PDF follow-ups: import layers, arcs and joins, names, background, page placement, snaps

- Unlayered raster images go with the PDF layer drawn last before them.
- Circular Bezier arcs import as arcs: bulged polyline segments with Join,
  ARC entities without it; straight chains still join.
- Imported PNGs go to "PDF Images" next to the PDF; import blocks are
  numbered imp31, imp32, ...
- Background darkness is its HSL lightness below one half.
- All chosen pages attach at the same insertion point.
- PDF geometry snaps as separate pieces: centre from the rim, intersections
  and perpendiculars.
- Automation: snap query with an optional base point.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/commands/pdf_import.rs

@@ -82,13 +82,8 @@ impl OpenCADStudio {
         };
         let mut result = pdf_import::convert(&content, &underlay, &area, &naming);
         let images = std::mem::take(&mut result.images);
-        let image_dir = self.tabs[i]
-            .current_path
-            .as_deref()
-            .and_then(|p| p.parent())
-            .map(|p| p.to_path_buf())
-            .or_else(|| std::path::Path::new(&path).parent().map(|p| p.to_path_buf()))
-            .map(|dir| dir.join("PDF Images"));
+        // PDFIMPORTIMAGEPATH "PDF Images": a folder next to the PDF.
+        let image_dir = std::path::Path::new(&path).parent().map(|dir| dir.join("PDF Images"));
 
         self.push_undo_snapshot(i, "PDFIMPORT");
         let scene = &mut self.tabs[i].scene;
@@ -101,7 +96,7 @@ impl OpenCADStudio {
             let _ = scene.document.line_types.add(lt);
         }
         // Raster images: each written as a PNG under "PDF Images" next to the
-        // drawing (or the PDF while the drawing is unsaved), then referenced.
+        // PDF, named after it with 8 hex digits, then referenced.
         let stem = std::path::Path::new(&path.replace('\\', "/"))
             .file_stem()
             .map(|s| s.to_string_lossy().into_owned())
@@ -128,7 +123,9 @@ impl OpenCADStudio {
             raster.u_vector = image.u;
             raster.v_vector = image.v;
             raster.flags = codec::entities::ImageDisplayFlags::SHOW_IMAGE
-                | codec::entities::ImageDisplayFlags::USE_CLIPPING_BOUNDARY;
+                | codec::entities::ImageDisplayFlags::SHOW_NOT_ALIGNED
+                | codec::entities::ImageDisplayFlags::USE_CLIPPING_BOUNDARY
+                | codec::entities::ImageDisplayFlags::TRANSPARENCY_ON;
             raster.common.layer = image.layer.clone();
             result.entities.push(codec::EntityType::RasterImage(raster));
         }
@@ -154,8 +151,8 @@ impl OpenCADStudio {
             let mut name = stem.clone();
             let mut n = 1;
             while scene.document.block_records.get(&name).is_some() {
+                name = format!("{stem}{n}");
                 n += 1;
-                name = format!("{stem}({n})");
             }
             if scene
                 .define_block_from_owned_entities(result.entities, &name, glam::DVec3::ZERO)

src/app/control/mod.rs

@@ -459,6 +459,7 @@ impl OpenCADStudio {
                 | "commands"
                 | "properties"
                 | "measure"
+                | "snap"
                 | "query"
                 | "records"
                 | "record_schema"
@@ -705,6 +706,28 @@ impl OpenCADStudio {
             let response = match op {
                 "properties" => self.control_properties(),
                 "measure" => self.control_measure(&req),
+                // The object snap a cursor over `point` (world) would get;
+                // `from` is the base for perpendicular and tangent.
+                "snap" => {
+                    let p = &req["point"];
+                    let world = glam::DVec3::new(
+                        p[0].as_f64().unwrap_or(0.0),
+                        p[1].as_f64().unwrap_or(0.0),
+                        p[2].as_f64().unwrap_or(0.0),
+                    );
+                    let from = req["from"].as_array().map(|f| {
+                        glam::DVec3::new(
+                            f.first().and_then(|v| v.as_f64()).unwrap_or(0.0),
+                            f.get(1).and_then(|v| v.as_f64()).unwrap_or(0.0),
+                            f.get(2).and_then(|v| v.as_f64()).unwrap_or(0.0),
+                        )
+                    });
+                    let i = self.active_tab;
+                    match self.snap_query(i, world, from) {
+                        Some(hit) => json!({"ok":true,"snap":format!("{:?}", hit.snap_type),"world":[hit.world.x,hit.world.y,hit.world.z]}),
+                        None => json!({"ok":true,"snap":null}),
+                    }
+                }
                 "xdata_get" => self.xdata_read(&req),
                 "get_selection" => self.control_get_selection(),
                 "history" => {

