Enhance next dev performance with placeholder=blur

packages/next/build/webpack-config.ts

@@ -1100,6 +1100,8 @@ export default async function getBaseWebpackConfig(
                 dependency: { not: ['url'] },
                 options: {
                   isServer,
+                  isDev: dev,
+                  assetPrefix: config.assetPrefix,
                 },
               },
             ]

packages/next/build/webpack/loaders/next-image-loader.js

@@ -9,14 +9,15 @@ const VALID_BLUR_EXT = ['jpeg', 'png', 'webp']
 function nextImageLoader(content) {
   const imageLoaderSpan = this.currentTraceSpan.traceChild('next-image-loader')
   return imageLoaderSpan.traceAsyncFn(async () => {
-    const isServer = loaderUtils.getOptions(this).isServer
+    const { isServer, isDev, assetPrefix } = loaderUtils.getOptions(this)
     const context = this.rootContext
     const opts = { context, content }
     const interpolatedName = loaderUtils.interpolateName(
       this,
       '/static/image/[path][name].[hash].[ext]',
       opts
     )
+    const outputPath = '/_next' + interpolatedName
 
     let extension = loaderUtils.interpolateName(this, '[ext]', opts)
     if (extension === 'jpg') {
@@ -26,31 +27,41 @@ function nextImageLoader(content) {
     const imageSizeSpan = imageLoaderSpan.traceChild('image-size-calculation')
     const imageSize = imageSizeSpan.traceFn(() => sizeOf(content))
     let blurDataURL
+
     if (VALID_BLUR_EXT.includes(extension)) {
-      // Shrink the image's largest dimension
-      const resizeOperationOpts =
-        imageSize.width >= imageSize.height
-          ? { type: 'resize', width: BLUR_IMG_SIZE }
-          : { type: 'resize', height: BLUR_IMG_SIZE }
+      if (isDev) {
+        const prefix = 'http://localhost'
+        const url = new URL('/_next/image', prefix)
+        url.searchParams.set('url', assetPrefix + outputPath)
+        url.searchParams.set('w', BLUR_IMG_SIZE)
+        url.searchParams.set('q', BLUR_QUALITY)
+        blurDataURL = url.href.slice(prefix.length)
+      } else {
+        // Shrink the image's largest dimension
+        const resizeOperationOpts =
+          imageSize.width >= imageSize.height
+            ? { type: 'resize', width: BLUR_IMG_SIZE }
+            : { type: 'resize', height: BLUR_IMG_SIZE }
 
-      const resizeImageSpan = imageLoaderSpan.traceChild('image-resize')
-      const resizedImage = await resizeImageSpan.traceAsyncFn(() =>
-        processBuffer(content, [resizeOperationOpts], extension, BLUR_QUALITY)
-      )
-      const blurDataURLSpan = imageLoaderSpan.traceChild(
-        'image-base64-tostring'
-      )
-      blurDataURL = blurDataURLSpan.traceFn(
-        () =>
-          `data:image/${extension};base64,${resizedImage.toString('base64')}`
-      )
+        const resizeImageSpan = imageLoaderSpan.traceChild('image-resize')
+        const resizedImage = await resizeImageSpan.traceAsyncFn(() =>
+          processBuffer(content, [resizeOperationOpts], extension, BLUR_QUALITY)
+        )
+        const blurDataURLSpan = imageLoaderSpan.traceChild(
+          'image-base64-tostring'
+        )
+        blurDataURL = blurDataURLSpan.traceFn(
+          () =>
+            `data:image/${extension};base64,${resizedImage.toString('base64')}`
+        )
+      }
     }
 
     const stringifiedData = imageLoaderSpan
       .traceChild('image-data-stringify')
       .traceFn(() =>
         JSON.stringify({
-          src: '/_next' + interpolatedName,
+          src: outputPath,
           height: imageSize.height,
           width: imageSize.width,
           blurDataURL,

packages/next/server/image-optimizer.ts

@@ -26,7 +26,7 @@ const CACHE_VERSION = 3
 const MODERN_TYPES = [/* AVIF, */ WEBP]
 const ANIMATABLE_TYPES = [WEBP, PNG, GIF]
 const VECTOR_TYPES = [SVG]
-
+const BLUR_IMG_SIZE = 8 // should match `next-image-loader`
 const inflightRequests = new Map<string, Promise<undefined>>()
 
 export async function imageOptimizer(
@@ -125,6 +125,10 @@ export async function imageOptimizer(
 
   const sizes = [...deviceSizes, ...imageSizes]
 
+  if (isDev) {
+    sizes.push(BLUR_IMG_SIZE)
+  }
+
   if (!sizes.includes(width)) {
     res.statusCode = 400
     res.end(`"w" parameter (width) of ${width} is not allowed`)

test/integration/image-optimizer/test/index.test.js

@@ -932,4 +932,31 @@ describe('Image Optimizer', () => {
       await expectWidth(res, 64)
     })
   })
+
+  describe('dev support for dynamic blur placeholder', () => {
+    beforeAll(async () => {
+      const json = JSON.stringify({
+        images: {
+          deviceSizes: [largeSize],
+          imageSizes: [],
+        },
+      })
+      nextConfig.replace('{ /* replaceme */ }', json)
+      appPort = await findPort()
+      app = await launchApp(appDir, appPort)
+    })
+    afterAll(async () => {
+      await killApp(app)
+      nextConfig.restore()
+      await fs.remove(imagesDir)
+    })
+
+    it('should support width 8 per BLUR_IMG_SIZE with next dev', async () => {
+      const query = { url: '/test.png', w: 8, q: 70 }
+      const opts = { headers: { accept: 'image/webp' } }
+      const res = await fetchViaHTTP(appPort, '/_next/image', query, opts)
+      expect(res.status).toBe(200)
+      await expectWidth(res, 8)
+    })
+  })
 })