Weld

The Weld command removes creases from a mesh by merging coincident mesh vertices.

Individual texture mapping, color, and normal vector data is averaged/created/destroyed for the affected vertices.

Weld will create new vertex normals at each welded vertex that is the average of the adjacent face normals. One vertex is shared by more than one face after welding.

The WeldEdge command:

• Merges coincident mesh vertices along selected unwelded mesh edges.

Supported input

• Mesh Vertex, Control point.

Why weld mesh vertices?

Welding polygon meshes affect rendering, texture mapping on mesh objects, and file export for stereolithography.

Smoother Rendering

In wireframe these two meshes look the same.

When shaded, the red mesh looks smooth and the blue mesh looks chunky.

Each triangle vertex in the red mesh is welded to its neighbors. This causes the vertices in the red mesh to look smooth. No points in the blue mesh are welded. This causes the vertices in the blue mesh to look sharp.

Suppose you want the blue mesh to look smoother.

First you have to determine the angle tolerance. The shaded creases of this angle should look sharp.

Texture Mapping

Texture mapping coordinates control how a texture wraps around the object. These are two-dimensional coordinates that are attached to a polygon vertex, and they determine which point on the texture bitmap maps to this vertex. The points between the vertices interpolate from the vertex values.

The bottom left corner of the bitmap is the origin, the bottom right is (1,0), top left is (0,1) and top right is (1,1). The texture mapping coordinates are always between these values.

Only one pair of texture mapping coordinates can attach to a single vertex. That is why if you weld vertices, thus removing the overlapping duplicates, you throw away all but one pair of texture mapping coordinates.

The loss of texture mapping coordinates is irreversible. You cannot recover the mapping coordinates from the remaining mesh information.

To remove welding (and to also lose the texture mapping coordinates), use the UnWeld or the UnWeldEdge command.

STL Mesh Export Diagnostics

In some rapid prototyping machines, STL files must contain completely closed (watertight) polygon mesh objects.

You might want to use the Join command, then Weld (angle=180), then UnifyMeshNormals to turn a group of meshes into a single watertight mesh object, and use SelNakedMeshEdgePt to find the open (naked) edges. Following this procedure will ensure that the meshes really do fit together before exporting them for use in an expensive STL job.

See: Stereolithography (.stl) Import/Export.

The Unweld command adds creases to a smooth mesh by creating coincident vertices.

The UnweldEdge command adds creases to a smooth mesh by creating coincident vertices along selected edges.

The UnweldVertex command unwelds selected mesh vertices.

Command-line options

ModifyNormals

What welding means

Internally, a mesh is defined by two lists:

• A list of vertices and their attributes (position, normal, texture coordinates, vertex-color).
• A list of face definitions describing how these vertices are connected to form faces.

Examples

Quadrangular mesh

This two-part definition describes two quadrangular faces that do not share vertices as shown in the illustration.

Vertex list

The vertex list is half of the mesh definition, describing the vertices.

In this example texture coordinates and vertex colors are left out, but those are per-vertex attributes as well.

Face definitions

This list describes how these vertices are connected to form faces.

Triangular mesh example 1

A mesh with a list like the following example describes two separate triangular faces, not touching each other, as shown in the illustration.

Triangular mesh example 2

In the next example, imagine two of those six vertices share the same position:

Here vertices 2 and 4 are at the same position and 3 and 6 are at the same position.

So we have a mesh consisting of two faces that share one edge, but vertices 2, 3, 4 and 6 all have their own individual attributes. So their normals can be different.

Joined mesh

When you Join a mesh, the vertices and faces are in no way merged; the lists are simply appended. The face normals retain their direction.

Welded mesh

When you Weld a mesh and the angle between the faces is below the threshold the common edge will be welded.

The two common vertices are replaced by a new vertex with new attributes:

New normal attributes for vertices 7 and 8 are averaged based on the normals of the faces they are part of, so there is no way to get the previous (pre-weld) normals back.

Summary

• Joining does not destroy any vertex attributes.
• Welding "merges" vertices on common edges below the angle threshold.
• The resulting new vertex has it's normal calculated based on the average of the faces it belongs to.

See also

Edit mesh objects