feat(exam): render template PDFs behind setup canvas

src/pages/exam/setup/pdfLoader.ts

@@ -0,0 +1,39 @@
+import * as pdfjsLib from "pdfjs-dist"
+import pdfWorkerSrc from "pdfjs-dist/build/pdf.worker.mjs?url"
+
+import { PAGE_WIDTH } from "../../../utils/exam-canvas/model"
+
+pdfjsLib.GlobalWorkerOptions.workerSrc = pdfWorkerSrc
+
+export interface PdfPageImage {
+  pageNumber: number
+  src: string
+  width: number
+  height: number
+}
+
+export async function loadPdfPageImages(pdfBytes: ArrayBuffer, targetWidth = PAGE_WIDTH): Promise<PdfPageImage[]> {
+  const pdf = await pdfjsLib.getDocument({ data: new Uint8Array(pdfBytes) }).promise
+  const pages: PdfPageImage[] = []
+
+  for (let pageNumber = 1; pageNumber <= pdf.numPages; pageNumber += 1) {
+    const page = await pdf.getPage(pageNumber)
+    const baseViewport = page.getViewport({ scale: 1 })
+    const scale = targetWidth / baseViewport.width
+    const viewport = page.getViewport({ scale })
+    const canvas = document.createElement("canvas")
+    canvas.width = Math.ceil(viewport.width)
+    canvas.height = Math.ceil(viewport.height)
+    const context = canvas.getContext("2d")
+    if (!context) throw new Error("Unable to create PDF render canvas")
+    await page.render({ canvasContext: context, viewport }).promise
+    pages.push({
+      pageNumber,
+      src: canvas.toDataURL("image/png"),
+      width: canvas.width,
+      height: canvas.height,
+    })
+  }
+
+  return pages
+}