Unflatten function - create an alpha channel and make white pixels fully transparent

lib/constructor.js

@@ -217,6 +217,7 @@ const Sharp = function (input, options) {
     tintB: 128,
     flatten: false,
     flattenBackground: [0, 0, 0],
+    unflatten: false,
     negate: false,
     negateAlpha: true,
     medianSize: 0,

lib/index.d.ts

@@ -427,6 +427,13 @@ declare namespace sharp {
          */
         flatten(flatten?: boolean | FlattenOptions): Sharp;
 
+        /**
+         * Unflatten - add an alpha channel to the image if required, and make white pixels fully transparent. Alpha for non-white pixels will be unchanged/opaque.
+         * @param unflatten true to enable and false to disable (defaults to true)
+         * @returns A sharp instance that can be used to chain operations
+         */
+        unflatten(unflatten?: boolean): Sharp;
+
         /**
          * 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.
          * This can improve the perceived brightness of a resized image in non-linear colour spaces.

lib/operation.js

@@ -405,6 +405,25 @@ function flatten (options) {
   return this;
 }
 
+/**
+ * Unflatten - add an alpha channel to the image if required, and make white pixels fully transparent. Alpha for non-white pixels will be unchanged/opaque.
+ *
+ * @example
+ * await sharp(rgbInput)
+ *   .unflatten()
+ *   .toBuffer();
+ *
+ * @example
+ * await sharp(rgbInput)
+ *   .threshold(128, { grayscale: false }) // converter bright pixels to white
+ *   .unflatten()
+ *   .toBuffer();
+ */
+function unflatten (options) {
+  this.options.unflatten = is.bool(options) ? options : true;
+  return this;
+}
+
 /**
  * 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`.
@@ -875,6 +894,7 @@ module.exports = function (Sharp) {
     median,
     blur,
     flatten,
+    unflatten,
     gamma,
     negate,
     normalise,

src/operations.cc

@@ -341,6 +341,19 @@ 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),

test/unit/unflatten.js

@@ -0,0 +1,37 @@
+'use strict';
+
+const sharp = require('../../');
+const fixtures = require('../fixtures');
+
+// const assert = require('assert');
+
+describe('Unflatten', function () {
+  it('unflatten white background', function (done) {
+    sharp(fixtures.inputPng).unflatten()
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('unflatten-white-transparent.png'), data, { threshold: 0 }, done);
+      });
+  });
+  it('unflatten transparent image', function (done) {
+    sharp(fixtures.inputPngTrimSpecificColourIncludeAlpha).unflatten()
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('unflatten-flag-white-transparent.png'), data, { threshold: 0 }, done);
+      });
+  });
+  it('unflatten using threshold', function (done) {
+    sharp(fixtures.inputPngPalette).unflatten(true).threshold(128, { grayscale: false })
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('unflatten-swiss.png'), data, { threshold: 1 }, done);
+      });
+  });
+  it('no unflatten', function (done) {
+    sharp(fixtures.inputPng).unflatten(false)
+      .toBuffer(function (err, data) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.inputPng, data, { threshold: 0 }, done);
+      });
+  });
+});