STL export welds shared vertices like the 3MF (#816)

STL (binary and ASCII) now welds each body's export mesh with the same tolerance as 3MF, so filleted and drilled bodies are closed. The weld grid's neighbour lookup no longer overflows for bodies far from the origin.

Fixes #784

crates/io/src/lib.rs

@@ -222,6 +222,11 @@ pub fn export_mesh(b: &solidcraft_kernel::Body) -> Result<Mesh> {
     Ok(b.tessellate((b.size() * 2e-4).max(1e-3))?)
 }
 
+/// Distance within which an exported mesh's vertices are welded (STL and 3MF alike).
+fn weld_eps(b: &solidcraft_kernel::Body) -> f64 {
+    (b.size() * 1e-6).max(1e-7)
+}
+
 /// Binary STL of several meshes (one solid).
 pub fn stl_binary(meshes: &[Mesh], name: &str) -> Vec<u8> {
     let n: usize = meshes.iter().map(|m| m.triangles.len()).sum();
@@ -292,13 +297,22 @@ pub fn export(state: &ModelState, bodies: &[String], format: Format, name: &str)
         return Err(IoError::Empty);
     }
     Ok(match format {
-        Format::StlBinary | Format::StlAscii | Format::Obj => {
+        Format::StlBinary | Format::StlAscii => {
+            // Each body welded on its own (as for 3MF), so shared vertices are written identically
+            // and the file is closed where faces meet.
+            let meshes: Vec<Mesh> = sel
+                .iter()
+                .map(|b| {
+                    let m = export_mesh(&b.body)?;
+                    let (positions, triangles) = weld(&m.positions, &m.triangles, weld_eps(&b.body));
+                    Ok(Mesh { positions, triangles, ..Mesh::default() })
+                })
+                .collect::<Result<_>>()?;
+            if format == Format::StlBinary { stl_binary(&meshes, name) } else { stl_ascii(&meshes, name).into_bytes() }
+        }
+        Format::Obj => {
             let meshes: Vec<(String, Mesh)> = sel.iter().map(|b| Ok((b.name.clone(), export_mesh(&b.body)?))).collect::<Result<_>>()?;
-            match format {
-                Format::StlBinary => stl_binary(&meshes.into_iter().map(|(_, m)| m).collect::<Vec<_>>(), name),
-                Format::StlAscii => stl_ascii(&meshes.into_iter().map(|(_, m)| m).collect::<Vec<_>>(), name).into_bytes(),
-                _ => obj(&meshes).into_bytes(),
-            }
+            obj(&meshes).into_bytes()
         }
         Format::Step => {
             let bs: Vec<solidcraft_kernel::ExportBody> =
@@ -317,7 +331,7 @@ pub fn export(state: &ModelState, bodies: &[String], format: Format, name: &str)
                 .iter()
                 .map(|b| {
                     let m = export_mesh(&b.body)?;
-                    let eps = (b.body.size() * 1e-6).max(1e-7);
+                    let eps = weld_eps(&b.body);
                     // Faces with colours of their own tag their triangles.
                     let paint = b.body.paint();
                     let mut looks = Vec::new();