Add experimental entropy field to stats response

docs/changelog.md

@@ -23,6 +23,8 @@ Requires libvips v8.6.1.
 * Improve install time error messages for FreeBSD users.
   [#1310](https://github.com/lovell/sharp/issues/1310)
 
+* Add experimental entropy field to stats response.
+
 #### v0.20.5 - 27<sup>th</sup> June 2018
 
 * Expose libjpeg optimize_coding flag.

lib/input.js

@@ -264,6 +264,7 @@ function metadata (callback) {
  *     - `maxX` (x-coordinate of one of the pixel where the maximum lies)
  *     - `maxY` (y-coordinate of one of the pixel where the maximum lies)
  * - `isOpaque`: Value to identify if the image is opaque or transparent, based on the presence and use of alpha channel
+ * - `entropy`: Histogram-based estimation of greyscale entropy, discarding alpha channel if any (experimental)
  *
  * @example
  * const image = sharp(inputJpg);

src/stats.cc

@@ -59,7 +59,6 @@ class StatsWorker : public Nan::AsyncWorker {
     using sharp::MaximumImageAlpha;
 
     vips::VImage image;
-    vips::VImage stats;
     sharp::ImageType imageType = sharp::ImageType::UNKNOWN;
 
     try {
@@ -69,9 +68,8 @@ class StatsWorker : public Nan::AsyncWorker {
     }
     if (imageType != sharp::ImageType::UNKNOWN) {
       try {
-        stats = image.stats();
-        int bands = image.bands();
-        double const max = MaximumImageAlpha(image.interpretation());
+        vips::VImage stats = image.stats();
+        int const bands = image.bands();
         for (int b = 1; b <= bands; b++) {
           ChannelStats cStats(static_cast<int>(stats.getpoint(STAT_MIN_INDEX, b).front()),
                               static_cast<int>(stats.getpoint(STAT_MAX_INDEX, b).front()),
@@ -83,11 +81,15 @@ class StatsWorker : public Nan::AsyncWorker {
                               static_cast<int>(stats.getpoint(STAT_MAXY_INDEX, b).front()));
           baton->channelStats.push_back(cStats);
         }
-
-        // alpha layer is there and the last band i.e. alpha has its max value greater than 0)
-        if (sharp::HasAlpha(image) && stats.getpoint(STAT_MIN_INDEX, bands).front() != max) {
+        // Image is not opaque when alpha layer is present and contains a non-mamixa value
+        if (sharp::HasAlpha(image)) {
+          double const minAlpha = static_cast<double>(stats.getpoint(STAT_MIN_INDEX, bands).front());
+          if (minAlpha != MaximumImageAlpha(image.interpretation())) {
             baton->isOpaque = false;
           }
+        }
+        // Estimate entropy via histogram of greyscale value frequency
+        baton->entropy = std::abs(image.colourspace(VIPS_INTERPRETATION_B_W)[0].hist_find().hist_entropy());
       } catch (vips::VError const &err) {
         (baton->err).append(err.what());
       }
@@ -130,6 +132,7 @@ class StatsWorker : public Nan::AsyncWorker {
 
       Set(info, New("channels").ToLocalChecked(), channels);
       Set(info, New("isOpaque").ToLocalChecked(), New<v8::Boolean>(baton->isOpaque));
+      Set(info, New("entropy").ToLocalChecked(), New<v8::Number>(baton->entropy));
       argv[1] = info;
     }
 

src/stats.h

@@ -51,12 +51,14 @@ struct StatsBaton {
   // Output
   std::vector<ChannelStats> channelStats;
   bool isOpaque;
+  double entropy;
 
   std::string err;
 
   StatsBaton():
     input(nullptr),
-    isOpaque(true)
+    isOpaque(true),
+    entropy(0.0)
     {}
 };
 

test/unit/stats.js

@@ -24,6 +24,7 @@ describe('Image Stats', function () {
       if (err) throw err;
 
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -82,6 +83,7 @@ describe('Image Stats', function () {
       if (err) throw err;
 
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0.3409031108021736));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -105,7 +107,9 @@ describe('Image Stats', function () {
   it('PNG with transparency', function (done) {
     sharp(fixtures.inputPngWithTransparency).stats(function (err, stats) {
       if (err) throw err;
+
       assert.strictEqual(false, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0.06778064835816622));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -180,6 +184,7 @@ describe('Image Stats', function () {
       if (err) throw err;
 
       assert.strictEqual(false, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0));
 
       // alpha channel
       assert.strictEqual(0, stats.channels[3]['min']);
@@ -204,7 +209,9 @@ describe('Image Stats', function () {
   it('Tiff', function (done) {
     sharp(fixtures.inputTiff).stats(function (err, stats) {
       if (err) throw err;
+
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0.3851250782608986));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -231,6 +238,7 @@ describe('Image Stats', function () {
       if (err) throw err;
 
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.51758075132966));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -289,6 +297,7 @@ describe('Image Stats', function () {
       if (err) throw err;
 
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 6.087309412541799));
 
       // red channel
       assert.strictEqual(35, stats.channels[0]['min']);
@@ -345,7 +354,9 @@ describe('Image Stats', function () {
   it('Grayscale GIF with alpha', function (done) {
     sharp(fixtures.inputGifGreyPlusAlpha).stats(function (err, stats) {
       if (err) throw err;
+
       assert.strictEqual(false, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 1));
 
       // gray channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -387,7 +398,9 @@ describe('Image Stats', function () {
     const readable = fs.createReadStream(fixtures.inputJpg);
     const pipeline = sharp().stats(function (err, stats) {
       if (err) throw err;
+
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -449,6 +462,7 @@ describe('Image Stats', function () {
 
     return pipeline.stats().then(function (stats) {
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);
@@ -505,6 +519,7 @@ describe('Image Stats', function () {
   it('File in, Promise out', function () {
     return sharp(fixtures.inputJpg).stats().then(function (stats) {
       assert.strictEqual(true, stats.isOpaque);
+      assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
 
       // red channel
       assert.strictEqual(0, stats.channels[0]['min']);