src/app/update/viewport.rs

@@ -2872,6 +2872,49 @@ impl OpenCADStudio {
         )
     }
 
+    /// The object snap the cursor would get over `world` in the model view,
+    /// through the same candidate gathering and snapper as a cursor move
+    /// (automation's `snap` query).
+    pub(in crate::app) fn snap_query(
+        &mut self,
+        i: usize,
+        world: glam::DVec3,
+        from: Option<glam::DVec3>,
+    ) -> Option<crate::snap::SnapResult> {
+        let (vw, vh) = self.tabs[i].scene.selection.borrow().vp_size;
+        if vw <= 1.0 || vh <= 1.0 {
+            return None;
+        }
+        let tile_b = self.tabs[i].scene.active_model_tile_bounds(vw, vh);
+        let bounds = iced::Rectangle {
+            x: 0.0,
+            y: 0.0,
+            width: tile_b.width,
+            height: tile_b.height,
+        };
+        let (view_rot, eye, px) = {
+            let cam = self.tabs[i].scene.camera.borrow();
+            (cam.view_proj_rte(bounds), cam.eye(), cam.project(world, bounds)?)
+        };
+        let p = Point::new(px.x, px.y);
+        let raw = self.cursor_model_point(i, &None, p, bounds);
+        let all_wires = self.tabs[i].scene.hit_test_wires();
+        let candidates = self.tabs[i].scene.interaction_candidates_near(
+            all_wires,
+            raw,
+            view_rot,
+            eye,
+            bounds,
+            self.snapper.osnap_radius_px,
+        );
+        // Perpendicular and tangent measure from `from`, else the running
+        // command's last point.
+        self.snapper.from_point = from.or(self.last_point).map(|p| p.as_vec3());
+        let (go, gr) = self.drafting_grid_basis(i);
+        self.snapper
+            .snap(raw, p, &candidates, view_rot, eye, bounds, go, gr, None)
+    }
+
     /// Apply one frame of a UCS-icon grip drag: map the cursor onto the active
     /// UCS plane (so the move stays in-plane) and either slide the origin there
     /// or rotate the chosen axis to point at it, keeping a right-handed frame

