Render: table body text on tinted cells uses the theme's dark colour, so it stays readable on dark themes

The medium2 (default), medium4 and banded light1 table styles tint body cells towards white
but gave their text no colour, so it inherited the theme's text colour: light on a dark theme
such as Nocturne, and nearly invisible on the light tint. PowerPoint's medium styles set that
text to dk1. DeckCraft's dark themes keep the dark colour in lt1 (Nocturne has dk1 white and
lt1 near-black, relying on the default tx1->dk1 map), so the look picks whichever of dk1 and
lt1 contrasts more with the cell's actual fill. Cells with no fill keep inheriting, and the
"grid" border keeps the text colour.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/render/src/table.rs

@@ -153,18 +153,28 @@ pub fn look(ctx: &Ctx, t: &Table, r: usize, c: usize) -> CellLook {
     let band_c = t.band_col && c.is_multiple_of(2);
     let tint = |k: i32| deckcraft_color::apply(a, &[ColorTransform::Tint(k)]);
     let white = Rgba::WHITE;
-    let dark_text = ctx.color(&ColorRef::scheme(SchemeSlot::Tx1), None);
+    let text_color = ctx.color(&ColorRef::scheme(SchemeSlot::Tx1), None);
+    // Text on a tinted cell never inherits the text colour, which a dark theme makes light and
+    // unreadable on the light tint: PowerPoint's medium styles set it to the theme's dark colour
+    // (dk1). A dark theme may keep that in dk1 and map tx1 away from it, or (Nocturne) put it in
+    // lt1, so of the two the one contrasting more with the cell's fill is used.
+    let dk1 = ctx.color(&ColorRef::scheme(SchemeSlot::Dk1), None);
+    let lt1 = ctx.color(&ColorRef::scheme(SchemeSlot::Lt1), None);
+    let on = |fill: Rgba| if dk1.contrast(fill) >= lt1.contrast(fill) { dk1 } else { lt1 };
     match kind {
         "none" => CellLook { fill: None, text: None, bold: false, border: None, sides: [None; 4], paint: None },
-        "grid" => CellLook { fill: None, text: None, bold: false, border: Some((dark_text, 0.75)), sides: [None; 4], paint: None },
-        "light1" => CellLook {
-            fill: if band || band_c { Some(tint(20000)) } else { None },
-            text: None,
-            bold: header || total || first_col || last_col,
-            border: if header { Some((a, 1.0)) } else { None },
-            sides: [None; 4],
-            paint: None,
-        },
+        "grid" => CellLook { fill: None, text: None, bold: false, border: Some((text_color, 0.75)), sides: [None; 4], paint: None },
+        "light1" => {
+            let fill = (band || band_c).then(|| tint(20000));
+            CellLook {
+                fill,
+                text: fill.map(on),
+                bold: header || total || first_col || last_col,
+                border: if header { Some((a, 1.0)) } else { None },
+                sides: [None; 4],
+                paint: None,
+            }
+        }
         "light2" => CellLook {
             fill: if header { Some(a) } else { None },
             text: if header { Some(white) } else { None },
@@ -187,29 +197,36 @@ pub fn look(ctx: &Ctx, t: &Table, r: usize, c: usize) -> CellLook {
             sides: [None; 4],
             paint: None,
         },
-        "medium4" => CellLook {
-            fill: Some(if header || band || band_c { tint(40000) } else { tint(20000) }),
-            text: None,
-            bold: header || total || first_col || last_col,
-            border: Some((a, 0.75)),
-            sides: [None; 4],
-            paint: None,
-        },
-        _ => CellLook {
+        "medium4" => {
+            let fill = if header || band || band_c { tint(40000) } else { tint(20000) };
+            CellLook {
+                fill: Some(fill),
+                text: Some(on(fill)),
+                bold: header || total || first_col || last_col,
+                border: Some((a, 0.75)),
+                sides: [None; 4],
+                paint: None,
+            }
+        }
+        _ => {
             // medium2: strong header, banded light tints, white grid.
-            fill: Some(if header || total || first_col || last_col {
+            let strong = header || total || first_col || last_col;
+            let fill = if strong {
                 a
             } else if band || band_c {
                 tint(40000)
             } else {
                 tint(20000)
-            }),
-            text: if header || total || first_col || last_col { Some(white) } else { None },
-            bold: header || total || first_col || last_col,
-            border: Some((white, 1.0)),
-            sides: [None; 4],
-            paint: None,
-        },
+            };
+            CellLook {
+                fill: Some(fill),
+                text: Some(if strong { white } else { on(fill) }),
+                bold: strong,
+                border: Some((white, 1.0)),
+                sides: [None; 4],
+                paint: None,
+            }
+        }
     }
 }
 

crates/render/src/tests.rs

@@ -1336,6 +1336,41 @@ fn a_table_fill_shows_behind_cells_without_their_own() {
     assert_eq!(img.pixel(50, 50)[..3], [255, 255, 255]);
 }
 
+#[test]
+fn table_body_text_is_dark_on_the_tinted_cells_of_a_dark_theme() {
+    // The default style (medium2) tints body cells towards white, so their text must be the
+    // theme's dark colour, as PowerPoint's medium styles set it (dk1): inheriting the text colour
+    // leaves it light, and unreadable, on a dark theme such as Nocturne.
+    let fill_and_darkest = |theme: &str| {
+        let mut t = Table::new(2, 2, 200.0, 40.0);
+        for (r, c) in [(0, 0), (0, 1), (1, 0), (1, 1)] {
+            t.cell_mut(r, c).unwrap().text = TextBody::from_text("Body");
+        }
+        let tbl = Shape { id: ShapeId(796), xfrm: Some(Xfrm::new(100.0, 100.0, 200.0, 80.0)), kind: ShapeKind::Table(t), ..Default::default() };
+        let mut p = deck_with(vec![tbl]);
+        let th = deckcraft_model::theme::builtin_themes().into_iter().find(|th| th.name == theme).unwrap();
+        std::sync::Arc::make_mut(&mut p.masters[0]).theme = th;
+        let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+        let lum = |x, y| {
+            let px = img.pixel(x, y);
+            deckcraft_color::Rgba::rgb(px[0], px[1], px[2]).luminance()
+        };
+        // Body cell (1, 1) covers (200, 140)–(300, 180); its top-left corner is clear of the text
+        // (cell margins) and of the white grid line.
+        let fill = lum(203, 143);
+        let darkest = (143..177).flat_map(|y| (203..297).map(move |x| (x, y))).map(|(x, y)| lum(x, y)).fold(1.0_f64, f64::min);
+        (fill, darkest)
+    };
+    for theme in ["Nocturne", "Harbor"] {
+        let (fill, darkest) = fill_and_darkest(theme);
+        assert!(fill > 0.4, "{theme}: the body cell's fill is a light tint, luminance {fill}");
+        assert!(
+            darkest < 0.15 && darkest < fill / 3.0,
+            "{theme}: darkest pixel in the body cell has luminance {darkest} on a fill of {fill}: its text is not dark"
+        );
+    }
+}
+
 /// A white 280x200 pt rectangle at (20, 20) with `effects`, rendered at 1 px/pt.
 fn effect_box(effects: deckcraft_model::Effects) -> Image {
     use deckcraft_model::style::{Fill, Line};