Diameter dimensions: the *D block's text keeps the sign as %%c (fixes #1645) (#1696)

The *D block and EXPLODE's MTEXT took the dimension's decoded text, so
%%c arrived as U+2300, which no SHX font has: other programs plotted a
diameter dimension in an SHX style without its sign. They now write
DimGeometry::mtext_contents(): the sign as %%c, which every font draws
(and CADCraft decodes), a % that would run into a code as %%%.

Co-authored-by: Brandon Thomas <echelon@gmail.com>

crates/engine/src/cmd/modify.rs

@@ -1403,7 +1403,7 @@ pub(crate) fn explode_kind(d: &cadcraft_doc::Drawing, e: &Entity) -> Option<Vec<
                         attach: 5,
                         rotation: g.text_angle,
                         style: style.text_style.clone(),
-                        contents: g.value.clone(),
+                        contents: g.mtext_contents(),
                         line_spacing: 1.0,
                         line_spacing_exact: false,
                         columns: None,

crates/engine/tests/dim_diameter_sign.rs

@@ -0,0 +1,70 @@
+//! #1645: a diameter dimension's text in its `*D` block, and in the MTEXT EXPLODE leaves, holds
+//! the diameter sign as `%%c`, as CADCraft's own TEXT and MTEXT keep it. The decoded ⌀ (U+2300)
+//! is in no SHX font, so other programs plotted the dimension in an SHX style ("romans.shx")
+//! without its sign. CADCraft still draws and reads the sign.
+
+use cadcraft_engine::Session;
+use cadcraft_engine::doc::{EntityKind, Handle};
+use serde_json::{Value, json};
+
+fn h(v: &Value) -> Handle {
+    v["handle"].as_str().and_then(Handle::parse_hex).unwrap()
+}
+
+/// The issue's drawing: an SHX text style, a dimension style using it, a diameter dimension
+/// of a circle of radius 30.
+fn session() -> (Session, Handle) {
+    let mut s = Session::new();
+    s.execute("units", &json!({"measurement": "metric"})).ok();
+    s.execute("style", &json!({"name": "R", "font": "romans.shx", "current": true})).unwrap();
+    s.execute("dimstyle", &json!({"name": "D", "current": true, "DIMTXSTY": "R", "DIMTXT": 5, "DIMASZ": 5, "DIMDEC": 0})).unwrap();
+    let c = h(&s.execute("circle", &json!({"center": [0, 0], "radius": 30})).unwrap());
+    let dim = h(&s.execute("dimdiameter", &json!({"handle": c.hex(), "at": [40, 40]})).unwrap());
+    (s, dim)
+}
+
+/// The MTEXT contents (group 1) inside each `*D…` block of a DXF.
+fn dblock_texts(dxf: &str) -> Vec<String> {
+    let lines: Vec<&str> = dxf.lines().map(str::trim).collect();
+    let mut out = Vec::new();
+    let (mut in_d, mut in_mtext) = (false, false);
+    for pair in lines.chunks(2) {
+        let [code, value] = pair else { continue };
+        match (*code, *value) {
+            ("0", "BLOCK") => in_d = false,
+            ("2", name) if !in_mtext && name.starts_with("*D") => in_d = true,
+            ("0", "MTEXT") => in_mtext = in_d,
+            ("0", _) => in_mtext = false,
+            ("1", text) if in_mtext => out.push(text.to_string()),
+            _ => {}
+        }
+    }
+    out
+}
+
+#[test]
+fn the_dimension_block_writes_the_sign_as_a_code() {
+    let (s, dim) = session();
+    let d = s.doc().unwrap();
+    let EntityKind::Dimension(dm) = &d.entity(dim).unwrap().kind else { panic!("not a dimension") };
+    assert!(matches!(dm.kind, cadcraft_engine::doc::DimKind::Diameter));
+    let texts = dblock_texts(&cadcraft_io::write_dxf(d));
+    assert_eq!(texts, vec!["%%c60".to_string()], "the *D block's MTEXT");
+    // Read back, CADCraft still shows the sign.
+    let read = cadcraft_io::read_dxf(cadcraft_io::write_dxf(d).as_bytes()).unwrap();
+    let dm = read.model.iter().find_map(|e| if let EntityKind::Dimension(dm) = &e.kind { Some(dm.clone()) } else { None }).unwrap();
+    let st = read.dim_styles.iter().find(|x| x.name.eq_ignore_ascii_case(&dm.style)).cloned().unwrap_or_default();
+    assert_eq!(cadcraft_render::dimension_geometry(&dm, &st, 1.0).value, "⌀60");
+}
+
+#[test]
+fn exploding_keeps_the_sign_as_a_code() {
+    let (mut s, dim) = session();
+    s.execute("explode", &json!({"handles": [dim.hex()]})).unwrap();
+    let d = s.doc().unwrap();
+    let texts: Vec<String> =
+        d.model.iter().filter_map(|e| if let EntityKind::MText(m) = &e.kind { Some(m.contents.clone()) } else { None }).collect();
+    assert_eq!(texts, vec!["%%c60".to_string()]);
+    let saved = cadcraft_io::write_dxf(d);
+    assert!(!saved.contains('⌀'), "no U+2300 is saved");
+}

