Prevent aliasing by using dynamic values for shrink(-on-load)

docs/changelog.md

@@ -41,6 +41,10 @@ Requires libvips v8.5.2.
   [#772](https://github.com/lovell/sharp/issues/772)
   [@ajiratech2](https://github.com/ajiratech2)
 
+* Prevent aliasing by using dynamic values for shrink(-on-load).
+  [#781](https://github.com/lovell/sharp/issues/781)
+  [@kleisauke](https://github.com/kleisauke)
+
 * Expose libvips' "squash" parameter to enable 1-bit TIFF output.
   [#783](https://github.com/lovell/sharp/pull/783)
   [@YvesBos](https://github.com/YvesBos)

src/pipeline.cc

@@ -242,6 +242,12 @@ class PipelineWorker : public Nan::AsyncWorker {
           shrink_on_load = 2;
         }
       }
+      // Help ensure a final kernel-based reduction to prevent shrink aliasing
+      if (shrink_on_load > 1 && (xresidual == 1.0 || yresidual == 1.0)) {
+        shrink_on_load = shrink_on_load / 2;
+        xfactor = xfactor * 2;
+        yfactor = yfactor * 2;
+      }
       if (shrink_on_load > 1) {
         // Reload input using shrink-on-load
         vips::VOption *option = VImage::option()->set("shrink", shrink_on_load);
@@ -285,6 +291,13 @@ class PipelineWorker : public Nan::AsyncWorker {
           std::swap(xresidual, yresidual);
         }
       }
+      // Help ensure a final kernel-based reduction to prevent shrink aliasing
+      if ((xshrink > 1 || yshrink > 1) && (xresidual == 1.0 || yresidual == 1.0)) {
+        xshrink = xshrink / 2;
+        yshrink = yshrink / 2;
+        xresidual = xresidual / 2.0;
+        yresidual = yresidual / 2.0;
+      }
 
       // Ensure we're using a device-independent colour space
       if (sharp::HasProfile(image)) {

test/unit/io.js

@@ -1335,7 +1335,7 @@ describe('Input/output', function () {
               assert.strictEqual(256, info.width);
               assert.strictEqual(192, info.height);
               assert.strictEqual(4, info.channels);
-              fixtures.assertSimilar(fixtures.inputPngOverlayLayer1, data, done);
+              fixtures.assertSimilar(fixtures.inputPngOverlayLayer1, data, { threshold: 7 }, done);
             });
         });
     });