Export: a converted JPG flattens translucent pixels before the conversion

A JPG exported in a profile other than the document's was converted while
still translucent and then composited over white, so translucent fills,
shadows and antialiased edges came out a few levels off what the document
shows. The render is now flattened over white in the document's profile
first. PNG and WebP keep their alpha and are unchanged.

crates/engine/src/cmd/file.rs

@@ -411,7 +411,19 @@ pub fn export_file(doc: &Document, ids: &[NodeId], setting: &ExportSetting, scal
             // The render is in the document's profile; the setting's profile converts the pixels
             // (out-of-gamut colors clip) and tags the file.
             let profile = setting.color_profile.resolve(doc.color_profile);
-            doc.color_profile.conversion_to(profile).apply_pixels(pix.data_mut());
+            let conversion = doc.color_profile.conversion_to(profile);
+            // JPG has no alpha: the render is flattened over white in the document's profile
+            // before the conversion, so translucent pixels match what the document shows.
+            if setting.format == ExportFormat::Jpg && !conversion.is_identity() {
+                for [r, g, b, a] in pix.data_mut().as_chunks_mut::<4>().0.iter_mut() {
+                    let under = 255 - *a;
+                    *r = r.saturating_add(under);
+                    *g = g.saturating_add(under);
+                    *b = b.saturating_add(under);
+                    *a = 255;
+                }
+            }
+            conversion.apply_pixels(pix.data_mut());
             Ok((figcraft_render::encode_export(&pix, setting.format, profile).map_err(EngineError::Other)?, Vec::new()))
         }
     }

crates/engine/src/export_profile_tests.rs

@@ -115,6 +115,19 @@ fn jpg_and_webp_follow_the_setting() {
     }
 }
 
+#[test]
+fn a_translucent_jpg_is_flattened_before_its_conversion() {
+    let (mut s, r) = session();
+    s.execute("fill.set", &json!({"ids": [r], "opacity": 0.5})).unwrap();
+    // The sRGB document shows 50% red over white; the Display P3 JPG holds that color converted.
+    let flat = export(&mut s, json!({"ids": [r], "format": "jpg", "colorProfile": "srgb"}));
+    let [fr, fg, fb] = middle(&flat);
+    let shown = Color::new(f32::from(fr) / 255.0, f32::from(fg) / 255.0, f32::from(fb) / 255.0, 1.0);
+    let want = rgb8(ColorProfile::Srgb.conversion_to(ColorProfile::DisplayP3).apply(shown));
+    let p3 = export(&mut s, json!({"ids": [r], "format": "jpg", "colorProfile": "displayP3"}));
+    assert!(close(middle(&p3), want, 2), "{:?} vs {want:?} (sRGB {:?})", middle(&p3), [fr, fg, fb]);
+}
+
 #[test]
 fn svg_pdf_and_xml_stay_srgb() {
     let (mut s, r) = session();