src/mcp.rs

@@ -35,6 +35,7 @@ const READ_OPS: &[&str] = &[
     "header",
     "properties",
     "measure",
+    "snap",
     "history",
     "commands",
     "events",
@@ -1198,7 +1199,7 @@ fn tool_definitions() -> Value {
         {
             "name":"ocs_read",
             "description":"Discover capabilities and record schemas, or read state, complete database records, command manifests, entities, properties, kernel measurements and spatial relationships, history, events or operation status from a live OCS session.",
-            "inputSchema":{"type":"object","properties":{"ocs_session_id":{"type":"string","minLength":1,"description":"Value of session_id returned by ocs_sessions."},"op":{"type":"string","enum":READ_OPS,"default":"state"},"parameters":{"type":"object","description":"Operation-specific filters.","properties":{"name":{"type":"string","description":"Command name or record name."},"search":{"type":"string","description":"Case-insensitive command or record-type search."},"document_id":{"type":"integer","minimum":0},"path":{"type":"string","description":"Optional intended output path for audit; extension determines target format."},"target_format":{"type":"string","enum":["dwg","dxf"],"description":"Intended output format for audit."},"target_version":{"type":"string","enum":["R14","2000","2004","2007","2010","2013","2018","AC1014","AC1015","AC1018","AC1021","AC1024","AC1027","AC1032"],"description":"Intended CAD output version for audit."},"collection":{"type":"string","description":"Record collection, all for records, or omit to discover collections and schema types."},"handle":{"type":"string"},"handles":{"type":"array","items":{"type":"string"},"description":"Exact entity or record handles."},"type":{"type":"string","description":"Entity or record type filter; for record_schema, returns its complete type graph and writable field paths."},"layer":{"type":"string","description":"Layer name filter for query."},"detail":{"type":"string","enum":["summary","geometry","full"],"default":"geometry","description":"Entity detail returned by query."},"fields":{"type":"array","items":{"type":"string"},"description":"Return only these entity fields plus handle."},"paths":{"type":"array","items":{"type":"string"},"description":"Project RFC 6901 JSON Pointer paths relative to record.properties."},"where":{"type":"array","description":"All property filters must match.","items":{"type":"object","properties":{"path":{"type":"string"},"op":{"type":"string","enum":["eq","ne","lt","lte","gt","gte","contains","starts_with","ends_with","in","exists","not_exists"],"default":"eq"},"value":{}},"required":["path"],"additionalProperties":false}},"near":{"type":"array","items":{"type":"number"},"minItems":2,"maxItems":3,"description":"Rank planar curves by exact kernel distance to this world XY point."},"contains_point":{"type":"array","items":{"type":"number"},"minItems":2,"maxItems":3,"description":"Return closed planar curves containing this world XY point."},"bounds":{"type":"array","items":{"type":"number"},"minItems":4,"maxItems":4,"description":"Filter entities whose world XY bounds overlap [min_x,min_y,max_x,max_y]."},"intersections":{"type":"array","items":{"type":"string"},"minItems":2,"maxItems":2,"description":"Return exact kernel intersections between two planar curve handles."},"after":{"type":"integer","minimum":0,"description":"Event cursor."},"request_id":{"type":"string","description":"Operation id to query."},"offset":{"type":"integer","minimum":0},"limit":{"type":"integer","minimum":1,"maximum":10000}},"additionalProperties":false}},"required":["ocs_session_id"],"additionalProperties":false},
+            "inputSchema":{"type":"object","properties":{"ocs_session_id":{"type":"string","minLength":1,"description":"Value of session_id returned by ocs_sessions."},"op":{"type":"string","enum":READ_OPS,"default":"state"},"parameters":{"type":"object","description":"Operation-specific filters.","properties":{"name":{"type":"string","description":"Command name or record name."},"search":{"type":"string","description":"Case-insensitive command or record-type search."},"document_id":{"type":"integer","minimum":0},"path":{"type":"string","description":"Optional intended output path for audit; extension determines target format."},"target_format":{"type":"string","enum":["dwg","dxf"],"description":"Intended output format for audit."},"target_version":{"type":"string","enum":["R14","2000","2004","2007","2010","2013","2018","AC1014","AC1015","AC1018","AC1021","AC1024","AC1027","AC1032"],"description":"Intended CAD output version for audit."},"collection":{"type":"string","description":"Record collection, all for records, or omit to discover collections and schema types."},"handle":{"type":"string"},"handles":{"type":"array","items":{"type":"string"},"description":"Exact entity or record handles."},"type":{"type":"string","description":"Entity or record type filter; for record_schema, returns its complete type graph and writable field paths."},"layer":{"type":"string","description":"Layer name filter for query."},"detail":{"type":"string","enum":["summary","geometry","full"],"default":"geometry","description":"Entity detail returned by query."},"fields":{"type":"array","items":{"type":"string"},"description":"Return only these entity fields plus handle."},"paths":{"type":"array","items":{"type":"string"},"description":"Project RFC 6901 JSON Pointer paths relative to record.properties."},"where":{"type":"array","description":"All property filters must match.","items":{"type":"object","properties":{"path":{"type":"string"},"op":{"type":"string","enum":["eq","ne","lt","lte","gt","gte","contains","starts_with","ends_with","in","exists","not_exists"],"default":"eq"},"value":{}},"required":["path"],"additionalProperties":false}},"near":{"type":"array","items":{"type":"number"},"minItems":2,"maxItems":3,"description":"Rank planar curves by exact kernel distance to this world XY point."},"point":{"type":"array","items":{"type":"number"},"minItems":2,"maxItems":3,"description":"World point whose object snap the snap op reports."},"from":{"type":"array","items":{"type":"number"},"minItems":2,"maxItems":3,"description":"Base point for perpendicular and tangent snaps (snap op)."},"contains_point":{"type":"array","items":{"type":"number"},"minItems":2,"maxItems":3,"description":"Return closed planar curves containing this world XY point."},"bounds":{"type":"array","items":{"type":"number"},"minItems":4,"maxItems":4,"description":"Filter entities whose world XY bounds overlap [min_x,min_y,max_x,max_y]."},"intersections":{"type":"array","items":{"type":"string"},"minItems":2,"maxItems":2,"description":"Return exact kernel intersections between two planar curve handles."},"after":{"type":"integer","minimum":0,"description":"Event cursor."},"request_id":{"type":"string","description":"Operation id to query."},"offset":{"type":"integer","minimum":0},"limit":{"type":"integer","minimum":1,"maximum":10000}},"additionalProperties":false}},"required":["ocs_session_id"],"additionalProperties":false},
             "outputSchema":read_output_schema(),
             "annotations":{"title":"Read OCS state","readOnlyHint":true,"destructiveHint":false,"idempotentHint":true,"openWorldHint":false}
         },

src/modules/insert/pdf_attach.rs

@@ -92,7 +92,7 @@ pub struct PdfAttachCommand {
     path: String,
     /// The path the definition stores (full, relative or file name only).
     stored_path: Option<String>,
-    /// Pages after the first, attached beside it (dialog selection).
+    /// Pages after the first, attached at the same point (dialog selection).
     extra_pages: Vec<String>,
     /// Scale and rotation fixed in the dialog: their prompts are skipped.
     preset_scale: Option<f64>,
@@ -301,7 +301,6 @@ impl PdfAttachCommand {
             .chain(self.extra_pages.iter().cloned())
             .collect();
         let placed = underlays_for_pages(
-            &self.path,
             &pages,
             self.plane.to_local(self.insertion),
             self.scale,
@@ -427,24 +426,19 @@ impl CadCommand for PdfAttachCommand {
     }
 }
 
