Skin Modifier - topology changes randomly when entering and exiting edit mode #42893

New Issue

Matt Heimlich · 2014-12-14T10:11:40+01:00

Matt Heimlich commented

2014-12-14 10:11:40 +01:00

System Information
Win7 x64

Blender Version
Broken: c51ca1b

Short description of error
Generated mesh topology changes randomly when changing modes

Exact steps for others to reproduce the error
Open file, hit Tab a bunch of times

skinBug.blend

**System Information** Win7 x64 **Blender Version** Broken: c51ca1b **Short description of error** Generated mesh topology changes randomly when changing modes **Exact steps for others to reproduce the error** Open file, hit Tab a bunch of times [skinBug.blend](https://archive.blender.org/developer/F131374/skinBug.blend)

Matt Heimlich commented

2014-12-14 10:11:40 +01:00

Changed status to: 'Open'

Matt Heimlich commented

2014-12-14 10:11:40 +01:00

Added subscriber: @MatthewHeimlich

Julian Eisel commented

2014-12-14 14:49:03 +01:00

Added subscriber: @JulianEisel

Julian Eisel commented

2014-12-14 14:49:03 +01:00

This seems to be a gooseberry only bug. We currently don't have a project dedicated to that, but I don't like having gooseberry branch bugs set to #bf_blender. Will check with others if it's worth adding a "BF Blender: Gooseberry" project.

Julian Eisel commented

2014-12-14 16:48:00 +01:00

Changed status from 'Open' to: 'Archived'

Julian Eisel closed this issue

2014-12-14 16:48:00 +01:00

Julian Eisel self-assigned this 2014-12-14 16:48:00 +01:00

Julian Eisel commented

2014-12-14 16:48:00 +01:00

Okay, so in todays meeting we decided to not allow gooseberry branch bugs. I'd recommend to get in contact with the gooseberry developers in those cases.

But as always: thanks for the report @MatthewHeimlich :)

Okay, so in todays meeting we decided to not allow gooseberry branch bugs. I'd recommend to get in contact with the gooseberry developers in those cases. But as always: thanks for the report @MatthewHeimlich :)

Julian Eisel commented

2015-01-20 18:35:16 +01:00

@MatthewHeimlich asked for reopening on IRC, since the bug seems to appear on master too.
I still cann't confirm here, but he said some people on #blender did.

@MatthewHeimlich asked for reopening on IRC, since the bug seems to appear on master too. I still cann't confirm here, but he said some people on #blender did.

Julian Eisel commented

2015-01-20 18:35:24 +01:00

Changed status from 'Archived' to: 'Open'

Julian Eisel reopened this issue

2015-01-20 18:35:24 +01:00

Julian Eisel removed their assignment 2015-01-20 18:36:57 +01:00

Karja Krähwald commented

2015-01-21 05:09:50 +01:00

Added subscriber: @karja

Karja Krähwald commented

2015-01-21 05:09:50 +01:00

Hey,
can confirm this - Skin is working continously and not consistent on branched edges.

Want to add that this topology changes occur for nearly every action that can change position or geometry of the skin object (except G/R/S in object mode).
Objects that are related somehow to the skin object (= modifier target, constraints, linked mesh) also cause this problem.

Further examples:

Give Skin mesh a Copy Location Constraint (no target needed), turn it on and off.
Edit-Objectmode switch on another linked mesh.
Moving a mirror-object (mirror modifier target) around.

Use point 3 for the file and you should spot the flickering better while moving the target.

Hey, can confirm this - Skin is working continously and not consistent __on branched edges__. Want to add that this topology changes occur for nearly every action that can change position or geometry of the skin object (except G/R/S in object mode). Objects that are related somehow to the skin object (= modifier target, constraints, linked mesh) also cause this problem. Further examples: - Give Skin mesh a Copy Location Constraint (no target needed), turn it on and off. - Edit-Objectmode switch on another linked mesh. - Moving a mirror-object (mirror modifier target) around. >Use point 3 for the file and you should spot the flickering better while moving the target.

