Add support for negating only non-alpha channels

Fixes #1035

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),