Ensure fit=contain resizing supports multiple alpha channels #4382

docs/src/content/docs/changelog.md

@@ -18,6 +18,9 @@ Requires libvips v8.16.1
   [#4375](https://github.com/lovell/sharp/pull/4375)
   [@hans00](https://github.com/hans00)
 
+* Ensure resizing with a `fit` of `contain` supports multiple alpha channels.
+  [#4382](https://github.com/lovell/sharp/issues/4382)
+
 * TypeScript: Ensure `metadata` response more closely matches reality.
   [#4383](https://github.com/lovell/sharp/issues/4383)
 

src/common.cc

@@ -1001,9 +1001,11 @@ namespace sharp {
         0.0722 * colour[2])
       };
     }
-    // Add alpha channel to alphaColour colour
+    // Add alpha channel(s) to alphaColour colour
     if (colour[3] < 255.0 || image.has_alpha()) {
+      do {
         alphaColour.push_back(colour[3] * multiplier);
+      } while (alphaColour.size() < static_cast<size_t>(image.bands()));
     }
     // Ensure alphaColour colour uses correct colourspace
     alphaColour = sharp::GetRgbaAsColourspace(alphaColour, image.interpretation(), premultiply);

test/unit/resize-contain.js

@@ -806,4 +806,33 @@ describe('Resize fit=contain', function () {
         fixtures.assertSimilar(fixtures.expected('./embedgravitybird/9-c.png'), data, done);
       });
   });
+
+  it('multiple alpha channels', async () => {
+    const create = {
+      width: 20,
+      height: 12,
+      channels: 4,
+      background: 'green'
+    };
+    const multipleAlphaChannels = await sharp({ create })
+      .joinChannel({ create })
+      .tiff({ compression: 'deflate' })
+      .toBuffer();
+
+    const data = await sharp(multipleAlphaChannels)
+      .resize({
+        width: 8,
+        height: 8,
+        fit: 'contain',
+        background: 'blue'
+      })
+      .tiff({ compression: 'deflate' })
+      .toBuffer();
+    const { format, width, height, space, channels } = await sharp(data).metadata();
+    assert.deepStrictEqual(format, 'tiff');
+    assert.deepStrictEqual(width, 8);
+    assert.deepStrictEqual(height, 8);
+    assert.deepStrictEqual(space, 'srgb');
+    assert.deepStrictEqual(channels, 8);
+  });
 });