Kévin Dietrich commented

2015-01-21 23:49:47 +01:00

Added subscribers: @NicholasBishop, @kevindietrich

Kévin Dietrich commented

2015-01-21 23:49:47 +01:00

Some steps to easily reproduce using @MatthewHeimlich file:

get rid of the mirror and subsurf modifiers,
ensure you are in edit mode,
select all vertices
move the mesh around

To quote some comments in MOD_skin.c :

Concave surfaces around branch nodes. The paper does not discuss
how to handle non-convex regions; this code adds a number of
cleanup operations to handle many (though not all) of these
cases.

As the glitch actually appears in one of those concave regions, it might be that this case is not handled properly. @NicholasBishop care to have a quick glance here?

Some steps to easily reproduce using @MatthewHeimlich file: - get rid of the mirror and subsurf modifiers, - ensure you are in edit mode, - select all vertices - move the mesh around To quote some [comments in MOD_skin.c ](https://developer.blender.org/diffusion/B/browse/master/source/blender/modifiers/intern/MOD_skin.c;dda355442dc7ba4f83f65cb792be3f27be8c9fee$54): > Concave surfaces around branch nodes. The paper does not discuss > how to handle non-convex regions; this code adds a number of > cleanup operations to handle many (though not all) of these > cases. As the glitch actually appears in one of those concave regions, it might be that this case is not handled properly. @NicholasBishop care to have a quick glance here?

Kévin Dietrich commented

2015-01-22 00:11:00 +01:00

Hmm, I've found another comment that could lead us in the right direction:

A frame could be merged diagonally across
another, would generate a weird (bad) T
junction

And that's exactly what appears to be going on:

Hmm, I've found another [comment ](https://developer.blender.org/diffusion/B/browse/master/source/blender/modifiers/intern/MOD_skin.c;dda355442dc7ba4f83f65cb792be3f27be8c9fee$375) that could lead us in the right direction: >A frame could be merged diagonally across >another, would generate a weird (bad) T >junction And that's exactly what appears to be going on: ![skinBug.blend.png](https://archive.blender.org/developer/F138642/skinBug.blend.png) ![skinBug.blend1.png](https://archive.blender.org/developer/F138644/skinBug.blend1.png)

Nicholas Bishop was assigned by Julian Eisel

2015-02-02 00:51:19 +01:00

Julian Eisel commented

2015-02-02 00:51:19 +01:00

Added subscriber: @ideasman42

Julian Eisel commented

2015-02-02 00:51:19 +01:00

Assigning to Nicholas to increase pressure a bit ;) Also @ideasman42 might have some input.

Grigore Florin commented

2015-02-25 02:22:48 +01:00

Added subscriber: @numarul7

Grigore Florin commented

2015-02-25 02:22:48 +01:00

I can`t reproduce on standard 2.71 build from blender.org install (I know I know ...i wait for the OpenGL :D refactor ) :)

marc dion commented

2015-03-13 08:51:09 +01:00

Added subscriber: @MarcClintDion

marc dion commented

2015-03-13 08:51:09 +01:00

For the following .gif, I removed the mirror and moved the model in Object mode then applied the Object's Location using Ctrl+a.

I then used Undo and Redo repeatedly to see if this was possibly due to the GPU doing internal triangulation differently based on how Object Mode vs. Edit Mode are passing info to OpenGL .

That is a known nuisance but apparently not the issue here because it's only a slight shading issue that presents itself when that is happening.

For this Skin Modifier problem, the geometry is actually being visually moved to different places in 3D space and the results are often very different just by Undoing and Redoing. After doing that about 20 times, I ended up seeing about 5-6 different configurations of the geometry.

That's very unusual since Undoing and Redoing repeatedly should give identical results for every iteration.

Something weird is happening here. It can be seen in the following animation.

For the following .gif, I removed the mirror and moved the model in Object mode then applied the Object's Location using Ctrl+a. I then used Undo and Redo repeatedly to see if this was possibly due to the GPU doing internal triangulation differently based on how Object Mode vs. Edit Mode are passing info to OpenGL . That is a known nuisance but apparently not the issue here because it's only a slight shading issue that presents itself when that is happening. ------ For this Skin Modifier problem, the geometry is actually being visually moved to different places in 3D space and the results are often very different just by Undoing and Redoing. After doing that about 20 times, I ended up seeing about 5-6 different configurations of the geometry. That's very unusual since Undoing and Redoing repeatedly should give identical results for every iteration. Something weird is happening here. It can be seen in the following animation. ![scale_UI.gif](https://archive.blender.org/developer/F150011/scale_UI.gif) ``` ```

Julian Eisel commented

2015-03-19 22:57:39 +01:00

@NicholasBishop, @ideasman42, did anybody find the time to look into this?