-/// Underlays for `pages` of a file: the first at `insertion`, each next one
-/// beside the previous along the rotated X axis (page width times scale).
-// ponytail: side-by-side placement of the pages after the first is not
-// measured against the reference.
+/// Underlays for `pages` of a file, all at `insertion` with the same scale
+/// and rotation (the reference stacks the chosen pages there).
 pub fn underlays_for_pages(
-    read_path: &str,
     pages: &[String],
     insertion: DVec3,
     scale: f64,
     rotation: f64,
 ) -> Vec<(String, EntityType)> {
-    let mut origin = insertion;
-    let (c, s) = (rotation.cos(), rotation.sin());
     pages
         .iter()
         .map(|page| {
             let mut underlay = Underlay::pdf();
-            underlay.insertion_point = Vector3::new(origin.x, origin.y, origin.z);
+            underlay.insertion_point = Vector3::new(insertion.x, insertion.y, insertion.z);
             underlay.set_scale(scale);
             underlay.rotation = rotation;
             // On, clipped by its boundary and colour-adjusted for the
@@ -452,9 +446,6 @@ pub fn underlays_for_pages(
             underlay.flags = UnderlayDisplayFlags::ON
                 | UnderlayDisplayFlags::CLIPPING
                 | UnderlayDisplayFlags::ADJUST_FOR_BACKGROUND;
-            let width = crate::scene::model::pdf_raster::page_size_inches(read_path, page)
-                .map_or(0.0, |(w, _)| w * scale);
-            origin += DVec3::new(c * width, s * width, 0.0);
             (page.clone(), EntityType::Underlay(underlay))
         })
         .collect()

src/modules/insert/pdf_import.rs

@@ -9,13 +9,14 @@
 // Content of a PDF layer goes to PDF_<layer>; the rest to PDF_Geometry,
 // PDF_Solid Fills, PDF_Text and PDF_Images (or those alone, or the current
 // layer, per the settings). Straight strokes that meet become polylines, a
-// closed four-arc Bézier loop that is a circle a CIRCLE, other curves
-// splines, filled triangles and quadrilaterals SOLIDs at 50% transparency
+// Bézier that is a circular arc an arc segment of them (with "Join" on) or
+// an ARC (off), a closed four-arc Bézier loop that is a circle a CIRCLE,
+// other curves splines, filled triangles and quadrilaterals SOLIDs at 50% transparency
 // (other fills solid hatches), text runs MTEXT in a "PDF <font>" style and
 // raster images PNG files referenced by IMAGE objects.
 
 use codec::entities::{
-    AttachmentPoint, BoundaryEdge, BoundaryPath, Circle, Hatch, LwPolyline, MText, PolylineEdge,
+    Arc, AttachmentPoint, BoundaryEdge, BoundaryPath, Circle, Hatch, LwPolyline, MText, PolylineEdge,
     Solid, Spline, Underlay, UnderlayType,
 };
 use codec::types::{Color, Handle, LineWeight, Vector2, Vector3};
@@ -557,27 +558,75 @@ pub struct ImageImport {
 /// space 0.2, scaled per object to the dash length.
 pub const DASH_LINETYPE: &str = "PDF_IMPORT";
 
-/// A stroke before it becomes an object: straight chains can still be joined
-/// or read as dashes.
+/// A stroke before it becomes an object: chains can still be joined or read
+/// as dashes. `bulges[i]` bends the segment from point i to the next (the
+/// closing one on a closed stroke; 0 after an open stroke's last point).
 struct Stroke {
     layer: String,
     color: Color,
     weight: LineWeight,
     points: Vec<[f64; 2]>,
+    bulges: Vec<f64>,
     closed: bool,
 }
 
+impl Stroke {
+    fn straight(&self) -> bool {
+        self.bulges.iter().all(|b| *b == 0.0)
+    }
+
+    fn reverse(&mut self) {
+        self.points.reverse();
+        let n = self.bulges.len();
+        let mut bulges: Vec<f64> = self.bulges[..n - 1].iter().rev().map(|b| -b).collect();
+        bulges.push(0.0);
+        self.bulges = bulges;
+    }
+}
+
+/// The bulge of a Bézier that is a circular arc (its quarter, half and
+/// three-quarter points on the circle through its ends and midpoint), from
+/// its midpoint's height over the chord; positive counter-clockwise.
+fn arc_bulge(seg: &Segment) -> Option<f64> {
+    let Segment::Cubic(p0, _, _, p3) = seg else {
+        return None;
+    };
+    let m = bezier(seg, 0.5);
+    let (ax, ay) = (m[0] - p0[0], m[1] - p0[1]);
+    let (bx, by) = (p3[0] - p0[0], p3[1] - p0[1]);
+    let d = 2.0 * (ax * by - ay * bx);
+    let chord = bx.hypot(by);
+    if d.abs() < 1e-12 || chord < 1e-9 {
+        return None;
+    }
+    // Circle through p0, m, p3 (relative to p0).
+    let (a2, b2) = (ax * ax + ay * ay, bx * bx + by * by);
+    let c = [(by * a2 - ay * b2) / d, (ax * b2 - bx * a2) / d];
+    let r = c[0].hypot(c[1]);
+    let tol = r * 2e-3;
+    let on = [0.25, 0.75].iter().all(|t| {
+        let q = bezier(seg, *t);
+        ((q[0] - p0[0] - c[0]).hypot(q[1] - p0[1] - c[1]) - r).abs() <= tol
+    });
+    if !on {
+        return None;
+    }
+    let h = (ax * by - ay * bx) / chord;
+    Some(2.0 * h / chord)
+}
+
 fn same_point(a: [f64; 2], b: [f64; 2]) -> bool {
     (a[0] - b[0]).abs() < 1e-7 && (a[1] - b[1]).abs() < 1e-7
 }
 
