Loop selection corner case #84906

New Issue

Stanislav Blinov · 2021-01-20T16:43:03+01:00

Stanislav Blinov commented

2021-01-20 16:43:03 +01:00

Blender Version
Broken: 2.93 master
Worked: never?

Short description of error

(pun almost intended)

In the left image, alt-clicking the highlighted edge only selects that edge, whereas alt-clicking any other edge along that loop selects the whole loop (including that edge). However, if faces are extruded to make the mesh in the right image, then alt-clicking that edge selects the whole loop.

Note that even though the edge does border an n-gon, there's still a clearly defined direction for the loop as the edge leads to a 4-pole.

loop_selection_issue.blend

**Blender Version** Broken: 2.93 master Worked: never? **Short description of error** (pun almost intended) ![image.png](https://archive.blender.org/developer/F9589686/image.png) In the left image, alt-clicking the highlighted edge only selects that edge, whereas alt-clicking any other edge along that loop selects the whole loop (including that edge). However, if faces are extruded to make the mesh in the right image, then alt-clicking that edge selects the whole loop. Note that even though the edge does border an n-gon, there's still a clearly defined direction for the loop as the edge leads to a 4-pole. [loop_selection_issue.blend](https://archive.blender.org/developer/F9589694/loop_selection_issue.blend)

Stanislav Blinov commented

2021-01-20 16:43:03 +01:00

Added subscriber: @Stan_Pancakes

Germano Cavalcante commented

2021-01-21 15:37:29 +01:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2021-01-21 15:37:29 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Germano Cavalcante commented

2021-01-21 15:37:29 +01:00

Thanks for the report I can confirm the problem.
Although there is an n-gon, in fact the behavior seems strange.
This is related to 01b3e9cc

Thanks for the report I can confirm the problem. Although there is an n-gon, in fact the behavior seems strange. This is related to 01b3e9cc

Falk David commented

2021-01-28 12:58:16 +01:00

Added subscribers: @ideasman42, @filedescriptor

Falk David commented

2021-01-28 12:58:16 +01:00

I looked into this and found a "fix" (I am not sure if it has any side effects):
P1924: Potential fix for #84906?

diff --git a/source/blender/bmesh/intern/bmesh_walkers_impl.c b/source/blender/bmesh/intern/bmesh_walkers_impl.c
index 7d56e560275..0a9f2d60cab 100644
--- a/source/blender/bmesh/intern/bmesh_walkers_impl.c
+++ b/source/blender/bmesh/intern/bmesh_walkers_impl.c
@@ -862,32 +862,7 @@ static void bmw_EdgeLoopWalker_begin(BMWalker *walker, void *data)
   lwalk->lastv = lwalk->startv = v;
   lwalk->is_boundary = BM_edge_is_boundary(e);
   lwalk->is_single = (lwalk->is_boundary && bm_edge_is_single(e));
-
-  /* could also check that vertex*/
-  if ((lwalk->is_boundary == false) && (vert_edge_count- [x] == 3 || vert_edge_count- [x] == 3)) {
-    BMIter iter;
-    BMFace *f_iter;
-    BMFace *f_best = NULL;
-
-    BM_ITER_ELEM (f_iter, &iter, e, BM_FACES_OF_EDGE) {
-      if (f_best == NULL || f_best->len < f_iter->len) {
-        f_best = f_iter;
-      }
-    }
-
-    if (f_best) {
-      /* only use hub selection for 5+ sides else this could
-       * conflict with normal edge loop selection. */
-      lwalk->f_hub = f_best->len > 4 ? f_best : NULL;
-    }
-    else {
-      /* edge doesn't have any faces connected to it */
-      lwalk->f_hub = NULL;
-    }
-  }
-  else {
-    lwalk->f_hub = NULL;
-  }
+  lwalk->f_hub = NULL;
 
   /* rewind */
   while ((owalk_pt = BMW_current_state(walker))) {

That if that I removed gets executed when the lower edge with the n-gon in the example is (loop) selected. Later in the step function, we only walk along the n-gon if the next vertex has exactly 3 neighbours. when it does not, the iteration ends. This seems very strange.

