mathutils.geometry.tessellate_polygon creates triangles with opposite normal from input ngon #70594

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Dealga McArdle · 2019-10-06T22:23:46+02:00

Dealga McArdle commented

2019-10-06 22:23:46 +02:00

Blender Version
any recent 2.8+

Short description of error

This script will generate a 5 vertex regular ngon, the face has indices [0, 1, 2, 3, 4], the output of tessellate_polygon will return polygons with reversed normals compared to the original input ngon. It seems to me that it would be more useful to return triangles where the order of indices will produce faces matching the normal of the input ngon.

import bpy
import math
from mathutils import Vector
from mathutils.geometry import tessellate_polygon

vectors = []
faces = []
for i in range(num_verts):
    theta = (2 * math.pi / num_verts) * i 
    vectors.append(Vector((math.sin(theta), math.cos(theta), 0.0)))

faces.append([0, 1, 2, 3, 4])
face = faces[0]

subcoords = [vectors[idx] for idx in face]
for pol in tessellate_polygon([subcoords]):
    print([face[i] for i in pol]) # opposite normal
    # print([face[i] for i in pol[::-1]]) # same normal

- [1, 0, 4]
- [2, 1, 4]
# [2, 4, 3]

**Blender Version** any recent 2.8+ **Short description of error** This script will generate a 5 vertex regular ngon, the face has indices [0, 1, 2, 3, 4], the output of `tessellate_polygon` will return polygons with reversed normals compared to the original input ngon. It seems to me that it would be more useful to return triangles where the order of indices will produce faces matching the normal of the input ngon. ``` import bpy import math from mathutils import Vector from mathutils.geometry import tessellate_polygon vectors = [] faces = [] for i in range(num_verts): theta = (2 * math.pi / num_verts) * i vectors.append(Vector((math.sin(theta), math.cos(theta), 0.0))) faces.append([0, 1, 2, 3, 4]) face = faces[0] subcoords = [vectors[idx] for idx in face] for pol in tessellate_polygon([subcoords]): print([face[i] for i in pol]) # opposite normal # print([face[i] for i in pol[::-1]]) # same normal - [1, 0, 4] - [2, 1, 4] # [2, 4, 3] ``` ![image.png](https://archive.blender.org/developer/F7793415/image.png)

Dealga McArdle commented

2019-10-06 22:23:46 +02:00

Added subscriber: @tetha.z

Dealga McArdle changed title from ~~mathtutils.geometry.tessellate_polygon returns indices in reverse order~~ to mathutils.geometry.tessellate_polygon returns indices in reverse order

2019-10-06 22:32:07 +02:00

Dealga McArdle changed title from ~~mathutils.geometry.tessellate_polygon returns indices in reverse order~~ to mathutils.geometry.tessellate_polygon creates triangles with opposite normal from input ngon

2019-10-06 22:33:27 +02:00

Germano Cavalcante commented

2019-10-08 20:28:11 +02:00

Added subscribers: @ideasman42, @mano-wii

Campbell Barton was assigned by Germano Cavalcante

2019-10-08 20:28:11 +02:00

Germano Cavalcante commented

2019-10-08 20:28:12 +02:00

I can confirm.
@ideasman42, other similar functions use axis_dominant_v3_to_m3_negate instead of axis_dominant_v3_to_m3.
Therefore, replacing this function seems to resolve in this case.
But it is important to test if this does not break other cases.

diff --git a/source/blender/blenlib/intern/scanfill.c b/source/blender/blenlib/intern/scanfill.c
index 08c3653153e..c9b0eb3ed5e 100644
--- a/source/blender/blenlib/intern/scanfill.c
+++ b/source/blender/blenlib/intern/scanfill.c
@@ -907,7 +907,7 @@ unsigned int BLI_scanfill_calc_ex(ScanFillContext *sf_ctx, const int flag, const
       return 0;
     }
 
-    axis_dominant_v3_to_m3(mat_2d, n);
+    axis_dominant_v3_to_m3_negate(mat_2d, n);
   }
 
   /* STEP 1: COUNT POLYS */

I can confirm. @ideasman42, other similar functions use `axis_dominant_v3_to_m3_negate` instead of `axis_dominant_v3_to_m3`. Therefore, replacing this function seems to resolve in this case. But it is important to test if this does not break other cases. ``` diff --git a/source/blender/blenlib/intern/scanfill.c b/source/blender/blenlib/intern/scanfill.c index 08c3653153e..c9b0eb3ed5e 100644 --- a/source/blender/blenlib/intern/scanfill.c +++ b/source/blender/blenlib/intern/scanfill.c @@ -907,7 +907,7 @@ unsigned int BLI_scanfill_calc_ex(ScanFillContext *sf_ctx, const int flag, const return 0; } - axis_dominant_v3_to_m3(mat_2d, n); + axis_dominant_v3_to_m3_negate(mat_2d, n); } /* STEP 1: COUNT POLYS */ ```

blender-admin commented

2019-10-09 07:53:53 +02:00

This issue was referenced by b718d659c9

This issue was referenced by b718d659c9a51a7218c161e4be1ff511abd436e8

Campbell Barton commented

2019-10-09 08:11:03 +02:00

Changed status from 'Open' to: 'Resolved'

Campbell Barton closed this issue

2019-10-09 08:11:03 +02:00

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mathutils.geometry.tessellate_polygon creates triangles with opposite normal from input ngon #70594