fix(io): preserve one-pixel export fringes

packages/core/src/io/formats/raster/render.ts

@@ -80,6 +80,23 @@ function findAlphaBounds(ck: CanvasKit, canvas: Canvas, width: number, height: n
   return { minX, minY, maxX, maxY }
 }
 
+const MIN_TRANSPARENT_TRIM_INSET = 2
+
+function shouldTrimAlphaBounds(
+  alphaBounds: NonNullable<ReturnType<typeof findAlphaBounds>>,
+  width: number,
+  height: number
+): boolean {
+  return (
+    Math.max(
+      alphaBounds.minX,
+      alphaBounds.minY,
+      width - alphaBounds.maxX,
+      height - alphaBounds.maxY
+    ) >= MIN_TRANSPARENT_TRIM_INSET
+  )
+}
+
 function renderToSurface(
   ck: CanvasKit,
   renderer: SkiaRenderer,
@@ -100,7 +117,11 @@ function renderToSurface(
     setup(canvas)
     renderer.renderSceneToCanvas(canvas, renderGraph, pageId)
     surface.flush()
-    const alphaBounds = trimTransparent ? findAlphaBounds(ck, canvas, width, height) : null
+    const foundAlphaBounds = trimTransparent ? findAlphaBounds(ck, canvas, width, height) : null
+    const alphaBounds =
+      foundAlphaBounds && shouldTrimAlphaBounds(foundAlphaBounds, width, height)
+        ? foundAlphaBounds
+        : null
     const image = alphaBounds
       ? surface.makeImageSnapshot([
           alphaBounds.minX,

tests/engine/render/canvas/raster-export.test.ts

@@ -8,11 +8,71 @@ import { expectDefined } from '#tests/helpers/assert'
 
 let ck: Awaited<ReturnType<typeof initCanvasKit>>
 
+function rectangleCommandsBlob(x: number, y: number, width: number, height: number): Uint8Array {
+  const blob = new Uint8Array(1 + 4 * 9 + 1)
+  const view = new DataView(blob.buffer)
+  const points = [
+    { command: 1, x, y },
+    { command: 2, x: x + width, y },
+    { command: 2, x: x + width, y: y + height },
+    { command: 2, x, y: y + height }
+  ]
+  let offset = 0
+  for (const point of points) {
+    blob[offset] = point.command
+    view.setFloat32(offset + 1, point.x, true)
+    view.setFloat32(offset + 5, point.y, true)
+    offset += 9
+  }
+  blob[offset] = 0
+  return blob
+}
+
 beforeAll(async () => {
   ck = await initCanvasKit()
 })
 
 describe('raster export', () => {
+  test('keeps one-pixel transparent fringes when trimming exports', async () => {
+    const graph = new SceneGraph()
+    const page = graph.getPages()[0]
+    const vector = graph.createNode('VECTOR', page.id, {
+      width: 10,
+      height: 10,
+      fillGeometry: [{ commandsBlob: rectangleCommandsBlob(1, 1, 8, 8) }],
+      fills: [
+        {
+          type: 'SOLID',
+          color: { r: 0, g: 0, b: 0, a: 1 },
+          opacity: 1,
+          visible: true
+        }
+      ]
+    })
+
+    const surface = expectDefined(ck.MakeSurface(1, 1), 'surface')
+    const renderer = new SkiaRenderer(ck, surface)
+
+    try {
+      const png = expectDefined(
+        renderNodesToImage(ck, renderer, graph, page.id, [vector.id], {
+          scale: 1,
+          format: 'PNG',
+          trimTransparent: true
+        }),
+        'png'
+      )
+      const image = expectDefined(ck.MakeImageFromEncoded(png), 'image')
+
+      expect(image.width()).toBe(10)
+      expect(image.height()).toBe(10)
+
+      image.delete()
+    } finally {
+      surface.delete()
+    }
+  })
+
   test('page exports can trim transparent text padding', async () => {
     const graph = new SceneGraph()
     const page = graph.getPages()[0]