Ensure composite replicates correct tiles with centre gravity #2626

docs/changelog.md

@@ -20,6 +20,9 @@ Requires libvips v8.10.6
   [#2612](https://github.com/lovell/sharp/issues/2612)
   [@edsilv](https://github.com/edsilv)
 
+* Ensure composite replicates the correct number of tiles for centred gravities.
+  [#2626](https://github.com/lovell/sharp/issues/2626)
+
 ## v0.27 - *avif*
 
 Requires libvips v8.10.5

src/pipeline.cc

@@ -562,9 +562,17 @@ class PipelineWorker : public Napi::AsyncWorker {
             // Use gravity in overlay
             if (compositeImage.width() <= baton->width) {
               across = static_cast<int>(ceil(static_cast<double>(image.width()) / compositeImage.width()));
+              // Ensure odd number of tiles across when gravity is centre, north or south
+              if (composite->gravity == 0 || composite->gravity == 1 || composite->gravity == 3) {
+                across |= 1;
+              }
             }
             if (compositeImage.height() <= baton->height) {
               down = static_cast<int>(ceil(static_cast<double>(image.height()) / compositeImage.height()));
+              // Ensure odd number of tiles down when gravity is centre, east or west
+              if (composite->gravity == 0 || composite->gravity == 2 || composite->gravity == 4) {
+                down |= 1;
+              }
             }
             if (across != 0 || down != 0) {
               int left;

test/unit/composite.js

@@ -225,6 +225,24 @@ describe('composite', () => {
       });
   });
 
+  it('centre gravity should replicate correct number of tiles', async () => {
+    const red = { r: 255, g: 0, b: 0 };
+    const [r, g, b] = await sharp({
+      create: {
+        width: 40, height: 40, channels: 4, background: red
+      }
+    })
+      .composite([{
+        input: fixtures.inputPngWithTransparency16bit,
+        gravity: 'centre',
+        tile: true
+      }])
+      .raw()
+      .toBuffer();
+
+    assert.deepStrictEqual({ r, g, b }, red);
+  });
+
   it('cutout via dest-in', done => {
     sharp(fixtures.inputJpg)
       .resize(300, 300)