Implements greyscale thresholding

.editorconfig

@@ -0,0 +1,12 @@
+# http://editorconfig.org
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.md]
+trim_trailing_whitespace = false

docs/api.md

@@ -328,6 +328,12 @@ When a `radius` is provided, performs a slower, more accurate sharpen of the L c
 * `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])
+
+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.
+
 #### 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

@@ -62,6 +62,7 @@ var Sharp = function(input) {
     sharpenRadius: 0,
     sharpenFlat: 1,
     sharpenJagged: 2,
+    threshold: 0,
     gamma: 0,
     greyscale: false,
     normalize: 0,
@@ -142,7 +143,18 @@ Sharp.prototype._write = function(chunk, encoding, callback) {
 };
 
 // Crop this part of the resized image (Center/Centre, North, East, South, West)
-module.exports.gravity = {'center': 0, 'centre': 0, 'north': 1, 'east': 2, 'south': 3, 'west': 4, 'northeast': 5, 'southeast': 6, 'southwest': 7, 'northwest': 8};
+module.exports.gravity = {
+  'center': 0,
+  'centre': 0,
+  'north': 1,
+  'east': 2,
+  'south': 3,
+  'west': 4,
+  'northeast': 5,
+  'southeast': 6,
+  'southwest': 7,
+  'northwest': 8
+};
 
 Sharp.prototype.crop = function(gravity) {
   this.options.canvas = 'crop';
@@ -328,6 +340,19 @@ Sharp.prototype.sharpen = function(radius, flat, jagged) {
   return this;
 };
 
+Sharp.prototype.threshold = function(threshold) {
+  if (typeof threshold === 'undefined') {
+    this.options.threshold = 128;
+  } else if (typeof threshold === 'boolean') {
+    this.options.threshold = threshold ? 128 : 0;
+  } else if (typeof threshold === 'number' && !Number.isNaN(threshold) && (threshold % 1 === 0) && threshold >= 0 && threshold <= 255) {
+    this.options.threshold = threshold;
+  } else {
+    throw new Error('Invalid threshold (0 to 255) ' + threshold);
+  }
+  return this;
+};
+
 /*
   Set the interpolator to use for the affine transformation
 */

package.json

@@ -17,7 +17,8 @@
     "Victor Mateevitsi <mvictoras@gmail.com>",
     "Alaric Holloway <alaric.holloway@gmail.com>",
     "Bernhard K. Weisshuhn <bkw@codingforce.com>",
-    "Chris Riley <criley@primedia.com>"
+    "Chris Riley <criley@primedia.com>",
+    "David Carley <dacarley@gmail.com>"
   ],
   "description": "High performance Node.js module to resize JPEG, PNG, WebP and TIFF images using the libvips library",
   "scripts": {

src/operations.cc

@@ -262,4 +262,21 @@ namespace sharp {
     *out = sharpened;
     return 0;
   }
+
+  int Threshold(VipsObject *context, VipsImage *image, VipsImage **out, int threshold) {
+      VipsImage *greyscale;
+      if (vips_colourspace(image, &greyscale, VIPS_INTERPRETATION_B_W, nullptr)) {
+        return -1;
+      }
+      vips_object_local(context, greyscale);
+      image = greyscale;
+
+      VipsImage *thresholded;
+      if (vips_moreeq_const1(image, &thresholded, threshold, nullptr)) {
+          return -1;
+      }
+      vips_object_local(context, thresholded);
+      *out = thresholded;
+      return 0;
+  }
 }  // namespace sharp

src/operations.h

@@ -24,6 +24,11 @@ namespace sharp {
    */
   int Sharpen(VipsObject *context, VipsImage *image, VipsImage **out, int radius, double flat, double jagged);
 
+  /*
+   * Perform thresholding on an image.  If the image is not greyscale, will convert before thresholding.
+   * Pixels with a greyscale value greater-than-or-equal-to `threshold` will be pure white.  All others will be pure black.
+   */
+  int Threshold(VipsObject *context, VipsImage *image, VipsImage **out, int threshold);
 }  // namespace sharp
 