Nicholas Bishop was unassigned by Brecht Van Lommel

2015-05-02 15:18:45 +02:00

Campbell Barton was assigned by Brecht Van Lommel

2015-05-02 15:18:45 +02:00

Brecht Van Lommel commented

2015-05-02 15:18:45 +02:00

Added subscriber: @brecht

Brecht Van Lommel commented

2015-05-02 15:18:45 +02:00

This bug was caused by 92733179ae.

I can see that hull_merge_triangles uses the normal, so presumably some intermediate recalc normal operations are necessary for that. Also, is it ok to use BMO_push and bmesh_edit_begin without BMO_pop and bmesh_edit_end?

Maybe the fix looks something like this, it works but I'm not sure it's correct.

diff --git a/source/blender/modifiers/intern/MOD_skin.c b/source/blender/modifiers/intern/MOD_skin.c
index bd16fd0..3b26b4a 100644
--- a/source/blender/modifiers/intern/MOD_skin.c
+++ b/source/blender/modifiers/intern/MOD_skin.c
@@ -1763,6 +1763,7 @@ static BMesh *build_skin(SkinNode *skin_nodes,

        /* Merge triangles here in the hope of providing better target
         * faces for skin_fix_hull_topology() to connect to */
+       BM_mesh_normals_update(so.bm);
        hull_merge_triangles(&so, smd);

        /* Using convex hulls may not generate a nice manifold mesh. Two
@@ -1781,8 +1782,12 @@ static BMesh *build_skin(SkinNode *skin_nodes,

        skin_output_end_nodes(&so, skin_nodes, totvert);
        skin_output_connections(&so, skin_nodes, medge, totedge);
+       BM_mesh_normals_update(so.bm);
        hull_merge_triangles(&so, smd);

+       bmesh_edit_end(so.bm, 0);
+       BMO_pop(so.bm);
+
        return so.bm;
 }

This bug was caused by 92733179ae. I can see that `hull_merge_triangles` uses the normal, so presumably some intermediate recalc normal operations are necessary for that. Also, is it ok to use `BMO_push` and `bmesh_edit_begin` without `BMO_pop` and `bmesh_edit_end`? Maybe the fix looks something like this, it works but I'm not sure it's correct. ``` diff --git a/source/blender/modifiers/intern/MOD_skin.c b/source/blender/modifiers/intern/MOD_skin.c index bd16fd0..3b26b4a 100644 --- a/source/blender/modifiers/intern/MOD_skin.c +++ b/source/blender/modifiers/intern/MOD_skin.c @@ -1763,6 +1763,7 @@ static BMesh *build_skin(SkinNode *skin_nodes, /* Merge triangles here in the hope of providing better target * faces for skin_fix_hull_topology() to connect to */ + BM_mesh_normals_update(so.bm); hull_merge_triangles(&so, smd); /* Using convex hulls may not generate a nice manifold mesh. Two @@ -1781,8 +1782,12 @@ static BMesh *build_skin(SkinNode *skin_nodes, skin_output_end_nodes(&so, skin_nodes, totvert); skin_output_connections(&so, skin_nodes, medge, totedge); + BM_mesh_normals_update(so.bm); hull_merge_triangles(&so, smd); + bmesh_edit_end(so.bm, 0); + BMO_pop(so.bm); + return so.bm; } ```

blender-admin commented

2015-05-04 12:56:57 +02:00

This issue was referenced by abd68c6e45

This issue was referenced by abd68c6e45a67ff682f13000d1ae2f8b79edea22

Campbell Barton commented

2015-05-04 13:03:27 +02:00

Changed status from 'Open' to: 'Resolved'

Campbell Barton closed this issue

2015-05-04 13:03:27 +02:00

Campbell Barton commented

2015-05-04 13:03:27 +02:00

Closed by commit abd68c6e45.

Closed by commit abd68c6e45.

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Skin Modifier - topology changes randomly when entering and exiting edit mode #42893