-/// Open straight strokes of one layer, colour and weight that meet end to
-/// end become one polyline (closed when the chain returns to its start).
-fn join_strokes(strokes: Vec<Stroke>) -> Vec<Stroke> {
+/// Open strokes of one layer, colour and weight that meet end to end become
+/// one polyline (closed when the chain returns to its start). Straight ones
+/// always do; ones with arcs only with `join` (off, their arcs stay apart).
+fn join_strokes(strokes: Vec<Stroke>, join: bool) -> Vec<Stroke> {
     let mut out: Vec<Stroke> = Vec::new();
     let mut open: Vec<Stroke> = Vec::new();
     for s in strokes {
-        if s.closed {
+        if s.closed || (!join && !s.straight()) {
             out.push(s);
         } else {
             open.push(s);
@@ -597,26 +646,32 @@ fn join_strokes(strokes: Vec<Stroke>) -> Vec<Stroke> {
             }) {
                 let mut next = open.remove(i);
                 if !same_point(next.points[0], end) {
-                    next.points.reverse();
+                    next.reverse();
                 }
                 chain.points.extend(next.points.into_iter().skip(1));
+                chain.bulges.pop();
+                chain.bulges.extend(next.bulges);
             } else if let Some(i) = open.iter().position(|o| {
                 fits(o)
                     && (same_point(*o.points.last().unwrap(), start) || same_point(o.points[0], start))
             }) {
                 let mut prev = open.remove(i);
                 if !same_point(*prev.points.last().unwrap(), start) {
-                    prev.points.reverse();
+                    prev.reverse();
                 }
                 prev.points.pop();
                 prev.points.extend(chain.points);
                 chain.points = prev.points;
+                prev.bulges.pop();
+                prev.bulges.extend(chain.bulges);
+                chain.bulges = prev.bulges;
             } else {
                 break;
             }
         }
         if chain.points.len() > 3 && same_point(chain.points[0], *chain.points.last().unwrap()) {
             chain.points.pop();
+            chain.bulges.pop();
             chain.closed = true;
         }
         out.push(chain);
@@ -624,13 +679,45 @@ fn join_strokes(strokes: Vec<Stroke>) -> Vec<Stroke> {
     out
 }
 
+/// An open stroke cut into its straight runs and one stroke per arc.
+fn split_at_arcs(stroke: Stroke) -> Vec<Stroke> {
+    let mut out = Vec::new();
+    let piece = |points: Vec<[f64; 2]>, bulges: Vec<f64>| Stroke {
+        layer: stroke.layer.clone(),
+        color: stroke.color,
+        weight: stroke.weight,
+        points,
+        bulges,
+        closed: false,
+    };
+    let mut run: Vec<[f64; 2]> = vec![stroke.points[0]];
+    for i in 0..stroke.points.len() - 1 {
+        let (a, b, bulge) = (stroke.points[i], stroke.points[i + 1], stroke.bulges[i]);
+        if bulge == 0.0 {
+            run.push(b);
+            continue;
+        }
+        if run.len() > 1 {
+            let n = run.len();
+            out.push(piece(std::mem::take(&mut run), vec![0.0; n]));
+        }
+        out.push(piece(vec![a, b], vec![bulge, 0.0]));
+        run = vec![b];
+    }
+    if run.len() > 1 {
+        let n = run.len();
+        out.push(piece(run, vec![0.0; n]));
+    }
+    out
+}
+
 /// Runs of three or more equal collinear dashes with equal gaps become one
 /// line from the first dash's start to the last's end, with the dash length
 /// (the linetype scale).
 // ponytail: dashes are matched along one direction at a time in drawing
 // order; patterns of mixed dash lengths stay separate lines.
 fn infer_dashes(strokes: Vec<Stroke>) -> Vec<(Stroke, Option<f64>)> {
-    let is_dash = |s: &Stroke| !s.closed && s.points.len() == 2;
+    let is_dash = |s: &Stroke| !s.closed && s.points.len() == 2 && s.straight();
     let len = |s: &Stroke| {
         let (a, b) = (s.points[0], s.points[1]);
         (b[0] - a[0]).hypot(b[1] - a[1])
@@ -777,19 +864,36 @@ pub fn convert(
                                 (common.layer, common.color, common.line_weight) =
                                     (layer.clone(), color, weight);
                                 curves.push(circle);
-                            } else if sp.segments.iter().all(|s| matches!(s, Segment::Line(..))) {
+                            } else if let Some(mut bulges) = sp
+                                .segments
+                                .iter()
+                                .map(|s| match s {
+                                    Segment::Line(..) => Some(0.0),
+                                    cubic => arc_bulge(cubic),
+                                })
+                                .collect::<Option<Vec<f64>>>()
+                            {
                                 let mut points = vec![sp.segments[0].start()];
                                 points.extend(sp.segments.iter().map(|s| s.end()));
                                 if sp.closed && points.len() > 2 {
                                     points.pop();
+                                } else {
+                                    bulges.push(0.0);
                                 }
-                                strokes.push(Stroke {
+                                let stroke = Stroke {
                                     layer: layer.clone(),
                                     color,
                                     weight,
                                     points,
+                                    bulges,
                                     closed: sp.closed,
-                                });
+                                };
+                                if settings.join || stroke.straight() || stroke.closed {
+                                    strokes.push(stroke);
+                                } else {
+                                    // Join off: each arc apart, straight runs between them.
+                                    strokes.extend(split_at_arcs(stroke));
+                                }
                             } else {
                                 let mut spline = EntityType::Spline(spline_of(sp, &world3));
                                 let common = spline.common_mut();
@@ -873,14 +977,42 @@ pub fn convert(
     }
 
