Add greyscale option to threshold operation (#480)

docs/api.md

@@ -391,12 +391,14 @@ When a `sigma` is provided, performs a slower, more accurate sharpen of the L ch
 * `flat`, if present, is a Number representing the level of sharpening to apply to "flat" areas, defaulting to a value of 1.0.
 * `jagged`, if present, is a Number representing the level of sharpening to apply to "jagged" areas, defaulting to a value of 2.0.
 
-#### threshold([threshold])
+#### threshold([threshold], [options])
 
 Converts all pixels in the image to greyscale white or black.  Any pixel greather-than-or-equal-to the threshold (0..255) will be white.  All others will be black.
 
 * `threshold`, if present, is a Number, representing the level above which pixels will be forced to white.
 
+* `options`, an options object that may contain a boolean `grayscale` or `greyscale`. When `grayscale` is `true`, `threshold` returns a black and white image, and when `false` a color image with each channel thresholded independently. The default is `grayscale: true` when omitted.
+
 #### gamma([gamma])
 
 Apply a gamma correction by reducing the encoding (darken) pre-resize at a factor of `1/gamma` then increasing the encoding (brighten) post-resize at a factor of `gamma`.

index.js

@@ -85,6 +85,7 @@ var Sharp = function(input, options) {
     sharpenFlat: 1,
     sharpenJagged: 2,
     threshold: 0,
+    thresholdGrayscale: true,
     gamma: 0,
     greyscale: false,
     normalize: 0,
@@ -487,7 +488,7 @@ Sharp.prototype.sharpen = function(sigma, flat, jagged) {
   return this;
 };
 
-Sharp.prototype.threshold = function(threshold) {
+Sharp.prototype.threshold = function(threshold, options) {
   if (typeof threshold === 'undefined') {
     this.options.threshold = 128;
   } else if (typeof threshold === 'boolean') {
@@ -497,6 +498,14 @@ Sharp.prototype.threshold = function(threshold) {
   } else {
     throw new Error('Invalid threshold (0 to 255) ' + threshold);
   }
+
+  if(typeof options === 'undefined' ||
+     options.greyscale === true || options.grayscale === true) {
+    this.options.thresholdGrayscale = true;
+  } else {
+    this.options.thresholdGrayscale = false;
+  }
+
   return this;
 };
 

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),

test/unit/threshold.js

@@ -52,8 +52,21 @@ describe('Threshold', function() {
         assert.strictEqual(240, info.height);
         fixtures.assertSimilar(fixtures.expected('threshold-128.jpg'), data, done);
       });
+   });
+
+  it('threshold grayscale: true (=128)', function(done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .threshold(128,{'grayscale':true})
+      .toBuffer(function(err, data, info) {
+        assert.strictEqual('jpeg', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('threshold-128.jpg'), data, done);
+      });    
   });
 
+
   it('threshold default jpeg', function(done) {
     sharp(fixtures.inputJpg)
       .resize(320, 240)
@@ -101,6 +114,18 @@ describe('Threshold', function() {
     });
   }
 
+  it('color threshold', function(done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .threshold(128,{'grayscale':false})
+      .toBuffer(function(err, data, info) {
+        assert.strictEqual('jpeg', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('threshold-color-128.jpg'), data, done);
+      });    
+  });
+
   it('invalid threshold -1', function() {
     assert.throws(function() {
       sharp(fixtures.inputJpg).threshold(-1);