Add support for negating only non-alpha channels

Fixes #1035

docs/api-operation.md

@@ -221,7 +221,9 @@ Produce the "negative" of the image.
 
 ### Parameters
 
-*   `negate` **[Boolean][6]**  (optional, default `true`)
+*   `options` **[Object][2]?** 
+
+    *   `options.alpha` **[Boolean][6]** Whether or not to negate any alpha channel (optional, default `true`)
 
 Returns **Sharp** 
 

lib/constructor.js

@@ -179,6 +179,7 @@ const Sharp = function (input, options) {
     flatten: false,
     flattenBackground: [0, 0, 0],
     negate: false,
+    negateAlpha: true,
     medianSize: 0,
     blurSigma: 0,
     sharpenSigma: 0,

lib/operation.js

@@ -325,11 +325,19 @@ function gamma (gamma, gammaOut) {
 
 /**
  * Produce the "negative" of the image.
- * @param {Boolean} [negate=true]
+ * @param {Object} [options]
+ * @param {Boolean} [options.alpha=true] Whether or not to negate any alpha channel
  * @returns {Sharp}
  */
-function negate (negate) {
-  this.options.negate = is.bool(negate) ? negate : true;
+function negate (options) {
+  this.options.negate = is.bool(options) ? options : true;
+  if (is.plainObject(options) && 'alpha' in options) {
+    if (!is.bool(options.alpha)) {
+      throw is.invalidParameterError('alpha', 'should be boolean value', options.alpha);
+    } else {
+      this.options.negateAlpha = options.alpha;
+    }
+  }
   return this;
 }
 

src/operations.cc

@@ -112,6 +112,19 @@ namespace sharp {
     }
   }
 
+  /**
+   * Produce the "negative" of the image.
+   */
+  VImage Negate(VImage image, bool const negateAlpha) {
+    if (HasAlpha(image) && !negateAlpha) {
+      // Separate alpha channel
+      VImage alpha = image[image.bands() - 1];
+      return RemoveAlpha(image).invert().bandjoin(alpha);
+    } else {
+      return image.invert();
+    }
+  }
+
   /*
    * Gaussian blur. Use sigma of -1.0 for fast blur.
    */

src/operations.h

@@ -45,6 +45,11 @@ namespace sharp {
    */
   VImage Gamma(VImage image, double const exponent);
 
+  /*
+   * Produce the "negative" of the image.
+   */
+  VImage Negate(VImage image, bool const negateAlpha);
+
   /*
    * Gaussian blur. Use sigma of -1.0 for fast blur.
    */

src/pipeline.cc

@@ -327,7 +327,7 @@ class PipelineWorker : public Napi::AsyncWorker {
 
       // Negate the colours in the image
       if (baton->negate) {
-        image = image.invert();
+        image = sharp::Negate(image, baton->negateAlpha);
       }
 
       // Gamma encoding (darken)
@@ -1320,6 +1320,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->flatten = sharp::AttrAsBool(options, "flatten");
   baton->flattenBackground = sharp::AttrAsVectorOfDouble(options, "flattenBackground");
   baton->negate = sharp::AttrAsBool(options, "negate");
+  baton->negateAlpha = sharp::AttrAsBool(options, "negateAlpha");
   baton->blurSigma = sharp::AttrAsDouble(options, "blurSigma");
   baton->brightness = sharp::AttrAsDouble(options, "brightness");
   baton->saturation = sharp::AttrAsDouble(options, "saturation");

src/pipeline.h

@@ -90,6 +90,7 @@ struct PipelineBaton {
   bool flatten;
   std::vector<double> flattenBackground;
   bool negate;
+  bool negateAlpha;
   double blurSigma;
   double brightness;
   double saturation;
@@ -220,6 +221,7 @@ struct PipelineBaton {
     flatten(false),
     flattenBackground{ 0.0, 0.0, 0.0 },
     negate(false),
+    negateAlpha(true),
     blurSigma(0.0),
     brightness(1.0),
     saturation(1.0),

test/unit/negate.js

@@ -107,4 +107,88 @@ describe('Negate', function () {
         done();
       });
   });
+
+  it('negate ({alpha: true})', function (done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .negate({ alpha: true })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('jpeg', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('negate.jpg'), data, done);
+      });
+  });
+
+  it('negate non-alpha channels (png)', function (done) {
+    sharp(fixtures.inputPng)
+      .resize(320, 240)
+      .negate({ alpha: false })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('negate-preserve-alpha.png'), data, done);
+      });
+  });
+
+  it('negate non-alpha channels (png, trans)', function (done) {
+    sharp(fixtures.inputPngWithTransparency)
+      .resize(320, 240)
+      .negate({ alpha: false })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('negate-preserve-alpha-trans.png'), data, done);
+      });
+  });
+
+  it('negate non-alpha channels (png, alpha)', function (done) {
+    sharp(fixtures.inputPngWithGreyAlpha)
+      .resize(320, 240)
+      .negate({ alpha: false })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('negate-preserve-alpha-grey.png'), data, done);
+      });
+  });
+
+  it('negate non-alpha channels (webp)', function (done) {
+    sharp(fixtures.inputWebP)
+      .resize(320, 240)
+      .negate({ alpha: false })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('webp', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('negate-preserve-alpha.webp'), data, done);
+      });
+  });
+
+  it('negate non-alpha channels (webp, trans)', function (done) {
+    sharp(fixtures.inputWebPWithTransparency)
+      .resize(320, 240)
+      .negate({ alpha: false })
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual('webp', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('negate-preserve-alpha-trans.webp'), data, done);
+      });
+  });
+
+  it('invalid alpha value', function () {
+    assert.throws(function () {
+      sharp(fixtures.inputWebPWithTransparency).negate({ alpha: 'non-bool' });
+    });
+  });
 });