     // The joined strokes, then (when asked) dashes read as one dashed line.
-    let strokes = join_strokes(strokes);
+    let strokes = join_strokes(strokes, settings.join);
     let strokes: Vec<(Stroke, Option<f64>)> = if settings.linetypes {
         infer_dashes(strokes)
     } else {
         strokes.into_iter().map(|s| (s, None)).collect()
     };
+    // A mirrored placement turns arcs the other way.
+    let turn = (underlay.x_scale * underlay.y_scale).signum();
     for (stroke, dash) in strokes {
+        // A lone arc is an ARC.
+        if !stroke.closed && stroke.points.len() == 2 && stroke.bulges[0] != 0.0 {
+            let (a, b) = (world2(stroke.points[0]), world2(stroke.points[1]));
+            let bulge = stroke.bulges[0] * turn;
+            if let Some(ba) = crate::entities::common::BulgeArc::from_bulge([a.x, a.y], [b.x, b.y], bulge) {
+                let (start, end) = if bulge > 0.0 {
+                    (ba.start_angle, ba.end_angle)
+                } else {
+                    (ba.end_angle, ba.start_angle)
+                };
+                let tau = std::f64::consts::TAU;
+                let mut arc = Arc::from_center_radius_angles(
+                    Vector3::new(ba.center[0], ba.center[1], z),
+                    ba.radius,
+                    start.rem_euclid(tau),
+                    end.rem_euclid(tau),
+                );
+                (arc.common.layer, arc.common.color, arc.common.line_weight) =
+                    (stroke.layer, stroke.color, stroke.weight);
+                result.entities.push(EntityType::Arc(arc));
+                continue;
+            }
+        }
         let mut pl = LwPolyline::from_points(stroke.points.iter().map(|p| world2(*p)).collect());
+        for (vertex, bulge) in pl.vertices.iter_mut().zip(&stroke.bulges) {
+            vertex.bulge = bulge * turn;
+        }
         pl.is_closed = stroke.closed;
         pl.elevation = z;
         (pl.common.layer, pl.common.color, pl.common.line_weight) =

src/scene/convert/tess.rs

@@ -1286,16 +1286,28 @@ fn tessellate_entity_inner(
             b.set_fixed_screen_width(2.0);
         }
     }
-    // PDF underlay geometry: a hidden, unplotted wire that object snaps
-    // (nearest, intersection, perpendicular) find, beside the page frame.
+    // PDF underlay geometry: hidden, unplotted wires that object snaps
+    // (nearest, intersection, perpendicular, centre) find, beside the page
+    // frame — one per segment or circle, so they intersect each other.
     if let EntityType::Underlay(underlay) = e {
-        let geometry = crate::scene::model::pdf_vector::underlay_snap_geometry(underlay, document);
-        if !geometry.is_empty() {
-            let (points, points_low) = convert::tessellate::points_to_ds(geometry);
+        for piece in crate::scene::model::pdf_vector::underlay_snap_geometry(underlay, document) {
+            let (points, points_low) = convert::tessellate::points_to_ds(piece.points);
             let mut wire = WireModel::solid(h.value().to_string(), points, entity_color, sel);
             wire.points_low = points_low;
             wire.display_visible = false;
             wire.plot_visible = false;
+            if let Some((center, radius)) = piece.circle {
+                wire.tangent_geoms = vec![crate::scene::model::wire_model::TangentGeom::PlanarCircle {
+                    center,
+                    axis_x: [1.0, 0.0, 0.0],
+                    axis_y: [0.0, 1.0, 0.0],
+                    radius,
+                }];
+                wire.snap_pts = vec![(
+                    glam::DVec3::from(center),
+                    crate::scene::model::wire_model::SnapHint::Center,
+                )];
+            }
             set_wire_aabb(&mut wire, aabb);
             bases.push(wire);
         }