crates/io/src/dxf_write.rs

@@ -1424,7 +1424,7 @@ fn dim_block_entities(d: &Drawing, dm: &Dimension, layer: &str) -> Vec<Entity> {
                 attach: 5,
                 rotation: g.text_angle,
                 style: style.text_style.clone(),
-                contents: g.value.clone(),
+                contents: g.mtext_contents(),
                 line_spacing: 1.0,
                 line_spacing_exact: false,
                 columns: None,

crates/render/src/dim.rs

@@ -53,6 +53,22 @@ pub struct DimGeometry {
 }
 
 impl DimGeometry {
+    /// `value` as the contents of a plain MTEXT (the dimension's `*D` block, its EXPLODE
+    /// pieces): the diameter sign as `%%c`, not the decoded ⌀ (U+2300), which no SHX font has,
+    /// so other programs plot nothing for it, while every font draws `%%c` (#1645). A `%`
+    /// that would run into a code is written `%%%`.
+    pub fn mtext_contents(&self) -> String {
+        let mut out = String::with_capacity(self.value.len() + 2);
+        let mut chars = self.value.chars().peekable();
+        while let Some(c) = chars.next() {
+            match c {
+                '⌀' => out.push_str("%%c"),
+                '%' if matches!(chars.peek(), Some('%' | '⌀')) => out.push_str("%%%"),
+                c => out.push(c),
+            }
+        }
+        out
+    }
     fn dim(&mut self, l: Vec<Vec2>) {
         if l.len() >= 2 {
             self.lines.push(l);
@@ -1381,6 +1397,7 @@ mod tests {
         d.defpt = Vec3::new(-3.0, 0.0, 0.0);
         let g = dimension_geometry(&d, &st_with(|s| s.decimals = 1), 1.0);
         assert_eq!(g.value, "⌀6.0");
+        assert_eq!(g.mtext_contents(), "%%c6.0", "the *D block's text (#1645)");
         let mut a = lin((10.0, 0.0), (0.0, 10.0), (3.0, 3.0), 0.0);
         a.kind = DimKind::Angular3P;
         a.p15 = Vec3::ZERO;
@@ -1430,6 +1447,28 @@ mod tests {
         let g = dimension_geometry(&d, &DimStyle::default(), 1.0);
         assert!(g.lines.is_empty() && g.fills.is_empty(), "{:?}", g.lines);
     }
+
+    /// #1645: the MTEXT contents give back the dimension's text when decoded, with the diameter
+    /// sign as `%%c` (no SHX font has U+2300) and a `%` that would start a code escaped.
+    #[test]
+    fn mtext_contents_write_the_diameter_sign_as_a_code() {
+        let with = |v: &str| DimGeometry { value: v.into(), ..DimGeometry::default() };
+        for (value, contents) in [
+            ("⌀60", "%%c60"),
+            ("2x ⌀8.5 THRU", "2x %%c8.5 THRU"),
+            ("R3.0", "R3.0"),
+            ("36.9° ±0.1", "36.9° ±0.1"),
+            ("50%", "50%"),
+            ("5%⌀6", "5%%%%%c6"),
+            ("%%", "%%%%"),
+            ("", ""),
+        ] {
+            let g = with(value);
+            assert_eq!(g.mtext_contents(), contents, "{value:?}");
+            assert_eq!(decode_value(&g.mtext_contents()), value, "{value:?} reads back");
+            assert!(!g.mtext_contents().contains('⌀'));
+        }
+    }
 }
 
