Skip JPEG shrink-on-load for known rounding errors #3066

Co-authored-by: Kleis Auke Wolthuizen <github@kleisauke.nl>

docs/changelog.md

@@ -4,6 +4,11 @@
 
 Requires libvips v8.12.2
 
+### v0.30.1 - TBD
+
+* Skip shrink-on-load for known libjpeg rounding errors.
+  [#3066](https://github.com/lovell/sharp/issues/3066)
+
 ### v0.30.0 - 1st February 2022
 
 * Add support for GIF output to prebuilt binaries.

src/pipeline.cc

@@ -193,6 +193,10 @@ class PipelineWorker : public Napi::AsyncWorker {
           } else if (shrink >= 2 * factor) {
             jpegShrinkOnLoad = 2;
           }
+          // Skip shrink-on-load for known libjpeg rounding errors
+          if (jpegShrinkOnLoad > 1 && shrink == jpegShrinkOnLoad) {
+            jpegShrinkOnLoad /= 2;
+          }
         } else if (inputImageType == sharp::ImageType::WEBP ||
                    inputImageType == sharp::ImageType::SVG ||
                    inputImageType == sharp::ImageType::PDF) {

test/unit/resize.js

@@ -780,6 +780,27 @@ describe('Resize dimensions', function () {
     assert.strictEqual(info.height, 1);
   });
 
+  it('Skip JPEG shrink-on-load for known libjpeg rounding errors', async () => {
+    const input = await sharp({
+      create: {
+        width: 1000,
+        height: 667,
+        channels: 3,
+        background: 'red'
+      }
+    })
+      .jpeg()
+      .toBuffer();
+
+    const output = await sharp(input)
+      .resize({ width: 500 })
+      .toBuffer();
+
+    const { width, height } = await sharp(output).metadata();
+    assert.strictEqual(width, 500);
+    assert.strictEqual(height, 334);
+  });
+
   it('unknown kernel throws', function () {
     assert.throws(function () {
       sharp().resize(null, null, { kernel: 'unknown' });