src/scene/model/image_model.rs

@@ -217,7 +217,11 @@ impl ImageModel {
         // Hidden PDF layers come from the underlay's layer overrides.
         let source = super::pdf_layers::underlay_source(u, &def.file_path);
         let raster = pdf_raster::rasterize_page_display(&source, page)?;
-        let bg_lum = 0.299 * background[0] + 0.587 * background[1] + 0.114 * background[2];
+        // Dark means an HSL lightness under one half: pure blue counts as
+        // light, (0, 128, 0) as dark.
+        let bg_max = background[0].max(background[1]).max(background[2]);
+        let bg_min = background[0].min(background[1]).min(background[2]);
+        let bg_lum = (bg_max + bg_min) / 2.0;
         let pixels = pdf_raster::adjusted_pixels(
             &source,
             page,
@@ -226,7 +230,7 @@ impl ImageModel {
                 contrast: u.contrast.min(100),
                 monochrome: u.flags.contains(UnderlayDisplayFlags::MONOCHROME),
                 adjust_for_background: u.flags.contains(UnderlayDisplayFlags::ADJUST_FOR_BACKGROUND),
-                dark_background: bg_lum <= 0.5,
+                dark_background: bg_lum < 0.5,
             },
         );
 

src/scene/model/pdf_raster.rs

@@ -252,8 +252,6 @@ pub fn adjusted_pixels(path: &str, page: &str, raster: &PdfPage, adjust: PageAdj
             // (plus 20/255, capped at full), hue and saturation kept: black
             // becomes white, dark red light pink. On a light background the
             // colours stay as drawn.
-            // ponytail: offset and the light-background rule measured on the
-            // default backgrounds only (pure white on paper not measured).
             let (h, s, l) = rgb_to_hsl(rgb);
             rgb = hsl_to_rgb(h, s, (1.0 - l + 20.0 / 255.0).min(1.0));
         }

src/scene/model/pdf_vector.rs

@@ -89,6 +89,16 @@ pub struct PageVectors {
     pub texts: Vec<PdfText>,
     /// Filled only when images are asked for (`page_content`).
     pub images: Vec<PdfImage>,
+    /// Paths, texts and images in the order the page draws them.
+    pub order: Vec<Drawn>,
+}
+
+/// One drawn item: an index into `paths`, `texts` or `images`.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Drawn {
+    Path(usize),
+    Text(usize),
+    Image(usize),
 }
 
 type Key = (String, String);
@@ -168,10 +178,54 @@ pub fn page_content_by_layer(
         only.images.retain(|i| !base.images.contains(i));
         out.push((Some(layer.name.clone()), only));
     }
+    let mut base = base;
+    images_to_last_layer(path, page, &mut base, &mut out);
     out.insert(0, (None, base));
     Some(out)
 }
 
+/// An image that no layer holds goes with the layer drawn last before it
+/// (with none before it, it stays unlayered): the page is read with every
+/// layer on for the drawing order.
+fn images_to_last_layer(
+    path: &str,
+    page: &str,
+    base: &mut PageVectors,
+    layered: &mut [(Option<String>, PageVectors)],
+) {
+    if base.images.is_empty() || layered.is_empty() {
+        return;
+    }
+    let Some(full) = page_content(&super::pdf_layers::source_with_layers(path, &[]), page) else {
+        return;
+    };
+    let key = |s: &dyn std::fmt::Debug| format!("{s:?}");
+    let owner = |item: String| {
+        layered.iter().position(|(_, pv)| {
+            pv.paths.iter().any(|p| key(p) == item) || pv.texts.iter().any(|t| key(t) == item)
+        })
+    };
+    let mut last: Option<usize> = None;
+    let mut moves: Vec<(usize, PdfImage)> = Vec::new();
+    for drawn in &full.order {
+        match *drawn {
+            Drawn::Path(i) => last = owner(key(&full.paths[i])).or(last),
+            Drawn::Text(i) => last = owner(key(&full.texts[i])).or(last),
+            Drawn::Image(i) => {
+                let image = &full.images[i];
+                if let Some(n) = layered.iter().position(|(_, pv)| pv.images.contains(image)) {
+                    last = Some(n);
+                } else if let (Some(n), Some(at)) = (last, base.images.iter().position(|b| b == image)) {
+                    moves.push((n, base.images.remove(at)));
+                }
+            }
+        }
+    }
+    for (n, image) in moves {
+        layered[n].1.images.push(image);
+    }
+}
+
 fn read_page(path: &str, page: &str) -> Option<PageVectors> {
     read_page_with(path, page, false)
 }
@@ -229,6 +283,7 @@ fn read_page_with(path: &str, page: &str, images: bool) -> Option<PageVectors> {
         paths: device.paths,
         texts: texts.into_iter().map(|t| t.text).collect(),
         images: device.images,
+        order: device.order,
     })
 }
 
