fix(color): only colour 7 swaps with the background; true colours draw as authored

Every near-white colour turned black on a light background (and near-black
white on a dark one), including true colours. A 255,255,255 solid used as a
white mask became a black block. Now only colour 7 swaps: true colours and
ACI 255 resolve a hair off pure white/black and keep their value on screen
and in plots.

Discussion #1500

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

src/scene/convert/tess.rs

@@ -794,10 +794,14 @@ fn tessellate_entity_inner(
                             a,
                         ),
                         ProxyColor::Rgb(r, g, b) => (
-                            view::render::adapt_to_bg(
-                                [r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0, 1.0],
-                                bg_color,
-                            ),
+                            {
+                                let [r, g, b] = convert::tess_util::authored_rgb([
+                                    r as f32 / 255.0,
+                                    g as f32 / 255.0,
+                                    b as f32 / 255.0,
+                                ]);
+                                [r, g, b, 1.0]
+                            },
                             0,
                         ),
                         ProxyColor::Inherit => (entity_color, aci),

src/scene/convert/tess_util.rs

@@ -31,8 +31,36 @@ pub type FallbackGeometry = (
 /// Convert an opencadcodec Color (ACI index or true-color) to a GPU RGBA value.
 pub fn aci_to_rgba(color: &AcadColor) -> [f32; 4] {
     if let Some((r, g, b)) = color.rgb() {
-        [r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0, 1.0]
+        let rgb = [r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0];
+        // Only colour 7 swaps white/black with the background; any other
+        // colour — a true colour, or ACI 255 — is drawn as authored.
+        let [r, g, b] = if matches!(color, AcadColor::Index(7)) {
+            rgb
+        } else {
+            authored_rgb(rgb)
+        };
+        [r, g, b, 1.0]
     } else {
         WireModel::WHITE
     }
 }
+
+/// Pure white and pure black are what the background adaptation swaps, and
+/// only colour 7 may be swapped. A colour chosen as-is (255,255,255 used as a
+/// white mask on a light sheet, say) is kept a hair off them so it is drawn
+/// exactly as authored; the difference is below one display step.
+pub fn authored_rgb(rgb: [f32; 3]) -> [f32; 3] {
+    const OFF: f32 = 1.0 / 1024.0;
+    if rgb == [1.0; 3] {
+        [1.0 - OFF; 3]
+    } else if rgb == [0.0; 3] {
+        [OFF; 3]
+    } else {
+        rgb
+    }
+}
+
+/// True for a colour [`authored_rgb`] moved off pure white.
+pub fn is_authored_white(rgb: [f32; 3]) -> bool {
+    rgb == authored_rgb([1.0; 3])
+}

src/scene/convert/tessellate.rs

@@ -2868,12 +2868,10 @@ impl DimGeom {
 /// `ByLayer` / `ByBlock` (assumes those are already resolved upstream).
 pub(crate) fn color_or_inherit(c: &AcadColor, inherited: [f32; 4]) -> [f32; 4] {
     match c.rgb() {
-        Some((r, g, b)) => [
-            r as f32 / 255.0,
-            g as f32 / 255.0,
-            b as f32 / 255.0,
-            inherited[3],
-        ],
+        Some(_) => {
+            let [r, g, b, _] = crate::scene::convert::tess_util::aci_to_rgba(c);
+            [r, g, b, inherited[3]]
+        }
         None => inherited,
     }
 }