 #endif  // SRC_OPERATIONS_H_

src/pipeline.cc

@@ -40,6 +40,7 @@ using sharp::Composite;
 using sharp::Normalize;
 using sharp::Blur;
 using sharp::Sharpen;
+using sharp::Threshold;
 
 using sharp::ImageType;
 using sharp::DetermineImageType;
@@ -104,6 +105,7 @@ struct PipelineBaton {
   int sharpenRadius;
   double sharpenFlat;
   double sharpenJagged;
+  int threshold;
   std::string overlayPath;
   double gamma;
   bool greyscale;
@@ -142,6 +144,7 @@ struct PipelineBaton {
     sharpenRadius(0),
     sharpenFlat(1.0),
     sharpenJagged(2.0),
+    threshold(0),
     gamma(0.0),
     greyscale(false),
     normalize(false),
@@ -502,6 +505,7 @@ class PipelineWorker : public AsyncWorker {
     bool shouldAffineTransform = xresidual != 0.0 || yresidual != 0.0;
     bool shouldBlur = baton->blurSigma != 0.0;
     bool shouldSharpen = baton->sharpenRadius != 0;
+    bool shouldThreshold = baton->threshold != 0;
     bool hasOverlay = !baton->overlayPath.empty();
     bool shouldPremultiplyAlpha = HasAlpha(image) && (shouldAffineTransform || shouldBlur || shouldSharpen || hasOverlay);
 
@@ -686,6 +690,15 @@ class PipelineWorker : public AsyncWorker {
       image = extractedPost;
     }
 
+    // Threshold - must happen before blurring, due to the utility of blurring after thresholding
+    if (shouldThreshold) {
+        VipsImage *thresholded;
+        if (Threshold(hook, image, &thresholded, baton->threshold)) {
+            return Error();
+        }
+        image = thresholded;
+    }
+
     // Blur
     if (shouldBlur) {
       VipsImage *blurred;
@@ -1216,6 +1229,7 @@ NAN_METHOD(pipeline) {
   baton->sharpenRadius = To<int32_t>(Get(options, New("sharpenRadius").ToLocalChecked()).ToLocalChecked()).FromJust();
   baton->sharpenFlat = To<double>(Get(options, New("sharpenFlat").ToLocalChecked()).ToLocalChecked()).FromJust();
   baton->sharpenJagged = To<double>(Get(options, New("sharpenJagged").ToLocalChecked()).ToLocalChecked()).FromJust();
+  baton->threshold = To<int32_t>(Get(options, New("threshold").ToLocalChecked()).ToLocalChecked()).FromJust();
   baton->gamma = To<int32_t>(Get(options, New("gamma").ToLocalChecked()).ToLocalChecked()).FromJust();
   baton->greyscale = To<bool>(Get(options, New("greyscale").ToLocalChecked()).ToLocalChecked()).FromJust();
   baton->normalize = To<bool>(Get(options, New("normalize").ToLocalChecked()).ToLocalChecked()).FromJust();

test/unit/threshold.js

@@ -0,0 +1,105 @@
+'use strict';
+
+var assert = require('assert');
+
+var sharp = require('../../index');
+var fixtures = require('../fixtures');
+
+sharp.cache(0);
+
+describe('Threshold', function() {
+  it('threshold 1 jpeg', function(done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .threshold(1)
+      .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-1.jpg'), data, done);
+      });
+  });
+
+  it('threshold 40 jpeg', function(done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .threshold(40)
+      .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-40.jpg'), data, done);
+      });
+  });
+
+  it('threshold 128', function(done) {
+    sharp(fixtures.inputJpg)
+      .resize(320, 240)
+      .threshold(128)
+      .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)
+      .threshold()
+      .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 png transparency', function(done) {
+    sharp(fixtures.inputPngWithTransparency)
+      .resize(320, 240)
+      .threshold()
+      .toBuffer(function(err, data, info) {
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('threshold-128-transparency.png'), data, done);
+      });
+  });
+
+  it('threshold default png alpha', function(done) {
+    sharp(fixtures.inputPngWithGreyAlpha)
+      .resize(320, 240)
+      .threshold()
+      .toBuffer(function(err, data, info) {
+        assert.strictEqual('png', info.format);
+        assert.strictEqual(320, info.width);
+        assert.strictEqual(240, info.height);
+        fixtures.assertSimilar(fixtures.expected('threshold-128-alpha.png'), data, done);
+      });
+  });
+
+  if (sharp.format.webp.output.file) {
+    it('threshold default webp transparency', function(done) {
+      sharp(fixtures.inputWebPWithTransparency)
+        .threshold()
+        .toBuffer(function(err, data, info) {
+          assert.strictEqual('webp', info.format);
+          fixtures.assertSimilar(fixtures.expected('threshold-128-transparency.webp'), data, done);
+        });
+    });
+  }
+
+  it('invalid threshold -1', function() {
+    assert.throws(function() {
+      sharp(fixtures.inputJpg).threshold(-1);
+    });
+  });
+
+  it('invalid threshold 256', function() {
+    assert.throws(function() {
+      sharp(fixtures.inputJpg).threshold(256);
+    });
+  });
+});