Add ensureAlpha op, adds alpha channel if missing #1153

src/operations.cc

@@ -38,6 +38,18 @@ namespace sharp {
     return image;
   }
 
+  /*
+    Ensures alpha channel, if missing.
+  */
+  VImage EnsureAlpha(VImage image) {
+    if (!HasAlpha(image)) {
+      std::vector<double> alpha;
+      alpha.push_back(sharp::MaximumImageAlpha(image.interpretation()));
+      image = image.bandjoin_const(alpha);
+    }
+    return image;
+  }
+
   /*
     Composite overlayImage over image at given position
     Assumes alpha channels are already premultiplied and will be unpremultiplied after

src/operations.h

@@ -30,6 +30,11 @@ namespace sharp {
   */
   VImage RemoveAlpha(VImage image);
 
+  /*
+    Ensures alpha channel, if missing.
+  */
+  VImage EnsureAlpha(VImage image);
+
   /*
     Alpha composite src over dst with given gravity.
     Assumes alpha channels are already premultiplied and will be unpremultiplied after.

src/pipeline.cc

@@ -677,6 +677,11 @@ class PipelineWorker : public Nan::AsyncWorker {
         image = sharp::RemoveAlpha(image);
       }
 
+      // Ensure alpha channel, if missing
+      if (baton->ensureAlpha) {
+        image = sharp::EnsureAlpha(image);
+      }
+
       // Convert image to sRGB, if not already
       if (sharp::Is16Bit(image.interpretation())) {
         image = image.cast(VIPS_FORMAT_USHORT);
@@ -1236,6 +1241,7 @@ NAN_METHOD(pipeline) {
   baton->extractChannel = AttrTo<int32_t>(options, "extractChannel");
 
   baton->removeAlpha = AttrTo<bool>(options, "removeAlpha");
+  baton->ensureAlpha = AttrTo<bool>(options, "ensureAlpha");
   if (HasAttr(options, "boolean")) {
     baton->boolean = CreateInputDescriptor(AttrAs<v8::Object>(options, "boolean"), buffersToPersist);
     baton->booleanOp = sharp::GetBooleanOperation(AttrAsStr(options, "booleanOp"));

src/pipeline.h

@@ -142,6 +142,7 @@ struct PipelineBaton {
   VipsOperationBoolean bandBoolOp;
   int extractChannel;
   bool removeAlpha;
+  bool ensureAlpha;
   VipsInterpretation colourspace;
   int tileSize;
   int tileOverlap;
@@ -237,6 +238,7 @@ struct PipelineBaton {
     bandBoolOp(VIPS_OPERATION_BOOLEAN_LAST),
     extractChannel(-1),
     removeAlpha(false),
+    ensureAlpha(false),
     colourspace(VIPS_INTERPRETATION_LAST),
     tileSize(256),
     tileOverlap(0),