#1786: ink is translucent by its width in points, while drawn and once committed

Addresses CodeRabbit review on #3229, "Use the same translucency
threshold as committed ink": the preview judged the pen by its width in
points, the committed stroke by its width on screen, so at 50% a 12 pt
highlighter turned opaque once committed and at 200% a 6 pt pen turned
translucent. Both now use chartview::ink_color, which goes by the
stroke's width in points, so the two agree and neither changes with
the zoom.

Test: ink_render::ink_translucency_follows_the_pen_not_the_zoom.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/chartview.rs

@@ -316,11 +316,12 @@ pub(crate) fn paint_objects(app: &SheetApp, p: &Painter, geo: &Geo, wb: &Workboo
                 p.add(PathShape::convex_polygon(vec![pos2(r.center().x, r.top()), r.right_bottom(), r.left_bottom()], fill, Stroke::new(1.0, line)));
             }
             ShapeKind::Ink => {
-                let w = gridcraft_engine::cmd::draw::ink_width(sp) * geo.z;
+                let pt = gridcraft_engine::cmd::draw::ink_width(sp);
+                let w = pt * geo.z;
                 let pts: Vec<Pos2> =
                     gridcraft_engine::cmd::draw::ink_points(sp).iter().map(|q| pos2(r.left() + q[0] * geo.z, r.top() + q[1] * geo.z)).collect();
                 if pts.len() >= 2 {
-                    let col = if w > 8.0 { line.gamma_multiply(0.45) } else { line };
+                    let col = ink_color(line, pt);
                     p.add(PathShape::line(pts, PathStroke::new(w, col)));
                 }
                 label = false;
@@ -556,3 +557,9 @@ pub fn hit(app: &SheetApp, geo: &Geo, sh: &Sheet, p: Pos2) -> Option<(ObjRef, Re
     }
     None
 }
+
+/// The colour ink is painted in: a highlighter (a stroke over 8 pt wide) is translucent, by its
+/// width in points, so the live pen stroke and the committed ink agree at every zoom.
+pub(crate) fn ink_color(line: Color32, width_pt: f32) -> Color32 {
+    if width_pt > 8.0 { line.gamma_multiply(0.45) } else { line }
+}

crates/ui-egui/src/grid/interact.rs

@@ -257,7 +257,7 @@ pub(crate) fn pen_ink(color: &str, width: f32, theme: &gridcraft_engine::model::
     let line = gridcraft_engine::model::Color::from_hex(color).unwrap_or(pen_blue);
     let width = if width <= 0.0 { 2.0 } else { width }.clamp(0.5, 40.0).round().max(1.0);
     let color = line.resolve(theme).map(crate::theme::color32).unwrap_or(Color32::from_rgb(0x1F, 0x5F, 0xC9));
-    (if width > 8.0 { color.gamma_multiply(0.45) } else { color }, width)
+    (crate::chartview::ink_color(color, width), width)
 }
 
 fn handle_header_autofit(app: &mut SheetApp, input: &InteractionInput<'_>) -> bool {

crates/ui-egui/tests/it/ink_render.rs

@@ -0,0 +1,45 @@
+//! Committed ink is painted as the pen previews it, at any zoom.
+
+use egui::Shape;
+use egui::epaint::ColorMode;
+use gridcraft_engine::Session;
+use gridcraft_ui_egui::SheetApp;
+use serde_json::json;
+
+/// The colour of the ink stroke drawn from (40, 40) to (200, 40) sheet points, `width` points
+/// wide: the two-point horizontal path whose width is `width` at its own scale (160 pt long).
+fn painted_ink(h: &egui_kittest::Harness<'static, SheetApp>, width: f32) -> Option<egui::Color32> {
+    h.output().shapes.iter().find_map(|s| match &s.shape {
+        Shape::Path(p) if p.points.len() == 2 && (p.points[0].y - p.points[1].y).abs() < 0.01 => {
+            let z = (p.points[1].x - p.points[0].x).abs() / 160.0;
+            match p.stroke.color {
+                ColorMode::Solid(c) if z > 0.0 && (p.stroke.width - width * z).abs() < 0.01 => Some(c),
+                _ => None,
+            }
+        }
+        _ => None,
+    })
+}
+
+#[test]
+fn ink_translucency_follows_the_pen_not_the_zoom() {
+    // A highlighter (over 8 pt) is translucent and a pen isn't, whatever the zoom: the committed
+    // stroke used its width on screen, so at 50% a 12 pt highlighter turned opaque and at 200% a
+    // 6 pt pen turned translucent, unlike the pen's preview.
+    for (width, zoom, translucent) in [(12.0_f32, 50, true), (6.0, 200, false), (12.0, 100, true), (6.0, 100, false)] {
+        let mut s = Session::new();
+        s.new_workbook();
+        s.execute("view.zoom", json!({"percent": zoom})).unwrap();
+        s.execute("draw.stroke", json!({"points": [[40, 40], [200, 40]], "color": "#FFFF00", "width": width})).unwrap();
+        let mut h = crate::common::builder().with_size(egui::vec2(1200.0, 800.0)).build_ui_state(
+            |ui, app: &mut SheetApp| {
+                app.logic(&ui.ctx().clone());
+                app.ui(ui);
+            },
+            crate::common::app(s),
+        );
+        h.run_steps(4);
+        let c = painted_ink(&h, width).unwrap_or_else(|| panic!("the {width} pt stroke is painted at {zoom}%"));
+        assert_eq!(c.a() < 255, translucent, "{width} pt at {zoom}%: {c:?}");
+    }
+}

crates/ui-egui/tests/it/main.rs

@@ -58,6 +58,7 @@ mod hyperlink_ctrl_click;
 mod hyperlink_dialog;
 mod in_cell_editor_size;
 mod in_cell_editor_wrap;
+mod ink_render;
 mod insert_copied_cells;
 mod interaction;
 mod interface_scale;