To summarize: I don't know what the use of the f_hub is or what the if in the bmw_EdgeLoopWalker_begin function should do. Sadly the comment on line 10 in the diff seems to incomplete. Removing that if (and therefore the use of f_hub) seems to fix the problem.

@ideasman42 what are your thoughts on this? Do you know what that code is for?

I looked into this and found a "fix" (I am not sure if it has any side effects): [P1924: Potential fix for #84906?](https://archive.blender.org/developer/P1924.txt) ```diff diff --git a/source/blender/bmesh/intern/bmesh_walkers_impl.c b/source/blender/bmesh/intern/bmesh_walkers_impl.c index 7d56e560275..0a9f2d60cab 100644 --- a/source/blender/bmesh/intern/bmesh_walkers_impl.c +++ b/source/blender/bmesh/intern/bmesh_walkers_impl.c @@ -862,32 +862,7 @@ static void bmw_EdgeLoopWalker_begin(BMWalker *walker, void *data) lwalk->lastv = lwalk->startv = v; lwalk->is_boundary = BM_edge_is_boundary(e); lwalk->is_single = (lwalk->is_boundary && bm_edge_is_single(e)); - - /* could also check that vertex*/ - if ((lwalk->is_boundary == false) && (vert_edge_count- [x] == 3 || vert_edge_count- [x] == 3)) { - BMIter iter; - BMFace *f_iter; - BMFace *f_best = NULL; - - BM_ITER_ELEM (f_iter, &iter, e, BM_FACES_OF_EDGE) { - if (f_best == NULL || f_best->len < f_iter->len) { - f_best = f_iter; - } - } - - if (f_best) { - /* only use hub selection for 5+ sides else this could - * conflict with normal edge loop selection. */ - lwalk->f_hub = f_best->len > 4 ? f_best : NULL; - } - else { - /* edge doesn't have any faces connected to it */ - lwalk->f_hub = NULL; - } - } - else { - lwalk->f_hub = NULL; - } + lwalk->f_hub = NULL; /* rewind */ while ((owalk_pt = BMW_current_state(walker))) { ``` That `if` that I removed gets executed when the lower edge with the n-gon in the example is (loop) selected. Later in the step function, we only walk along the n-gon if the next vertex has exactly 3 neighbours. when it does not, the iteration ends. This seems very strange. To summarize: I don't know what the use of the `f_hub` is or what the `if` in the `bmw_EdgeLoopWalker_begin` function should do. Sadly the comment on line 10 in the diff seems to incomplete. Removing that `if` (and therefore the use of `f_hub`) seems to fix the problem. @ideasman42 what are your thoughts on this? Do you know what that code is for?

blender-admin commented

2021-01-28 23:47:17 +01:00

This issue was referenced by 213f8294b5

This issue was referenced by 213f8294b5c801c367fbddd6c1b4e6656908e71e

Campbell Barton commented

2021-01-29 00:09:29 +01:00

Changed status from 'Confirmed' to: 'Resolved'

Campbell Barton closed this issue

2021-01-29 00:09:29 +01:00

Campbell Barton self-assigned this 2021-01-29 00:09:29 +01:00

Campbell Barton commented

2021-01-29 00:10:39 +01:00

@mano-wii & @filedescriptor, thanks for looking into this.

The f_hub was introduced in 01b3e9cc9f.

This is the image linked (recovered from the archive) although the image along is still not enough to explain the behavior.

Committed more detailed description of this feature 1e355b0992 to avoid this kind of confusion in the future.

@mano-wii & @filedescriptor, thanks for looking into this. The `f_hub` was introduced in 01b3e9cc9f. This is the image linked (recovered from the archive) ![BMesh_NGon_Loop_Select.png](https://archive.blender.org/developer/F9603290/BMesh_NGon_Loop_Select.png) although the image along is still not enough to explain the behavior. Committed more detailed description of this feature 1e355b0992 to avoid this kind of confusion in the future.

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Loop selection corner case #84906