fix(pptx/render): emit valid SVG <image> for image-fill backgrounds (#100)

* fix(pptx/render): emit valid SVG <image> for image-fill backgrounds

renderDeckToSvg rendered a slide's image-fill background as a CSS
background shorthand inside fill="…" (center / cover no-repeat
url("data:image…")) — invalid SVG that strict rasterisers (resvg,
librsvg) reject, blocking a Chromium-free render path. The renderer now
recognises a url(...) anywhere in the value and emits a real <image>
element (slice for cover, meet for contain). Adds a strict-XML-parser
lock-in test over an image-background fixture.

* test(render): rasterise image-background slide through real @resvg/resvg-js

XMLValidator proves well-formedness only; resvg is the actual consumer.
Add @resvg/resvg-js as a devDependency and drive an image-background
slide through the package's default rasteriser (no injected hook),
asserting a valid PNG of the expected dimensions (320x180). resvg threw
on the old fill="...url(data:...)..." output, so this is a true guard.

* Revert "test(render): rasterise image-background slide through real @resvg/resvg-js"

This reverts commit 3f9d393. Keep the resvg renderability check on the
consumer side (the host serializer already ships @resvg/resvg-js) rather
than dragging a platform-native binary into the package's CI. The bug was
an SVG-validity (well-formedness) regression, which the pure-JS
XMLValidator lock-in already catches — it was confirmed to reject the old
malformed output. The package CI stays native-dep-free.

Author: karthikmudunuri

.changeset/render-image-bg-valid-svg.md

@@ -0,0 +1,20 @@
+---
+"@textcortex/slidewise": patch
+---
+
+fix(render): emit a valid SVG `<image>` for image-fill backgrounds
+
+`renderDeckToSvg` rendered a slide's image-fill background as a CSS `background`
+shorthand inside an SVG `fill="…"` attribute — `fill="center / cover no-repeat
+url("data:image…")"`. That is not valid SVG (a non-paint value plus nested
+unescaped quotes); browsers tolerate it, but strict rasterisers
+(`@resvg/resvg-js`, librsvg, batik) reject it, blocking a Chromium-free
+`parsePptx → renderDeckToSvg → resvg → PNG` path.
+
+The pptx importer stores image backgrounds as a CSS shorthand, but the
+renderer's image-ref detection only inspected the *start* of the value, so the
+shorthand fell through to the `fill` path. The renderer now recognises a
+`url(...)` anywhere in the value and emits a real `<image>` element
+(`preserveAspectRatio` = `slice` for `cover`, `meet` for `contain`). A lock-in
+test asserts every rendered slide is valid SVG a strict XML parser accepts,
+with an image-background slide as the regression case.

packages/slidewise/src/lib/render/__tests__/render-deck.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect } from "vitest";
 import { readFileSync } from "node:fs";
 import { fileURLToPath } from "node:url";
 import path from "node:path";
+import { XMLValidator } from "fast-xml-parser";
 import {
   renderDeckToSvg,
   renderDeckToImages,
@@ -146,6 +147,57 @@ describe("renderDeckToSvg / renderDeckToImages", () => {
     expect(w).toBe(480);
   });
 
