M7: a slice's export preview shows the page content under it

A slice paints nothing itself, so thumbnail() rendered an empty pixmap of the slice's own box: expanding Preview for a slice showed only the transparency checkerboard while exporting the same slice wrote the artwork correctly.

Give the preview the export's slice rule (drawn_layers + export_area in cmd/file.rs): draw every top-level layer of the slice's page over its world bounds. Regression tests cover a slice over artwork, a slice with nothing under it, and a plain layer.

Signed-off-by: Ahmed Mahmoud <ahmedmhmouad41@gmail.com>

crates/ui-egui/src/assets_panel.rs

@@ -190,15 +190,27 @@ fn same_layers(a: &Document, b: &Document, id: NodeId) -> bool {
 }
 
 pub(crate) fn thumbnail(ctx: &egui::Context, doc: &Document, id: NodeId, px: f32) -> Option<egui::TextureHandle> {
-    let b = figcraft_render::export_bounds(doc, id)?;
+    let pix = thumbnail_pixmap(doc, id, px)?;
+    let img = crate::render_worker::screen_image(&pix, doc.color_profile);
+    Some(ctx.load_texture(format!("asset-thumb-{}", id.0), img, egui::TextureOptions::LINEAR))
+}
+
+/// The pixels a thumbnail shows. A slice paints nothing itself, so it shows the page content
+/// under it, exactly as exporting one does (`figcraft_engine::cmd::file::drawn_layers`): without
+/// this a slice's preview is a blank checkerboard next to a correct export.
+pub(crate) fn thumbnail_pixmap(doc: &Document, id: NodeId, px: f32) -> Option<figcraft_render::Pixmap> {
+    let slice = doc.node(id).is_some_and(|n| matches!(n.kind, NodeKind::Slice));
+    let b = if slice { doc.world_bounds(id)? } else { figcraft_render::export_bounds(doc, id)? };
     let side = b.width().max(b.height());
     if !(side.is_finite() && side > 0.0) {
         return None;
     }
     let scale = (f64::from(px) / side).min(2.0);
-    let pix = figcraft_render::render_node(doc, id, scale)?;
-    let img = crate::render_worker::screen_image(&pix, doc.color_profile);
-    Some(ctx.load_texture(format!("asset-thumb-{}", id.0), img, egui::TextureOptions::LINEAR))
+    if slice {
+        let layers = doc.page_of(id).and_then(|p| doc.node(p)).map(|p| p.children.clone()).unwrap_or_default();
+        return figcraft_render::render_area(doc, &layers, b, scale);
+    }
+    figcraft_render::render_node(doc, id, scale)
 }
 
 fn matches(name: &str, q: &str) -> bool {

crates/ui-egui/src/export_preview_tests.rs

@@ -0,0 +1,52 @@
+//! The Export section's Preview row: a slice paints nothing itself, so its preview must show the
+//! page content under it — the same thing exporting the slice writes — and not a blank
+//! transparency checkerboard next to a correct export.
+
+use figcraft_doc::{Document, NodeId};
+use figcraft_engine::Session;
+use serde_json::{Value, json};
+
+use crate::assets_panel::thumbnail_pixmap;
+
+/// Run `commands`, returning the document and the id each one made.
+fn doc_after(commands: &[(&str, Value)]) -> (Document, Vec<NodeId>) {
+    let mut s = Session::new();
+    let ids = commands.iter().map(|(cmd, params)| NodeId(s.execute(cmd, params).unwrap()["id"].as_u64().unwrap())).collect();
+    (s.doc().unwrap().clone(), ids)
+}
+
+/// Whether the pixmap paints anything (any pixel not fully transparent).
+fn paints(pix: &figcraft_render::Pixmap) -> bool {
+    figcraft_render::to_rgba8(pix).as_chunks::<4>().0.iter().any(|p| p.get(3).copied().unwrap_or(0) > 0)
+}
+
+#[test]
+fn a_slice_previews_the_content_under_it() {
+    let (doc, ids) = doc_after(&[
+        ("shape.rectangle", json!({"x": 0, "y": 0, "width": 200, "height": 100})),
+        ("node.create", json!({"type": "slice", "x": 0, "y": 0, "width": 200, "height": 100})),
+    ]);
+    let slice = ids[1];
+    let pix = thumbnail_pixmap(&doc, slice, 200.0).expect("the slice previews");
+    assert_eq!([pix.width(), pix.height()], [200, 100], "the slice's area, not an empty node");
+    assert!(paints(&pix), "the rectangle under the slice shows");
+}
+
+#[test]
+fn a_slice_with_nothing_under_it_is_blank() {
+    let (doc, ids) = doc_after(&[("node.create", json!({"type": "slice", "x": 0, "y": 0, "width": 200, "height": 100}))]);
+    let pix = thumbnail_pixmap(&doc, ids[0], 200.0).expect("the preview still has the slice's area");
+    assert_eq!([pix.width(), pix.height()], [200, 100]);
+    assert!(!paints(&pix), "nothing under it stays transparent");
+}
+
+#[test]
+fn other_layers_preview_themselves() {
+    let (doc, ids) = doc_after(&[
+        ("shape.rectangle", json!({"x": 0, "y": 0, "width": 200, "height": 100})),
+        ("node.create", json!({"type": "slice", "x": 0, "y": 0, "width": 200, "height": 100})),
+    ]);
+    let pix = thumbnail_pixmap(&doc, ids[0], 200.0).expect("the rectangle previews");
+    assert_eq!([pix.width(), pix.height()], [200, 100], "the layer's own area");
+    assert!(paints(&pix));
+}

crates/ui-egui/src/lib.rs

@@ -78,6 +78,8 @@ mod cursors_tests;
 #[cfg(test)]
 mod dev_mode_tests;
 #[cfg(test)]
+mod export_preview_tests;
+#[cfg(test)]
 mod field_tests;
 #[cfg(test)]
 mod frame_label_tests;