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Tweak comment regarding Sculpt mode undo issues in object update code. 

Original comment from rB8a9dedf82954. See also T84084.
This commit is contained in:
Bastien Montagne 2020-12-28 14:16:14 +01:00
parent 823f0da702
commit 26c34a2a3a
1 changed files with 24 additions and 19 deletions
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43
source/blender/blenkernel/intern/multires_reshape_ccg.c
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@ -60,25 +60,30 @@ bool multires_reshape_assign_final_coords_from_ccg(const MultiresReshapeContext
CCG_grid_elem_co(&reshape_level_key, ccg_grid, x, y),
sizeof(float[3]));
if (reshape_level_key.has_mask) {
/* Assert about a non-NULL `grid_element.mask` may fail here, this code may be called
* from cleanup code during COW evaluation phase by depsgraph (e.g.
* `object_update_from_subsurf_ccg` call in `BKE_object_free_derived_caches`).
*
* `reshape_level_key.has_mask` is ultimately set from MultiRes modifier apply code
* (through `multires_as_ccg` -> `multires_ccg_settings_init`), when object is in sculpt
* mode only, and there is matching loop cdlayer.
*
* `grid_element.mask` is directly set from existing matching loop cdlayer during
* initialization of `MultiresReshapeContext` struct.
*
* Since ccg data is preserved during undos, we may end up with a state where there is no
* mask data in mesh loops' cdlayer, while ccg's `has_mask` is still set to true.
*/
// BLI_assert(grid_element.mask != NULL);
if (grid_element.mask != NULL) {
*grid_element.mask = *CCG_grid_elem_mask(&reshape_level_key, ccg_grid, x, y);
}
/* NOTE: The sculpt mode might have SubdivCCG's data out of sync from what is stored in
* the original object. This happens upon the following scenario:
*
* - User enters sculpt mode of the default cube object.
* - Sculpt mode creates new `layer`
* - User does some strokes.
* - User used undo until sculpt mode is exited.
*
* In an ideal world the sculpt mode will take care of keeping CustomData and CCG layers in
* sync by doing proper pushes to a local sculpt undo stack.
*
* Since the proper solution needs time to be implemented, consider the target object
* the source of truth of which data layers are to be updated during reshape. This means,
* for example, that if the undo system says object does not have paint mask layer, it is
* not to be updated.
*
* This is a fragile logic, and is only working correctly because the code path is only
* used by sculpt changes. In other usecases the code might not catch inconsistency and
* silently do wrong decision. */
/* NOTE: There is a known bug in Undo code that results in first Sculpt step
* after a Memfile one to never be undone (see T83806). This might be the root cause of
* this inconsistency. */
if (reshape_level_key.has_mask && grid_element.mask != NULL) {
*grid_element.mask = *CCG_grid_elem_mask(&reshape_level_key, ccg_grid, x, y);
}
}
}