+  it("emits a real <image> for an image-fill background, not a CSS-shorthand fill", async () => {
+    // The pptx importer stores image backgrounds as a CSS `background`
+    // shorthand: `center / cover no-repeat url("data:image…")`. The renderer
+    // must turn that into a valid SVG <image>, never `fill="…url(data:…)…"`
+    // (nested quotes + a non-paint value that strict rasterisers reject).
+    const bg = `center / cover no-repeat url("${IMG_SRC}")`;
+    const deck = {
+      version: 1,
+      title: "ImageBg",
+      slides: [{ id: "s1", background: bg, elements: [] }],
+    } as Deck;
+
+    const [svg] = await renderDeckToSvg(deck);
+    expect(svg).toContain(`<image`);
+    expect(svg).toContain(`xlink:href="${IMG_SRC}"`);
+    expect(svg).toContain(`preserveAspectRatio="xMidYMid slice"`); // cover
+    // The malformed shorthand-as-fill must NOT appear.
+    expect(svg).not.toContain("no-repeat");
+    expect(svg).not.toMatch(/fill="[^"]*url\(/);
+  });
+
+  it("renders `contain` image backgrounds with preserveAspectRatio=meet", async () => {
+    const bg = `center / contain no-repeat url("${IMG_SRC}")`;
+    const deck = {
+      version: 1,
+      title: "ContainBg",
+      slides: [{ id: "s1", background: bg, elements: [] }],
+    } as Deck;
+    const [svg] = await renderDeckToSvg(deck);
+    expect(svg).toContain(`preserveAspectRatio="xMidYMid meet"`);
+  });
+
+  it("every rendered slide is valid SVG a strict XML parser accepts", async () => {
+    // Lock-in for the resvg/librsvg path: a strict (non-browser) parser must
+    // accept every slide. An image-background slide is the regression case.
+    const deck = {
+      version: 1,
+      title: "Strict",
+      slides: [
+        { id: "s1", background: `center / cover no-repeat url("${IMG_SRC}")`, elements: [] },
+        ...buildDeck().slides,
+      ],
+    } as Deck;
+    const svgs = await renderDeckToSvg(deck);
+    for (const svg of svgs) {
+      const result = XMLValidator.validate(svg);
+      expect(result, typeof result === "object" ? JSON.stringify(result.err) : "")
+        .toBe(true);
+    }
+  });
+
   it("stays browser-free (no Playwright/Puppeteer/jsdom in the source)", () => {
     const __dirname = path.dirname(fileURLToPath(import.meta.url));
     const src = readFileSync(path.resolve(__dirname, "../renderDeck.ts"), "utf8");

packages/slidewise/src/lib/render/renderDeck.ts

@@ -163,9 +163,10 @@ function renderBackground(background: string | undefined): string {
   const fill = background ? solidFrom(background) : "#FFFFFF";
   if (isImageRef(background)) {
     const href = imageHref(background!);
+    const preserve = imageFitPreserve(background);
     return (
       `<rect x="0" y="0" width="${SLIDE_W}" height="${SLIDE_H}" fill="#FFFFFF"/>` +
-      `<image x="0" y="0" width="${SLIDE_W}" height="${SLIDE_H}" preserveAspectRatio="xMidYMid slice" xlink:href="${escAttr(href)}"/>`
+      `<image x="0" y="0" width="${SLIDE_W}" height="${SLIDE_H}" preserveAspectRatio="${preserve}" xlink:href="${escAttr(href)}"/>`
     );
   }
   return `<rect x="0" y="0" width="${SLIDE_W}" height="${SLIDE_H}" fill="${fill ?? "#FFFFFF"}"/>`;
@@ -499,12 +500,24 @@ async function defaultRasterize(
 // -- colour / string helpers -------------------------------------------------
 
 function isImageRef(s: string | undefined): boolean {
-  return !!s && (s.startsWith("data:image") || /^url\(/i.test(s) || /^https?:\/\//i.test(s));
+  if (!s) return false;
+  const v = s.trim();
+  // A bare data/http URL, OR a CSS `background` shorthand that embeds a
+  // `url(...)` anywhere (e.g. `center / cover no-repeat url("data:image…")`,
+  // as produced by the pptx importer for image-fill backgrounds).
+  return v.startsWith("data:image") || /url\(/i.test(v) || /^https?:\/\//i.test(v);
 }
 
 function imageHref(s: string): string {
-  const m = /^url\(["']?(.*?)["']?\)$/i.exec(s);
-  return m ? m[1] : s;
+  // Pull the URL out of a `url(...)` wherever it appears in the value; data
+  // URLs use a `)`-free base64 alphabet, so non-greedy-to-first-`)` is safe.
+  const m = /url\(\s*["']?(.*?)["']?\s*\)/i.exec(s);
+  return m ? m[1] : s.trim();
+}
+
+/** `cover` → `slice`, `contain` → `meet`, for an image-fill shorthand. */
+function imageFitPreserve(s: string | undefined): string {
+  return s && /\bcontain\b/i.test(s) ? "xMidYMid meet" : "xMidYMid slice";
 }
 
 /** Best-effort single colour for SVG: pass hex through, pull the first hex out