@@ -250,12 +305,14 @@ struct Collector {
     font_order: Vec<u128>,
     want_images: bool,
     images: Vec<PdfImage>,
+    order: Vec<Drawn>,
 }
 
 impl Collector {
     fn finish_text(&mut self) {
         if let Some(text) = self.current.take() {
             if !text.text.text.trim().is_empty() {
+                self.order.push(Drawn::Text(self.texts.len()));
                 self.texts.push(text);
             }
         }
@@ -351,6 +408,7 @@ impl<'a> Device<'a> for Collector {
             }
             PathDrawMode::Fill(_) => (None, Some(rgb(paint))),
         };
+        self.order.push(Drawn::Path(self.paths.len()));
         self.paths.push(PdfPath { subpaths, stroke, fill });
     }
     fn push_clip_path(&mut self, _: &ClipPath) {}
@@ -482,7 +540,10 @@ impl<'a> Device<'a> for Collector {
             },
             None,
         );
-        self.images.extend(decoded);
+        if let Some(image) = decoded {
+            self.order.push(Drawn::Image(self.images.len()));
+            self.images.push(image);
+        }
     }
     fn pop_clip_path(&mut self) {}
     fn pop_transparency_group(&mut self) {}
@@ -605,13 +666,21 @@ pub fn underlay_snap_points(
 // ponytail: flat cap; a spatial index over page segments for huge drawings.
 const MAX_GEOMETRY_POINTS: usize = 200_000;
 
-/// The underlay's PDF geometry as world polylines (NaN-separated, curves
-/// flattened) for nearest, intersection and perpendicular snaps. Empty when
+/// One piece of an underlay's PDF geometry for snapping: a world polyline
+/// (curves flattened), and for a circle its centre and radius.
+pub struct SnapPiece {
+    pub points: Vec<[f64; 3]>,
+    pub circle: Option<([f64; 3], f64)>,
+}
+
+/// The underlay's PDF geometry for nearest, intersection, perpendicular and
+/// centre snaps: each straight segment, each curve and each circle its own
+/// piece, so pieces cross each other as separate objects do. Empty when
 /// PDFOSNAP is off or the page is not shown.
 pub fn underlay_snap_geometry(
     u: &codec::entities::Underlay,
     document: &codec::CadDocument,
-) -> Vec<[f64; 3]> {
+) -> Vec<SnapPiece> {
     if !pdf_osnap() || !u.flags.contains(codec::entities::UnderlayDisplayFlags::ON) {
         return Vec::new();
     }
@@ -626,21 +695,35 @@ pub fn underlay_snap_geometry(
         return Vec::new();
     };
     let world = |p: [f64; 2]| crate::entities::underlay::local_to_world(u, p);
-    let mut out: Vec<[f64; 3]> = Vec::new();
+    // A circle stays one only while the placement scales both axes alike.
+    let uniform = (u.x_scale.abs() - u.y_scale.abs()).abs() <= 1e-9 * u.x_scale.abs().max(1e-12);
+    let flatten = |seg: &Segment, points: &mut Vec<[f64; 3]>| {
+        let steps = if matches!(seg, Segment::Cubic(..)) { 12 } else { 1 };
+        for k in 1..=steps {
+            points.push(world(bezier(seg, k as f64 / steps as f64)));
+        }
+    };
+    let mut out: Vec<SnapPiece> = Vec::new();
+    let mut total = 0usize;
     for path in &vectors.paths {
         for sp in &path.subpaths {
-            if out.len() >= MAX_GEOMETRY_POINTS {
+            if total >= MAX_GEOMETRY_POINTS {
                 return out;
             }
-            if !out.is_empty() {
-                out.push([f64::NAN; 3]);
-            }
-            out.push(world(sp.segments[0].start()));
-            for seg in &sp.segments {
-                let steps = if matches!(seg, Segment::Cubic(..)) { 12 } else { 1 };
-                for k in 1..=steps {
-                    out.push(world(bezier(seg, k as f64 / steps as f64)));
+            if let (Some((c, r)), true) = (circle_of(sp), uniform) {
+                let mut points = vec![world(sp.segments[0].start())];
+                for seg in &sp.segments {
+                    flatten(seg, &mut points);
                 }
+                total += points.len();
+                out.push(SnapPiece { points, circle: Some((world(c), r * u.x_scale.abs())) });
+                continue;
+            }
+            for seg in &sp.segments {
+                let mut points = vec![world(seg.start())];
+                flatten(seg, &mut points);
+                total += points.len();
+                out.push(SnapPiece { points, circle: None });
             }
         }
     }