Add greyscale option to threshold operation (#480)

src/operations.cc

@@ -327,4 +327,11 @@ namespace sharp {
     );
   }
 
+  VImage Threshold(VImage image, double const threshold, bool const thresholdGrayscale) {
+    if(!thresholdGrayscale) {
+      return image >= threshold;
+    }
+    return image.colourspace(VIPS_INTERPRETATION_B_W) >= threshold;
+  }
+
 }  // namespace sharp

src/operations.h

@@ -54,6 +54,11 @@ namespace sharp {
   */
   VImage TileCache(VImage image, double const factor);
 
+  /*
+    Threshold an image
+  */
+  VImage Threshold(VImage image, double const threshold, bool const thresholdColor);
+
 }  // namespace sharp
 
 #endif  // SRC_OPERATIONS_H_

src/pipeline.cc

@@ -52,6 +52,7 @@ using sharp::Blur;
 using sharp::Sharpen;
 using sharp::EntropyCrop;
 using sharp::TileCache;
+using sharp::Threshold;
 
 using sharp::ImageType;
 using sharp::ImageTypeId;
@@ -625,7 +626,7 @@ class PipelineWorker : public AsyncWorker {
 
       // Threshold - must happen before blurring, due to the utility of blurring after thresholding
       if (shouldThreshold) {
-        image = image.colourspace(VIPS_INTERPRETATION_B_W) >= baton->threshold;
+        image = Threshold(image, baton->threshold, baton->thresholdGrayscale);
       }
 
       // Blur
@@ -1107,6 +1108,7 @@ NAN_METHOD(pipeline) {
   baton->sharpenFlat = attrAs<double>(options, "sharpenFlat");
   baton->sharpenJagged = attrAs<double>(options, "sharpenJagged");
   baton->threshold = attrAs<int32_t>(options, "threshold");
+  baton->thresholdGrayscale = attrAs<bool>(options, "thresholdGrayscale");
   baton->gamma = attrAs<double>(options, "gamma");
   baton->greyscale = attrAs<bool>(options, "greyscale");
   baton->normalize = attrAs<bool>(options, "normalize");

src/pipeline.h

@@ -58,6 +58,7 @@ struct PipelineBaton {
   double sharpenFlat;
   double sharpenJagged;
   int threshold;
+  bool thresholdGrayscale;
   double gamma;
   bool greyscale;
   bool normalize;
@@ -113,6 +114,7 @@ struct PipelineBaton {
     sharpenFlat(1.0),
     sharpenJagged(2.0),
     threshold(0),
+    thresholdGrayscale(true),
     gamma(0.0),
     greyscale(false),
     normalize(false),