/draw/intern/draw_volume.cc
@@ -129,12 +129,14 @@ static DRWShadingGroup *drw_volume_object_grids_init(Object *ob,
   volume_infos.temperature_bias = 0.0f;
 
   /* Bind volume grid textures. */
-  int grid_id = 0;
+  int grid_id = 0, grids_len = 0;
   LISTBASE_FOREACH (GPUMaterialAttribute *, attr, attrs) {
     const VolumeGrid *volume_grid = BKE_volume_grid_find_for_read(volume, attr->name);
     const DRWVolumeGrid *drw_grid = (volume_grid) ?
                                         DRW_volume_batch_cache_get_grid(volume, volume_grid) :
                                         nullptr;
+    /* Count number of valid attributes. */
+    grids_len += int(volume_grid != nullptr);
 
     /* Handle 3 cases here:
      * - Grid exists and texture was loaded -> use texture.
@@ -145,7 +147,13 @@ static DRWShadingGroup *drw_volume_object_grids_init(Object *ob,
                                                  grid_default_texture(attr->default_value);
     DRW_shgroup_uniform_texture(grp, attr->input_name, grid_tex);
 
-    copy_m4_m4(volume_infos.grids_xform[grid_id++].ptr(), drw_grid->object_to_texture);
+    copy_m4_m4(volume_infos.grids_xform[grid_id++].ptr(),
+               (drw_grid) ? drw_grid->object_to_texture : g_data.dummy_grid_mat);
+  }
+  /* Render nothing if there is no attribute for the shader to render.
+   * This also avoids an assert caused by the bounding box being zero in size. */
+  if (grids_len == 0) {
+    return nullptr;
   }
 
   volume_infos.push_update();