Ensure image is unpremultiplied before composite #3334

docs/changelog.md

@@ -52,6 +52,9 @@ Requires libvips v8.13.0
   Emit warnings when previous calls in the same pipeline will be ignored.
   [#3319](https://github.com/lovell/sharp/issues/3319)
 
+* Ensure resized image is unpremultiplied before composite.
+  [#3334](https://github.com/lovell/sharp/issues/3334)
+
 ## v0.30 - *dresser*
 
 Requires libvips v8.12.2

src/pipeline.cc

@@ -590,6 +590,18 @@ class PipelineWorker : public Napi::AsyncWorker {
           baton->sharpenX1, baton->sharpenY2, baton->sharpenY3);
       }
 
+      // Reverse premultiplication after all transformations
+      if (shouldPremultiplyAlpha) {
+        image = image.unpremultiply();
+        // Cast pixel values to integer
+        if (sharp::Is16Bit(image.interpretation())) {
+          image = image.cast(VIPS_FORMAT_USHORT);
+        } else {
+          image = image.cast(VIPS_FORMAT_UCHAR);
+        }
+      }
+      baton->premultiplied = shouldPremultiplyAlpha;
+
       // Composite
       if (shouldComposite) {
         std::vector<VImage> images = { image };
@@ -670,18 +682,6 @@ class PipelineWorker : public Napi::AsyncWorker {
         image = VImage::composite(images, modes, VImage::option()->set("x", xs)->set("y", ys));
       }
 
-      // Reverse premultiplication after all transformations:
-      if (shouldPremultiplyAlpha) {
-        image = image.unpremultiply();
-        // Cast pixel values to integer
-        if (sharp::Is16Bit(image.interpretation())) {
-          image = image.cast(VIPS_FORMAT_USHORT);
-        } else {
-          image = image.cast(VIPS_FORMAT_UCHAR);
-        }
-      }
-      baton->premultiplied = shouldPremultiplyAlpha;
-
       // Gamma decoding (brighten)
       if (baton->gammaOut >= 1 && baton->gammaOut <= 3) {
         image = sharp::Gamma(image, baton->gammaOut);

test/unit/composite.js

@@ -447,4 +447,26 @@ describe('composite', () => {
     assert.strictEqual(info.width, 40);
     assert.strictEqual(info.height, 40);
   });
+
+  it('Ensure implict unpremultiply after resize but before composite', async () => {
+    const [r, g, b, a] = await sharp({
+      create: {
+        width: 1, height: 1, channels: 4, background: 'saddlebrown'
+      }
+    })
+      .resize({ width: 8 })
+      .composite([{
+        input: Buffer.from([255, 255, 255, 128]),
+        raw: { width: 1, height: 1, channels: 4 },
+        tile: true,
+        blend: 'dest-in'
+      }])
+      .raw()
+      .toBuffer();
+
+    assert.strictEqual(r, 139);
+    assert.strictEqual(g, 69);
+    assert.strictEqual(b, 19);
+    assert.strictEqual(a, 128);
+  });
 });