 #[cfg(test)]

docs/file-format-parity.md

@@ -1,6 +1,6 @@
 # File-format parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-19 · **Change:** minor (a file's own standard-named arrowhead blocks are kept) · **Target:** Autodesk AutoCAD 2027
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-20 · **Change:** minor (a diameter dimension's `*D` text keeps the sign as `%%c`) · **Target:** Autodesk AutoCAD 2027
 
 Every format AutoCAD 2027 reads or writes, with CADCraft's support, fidelity and tests. Work
 items are in [gaps.md](gaps.md#file-formats); the weighted score feeds
@@ -77,6 +77,7 @@ on read with a note, so such files no longer exceed the DXF limit (#1482). **DWG
 | POLYLINE 2D/3D | yes | yes (as LWPOLYLINE / 3D POLYLINE) | polyface and polygon meshes flattened: lossy |
 | TEXT, MTEXT, ATTDEF, INSERT + ATTRIB | yes; multiline attributes (embedded MTEXT) read and drawn | yes; multiline attributes written as embedded MTEXT (DXF 2007 and later; older versions: one line) | ATTDEF prompt kept (#137) |
 | DIMENSION (all 7 kinds) | yes | yes, with anonymous `*D` blocks | a reopened dimension's `*D` block is regenerated under its name, unreferenced ones dropped (#495); DIMASSOC objects written; angular block text honors DIMDSEP, including per-dimension overrides (#1646) |
+| DIMENSION (all 7 kinds) | yes | yes, with anonymous `*D` blocks; their text keeps the diameter sign as `%%c`, so SHX styles plot it (#1645) | a reopened dimension's `*D` block is regenerated under its name, unreferenced ones dropped (#495); DIMASSOC objects written |
 | LEADER | yes | yes | |
 | MULTILEADER | yes (#459) | yes, native MULTILEADER with MLEADERSTYLE (#459) | several landings per multileader are folded into one; per-object overrides and annotative scale contexts not kept |
 | HATCH pattern / solid | yes | yes; associative hatches keep their boundary objects in blocks too, and the boundary carries the hatch reactor (stretched dynamic blocks drag the fill in AutoCAD) | |
@@ -140,6 +141,7 @@ our output in AutoCAD black-box.
 |---|---|---|
 | 2026-10-19 | minor | DXF/DWG save: angular dimension block text honors DIMDSEP, including per-dimension overrides (#1646) |
 | 2026-10-19 | minor | A file's own arrowhead blocks with standard names (AutoCAD's `_Dot`, `_DotSmall`), named only by a dimension style, are kept and saved as they were instead of being replaced by CADCraft's solids; PURGE keeps arrowhead blocks dimension styles name |
+| 2026-10-20 | minor | DIMENSION: the `*D` block's MTEXT, and the MTEXT EXPLODE leaves, hold the diameter sign as `%%c` instead of the decoded U+2300, which no SHX font has, so other programs plotted diameter dimensions in an SHX style without the sign (#1645) |
 | 2026-10-17 | minor | DWG read: DGN line style components no line style definition reaches are left out before the DXF bridge, with a read note; an 83 MB site plan with 888,483 of them (1.2 GB of DXF, over the limit) now opens (#1482) |
 | 2026-10-12 | minor | PDF output: object transparency plotted as PDF alpha (`/ExtGState` `/ca` `/CA`) when the page setup's or the plot's "Plot transparency" is on; PNG plots blend it, SVG uses `opacity`; plots without it are unchanged byte for byte |
 | 2026-10-12 | minor | DXF/DWG save: the Model layout's plot settings are written as CADCraft plots model space, drawing extents scaled to fit (PLOTSETTINGS 74 = 1, 75 = 0), instead of plot type 5 "Layout information", which other programs' Plot dialogs can't use for model space |