Add premultiplied option to composite operation (#1835)

docs/api-composite.md

@@ -31,6 +31,7 @@ and [https://www.cairographics.org/operators/][2]
     -   `images[].top` **[Number][7]?** the pixel offset from the top edge.
     -   `images[].left` **[Number][7]?** the pixel offset from the left edge.
     -   `images[].tile` **[Boolean][9]** set to true to repeat the overlay image across the entire image with the given `gravity`. (optional, default `false`)
+    -   `images[].premultiplied` **[Boolean][9]** set to true to avoid premultipling the image below. Equivalent to the `--premultiplied` vips option. (optional, default `false`)
     -   `images[].density` **[Number][7]** number representing the DPI for vector overlay image. (optional, default `72`)
     -   `images[].raw` **[Object][4]?** describes overlay when using raw pixel data.
         -   `images[].raw.width` **[Number][7]?** 

lib/composite.js

@@ -105,7 +105,8 @@ function composite (images) {
       tile: false,
       left: -1,
       top: -1,
-      gravity: 0
+      gravity: 0,
+      premultiplied: false
     };
     if (is.defined(image.blend)) {
       if (is.string(blend[image.blend])) {
@@ -147,6 +148,14 @@ function composite (images) {
         throw is.invalidParameterError('gravity', 'valid gravity', image.gravity);
       }
     }
+    if (is.defined(image.premultiplied)) {
+      if (is.bool(image.premultiplied)) {
+        composite.premultiplied = image.premultiplied;
+      } else {
+        throw is.invalidParameterError('premultiplied', 'boolean', image.premultiplied);
+      }
+    }
+
     return composite;
   });
   return this;

src/pipeline.cc

@@ -591,7 +591,7 @@ class PipelineWorker : public Nan::AsyncWorker {
           if (!HasAlpha(compositeImage)) {
             compositeImage = sharp::EnsureAlpha(compositeImage);
           }
-          compositeImage = compositeImage.premultiply();
+          if (!composite->premultiplied) compositeImage = compositeImage.premultiply();
           // Calculate position
           int left;
           int top;
@@ -1230,6 +1230,7 @@ NAN_METHOD(pipeline) {
     composite->left = AttrTo<int32_t>(compositeObject, "left");
     composite->top = AttrTo<int32_t>(compositeObject, "top");
     composite->tile = AttrTo<bool>(compositeObject, "tile");
+    composite->premultiplied = AttrTo<bool>(compositeObject, "premultiplied");
     baton->composite.push_back(composite);
   }
   // Resize options

src/pipeline.h

@@ -41,6 +41,7 @@ struct Composite {
   int left;
   int top;
   bool tile;
+  bool premultiplied;
 
   Composite():
     input(nullptr),
@@ -48,7 +49,8 @@ struct Composite {
     gravity(0),
     left(-1),
     top(-1),
-    tile(false) {}
+    tile(false),
+    premultiplied(false) {}
 };
 
 struct PipelineBaton {

test/unit/composite.js

@@ -62,6 +62,65 @@ describe('composite', () => {
     })
   ));
 
+  it('premultiplied true', () => {
+    const filename = 'composite.premultiplied.png';
+    const below = fixtures.path(`input.below.${filename}`);
+    const above = fixtures.path(`input.above.${filename}`);
+    const actual = fixtures.path(`output.true.${filename}`);
+    const expected = fixtures.expected(`expected.true.${filename}`);
+    return sharp(below)
+      .composite([{
+        input: above,
+        blend: 'color-burn',
+        top: 0,
+        left: 0,
+        premultiplied: true
+      }])
+      .toFile(actual)
+      .then(() => {
+        fixtures.assertMaxColourDistance(actual, expected);
+      });
+  });
+
+  it('premultiplied false', () => {
+    const filename = 'composite.premultiplied.png';
+    const below = fixtures.path(`input.below.${filename}`);
+    const above = fixtures.path(`input.above.${filename}`);
+    const actual = fixtures.path(`output.false.${filename}`);
+    const expected = fixtures.expected(`expected.false.${filename}`);
+    return sharp(below)
+      .composite([{
+        input: above,
+        blend: 'color-burn',
+        top: 0,
+        left: 0,
+        premultiplied: false
+      }])
+      .toFile(actual)
+      .then(() => {
+        fixtures.assertMaxColourDistance(actual, expected);
+      });
+  });
+
+  it('premultiplied absent', () => {
+    const filename = 'composite.premultiplied.png';
+    const below = fixtures.path(`input.below.${filename}`);
+    const above = fixtures.path(`input.above.${filename}`);
+    const actual = fixtures.path(`output.absent.${filename}`);
+    const expected = fixtures.expected(`expected.absent.${filename}`);
+    return sharp(below)
+      .composite([{
+        input: above,
+        blend: 'color-burn',
+        top: 0,
+        left: 0
+      }])
+      .toFile(actual)
+      .then(() => {
+        fixtures.assertMaxColourDistance(actual, expected);
+      });
+  });
+
   it('multiple', () => {
     const filename = 'composite-multiple.png';
     const actual = fixtures.path(`output.${filename}`);
@@ -265,6 +324,12 @@ describe('composite', () => {
       }, /Expected boolean for tile but received invalid of type string/);
     });
 
+    it('invalid premultiplied', () => {
+      assert.throws(() => {
+        sharp().composite([{ input: 'test', premultiplied: 'invalid' }]);
+      }, /Expected boolean for premultiplied but received invalid of type string/);
+    });
+
     it('invalid left', () => {
       assert.throws(() => {
         sharp().composite([{ input: 'test', left: 0.5 }]);