Add unflatten operation to create an alpha channel (#3461)

src/operations.cc

@@ -342,6 +342,19 @@ namespace sharp {
     }
   }
 
+  /*
+   * Unflatten
+   */
+  VImage Unflatten(VImage image) {
+    if (HasAlpha(image)) {
+      VImage alpha = image[image.bands() - 1];
+      VImage noAlpha = RemoveAlpha(image);
+      return noAlpha.bandjoin(alpha & (noAlpha.colourspace(VIPS_INTERPRETATION_B_W) < 255));
+    } else {
+      return image.bandjoin(image.colourspace(VIPS_INTERPRETATION_B_W) < 255);
+    }
+  }
+
   /*
    * Ensure the image is in a given colourspace
    */

src/operations.h

@@ -86,6 +86,11 @@ namespace sharp {
    */
   VImage Linear(VImage image, std::vector<double> const a,  std::vector<double> const b);
 
+  /*
+   * Unflatten
+   */
+  VImage Unflatten(VImage image);
+
   /*
    * Recomb with a Matrix of the given bands/channel size.
    * Eg. RGB will be a 3x3 matrix.

src/pipeline.cc

@@ -550,7 +550,9 @@ class PipelineWorker : public Napi::AsyncWorker {
       if (baton->medianSize > 0) {
         image = image.median(baton->medianSize);
       }
+
       // Threshold - must happen before blurring, due to the utility of blurring after thresholding
+      // Threshold - must happen before unflatten to enable non-white unflattening
       if (baton->threshold != 0) {
         image = sharp::Threshold(image, baton->threshold, baton->thresholdGrayscale);
       }
@@ -560,6 +562,11 @@ class PipelineWorker : public Napi::AsyncWorker {
         image = sharp::Blur(image, baton->blurSigma);
       }
 
+      // Unflatten the image
+      if (baton->unflatten) {
+        image = sharp::Unflatten(image);
+      }
+
       // Convolve
       if (shouldConv) {
         image = sharp::Convolve(image,
@@ -1460,6 +1467,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   // Operators
   baton->flatten = sharp::AttrAsBool(options, "flatten");
   baton->flattenBackground = sharp::AttrAsVectorOfDouble(options, "flattenBackground");
+  baton->unflatten = sharp::AttrAsBool(options, "unflatten");
   baton->negate = sharp::AttrAsBool(options, "negate");
   baton->negateAlpha = sharp::AttrAsBool(options, "negateAlpha");
   baton->blurSigma = sharp::AttrAsDouble(options, "blurSigma");

src/pipeline.h

@@ -73,6 +73,7 @@ struct PipelineBaton {
   double tintB;
   bool flatten;
   std::vector<double> flattenBackground;
+  bool unflatten;
   bool negate;
   bool negateAlpha;
   double blurSigma;
@@ -239,6 +240,7 @@ struct PipelineBaton {
     tintB(128.0),
     flatten(false),
     flattenBackground{ 0.0, 0.0, 0.0 },
+    unflatten(false),
     negate(false),
     negateAlpha(true),
     blurSigma(0.0),