Add experimental sharpness calc to stats lovell#2251

docs/api-input.md

@@ -71,6 +71,7 @@ A `Promise` is returned when `callback` is not provided.
     -   `maxY` (y-coordinate of one of the pixel where the maximum lies)
 -   `isOpaque`: Is the image fully opaque? Will be `true` if the image has no alpha channel or if every pixel is fully opaque.
 -   `entropy`: Histogram-based estimation of greyscale entropy, discarding alpha channel if any (experimental)
+-   `sharpness`: Estimation of greyscale sharpness based on the standard deviation of a Laplacian convolution, discarding alpha channel if any (experimental)
 
 ### Parameters
 
@@ -87,6 +88,10 @@ image
   });
 ```
 
+```javascript
+const { entropy, sharpness } = await sharp(input).stats();
+```
+
 Returns **[Promise][5]<[Object][6]>** 
 
 [1]: https://libvips.github.io/libvips/API/current/VipsImage.html#VipsInterpretation

docs/changelog.md

@@ -21,6 +21,9 @@ Requires libvips v8.9.1
 * Add support for named `alpha` channel to `extractChannel` operation.
   [#2138](https://github.com/lovell/sharp/issues/2138)
 
+* Add experimental `sharpness` calculation to `stats()` response.
+  [#2251](https://github.com/lovell/sharp/issues/2251)
+
 ### v0.25.3 - 17th May 2020
 
 * Ensure libvips is initialised only once, improves worker thread safety.

lib/input.js

@@ -299,6 +299,7 @@ function metadata (callback) {
  *     - `maxY` (y-coordinate of one of the pixel where the maximum lies)
  * - `isOpaque`: Is the image fully opaque? Will be `true` if the image has no alpha channel or if every pixel is fully opaque.
  * - `entropy`: Histogram-based estimation of greyscale entropy, discarding alpha channel if any (experimental)
+ * - `sharpness`: Estimation of greyscale sharpness based on the standard deviation of a Laplacian convolution, discarding alpha channel if any (experimental)
  *
  * @example
  * const image = sharp(inputJpg);
@@ -308,6 +309,9 @@ function metadata (callback) {
  *      // stats contains the channel-wise statistics array and the isOpaque value
  *   });
  *
+ * @example
+ * const { entropy, sharpness } = await sharp(input).stats();
+ *
  * @param {Function} [callback] - called with the arguments `(err, stats)`
  * @returns {Promise<Object>}
  */

src/stats.cc

@@ -75,8 +75,17 @@ class StatsWorker : public Napi::AsyncWorker {
             baton->isOpaque = false;
           }
         }
+        // Convert to greyscale
+        vips::VImage greyscale = image.colourspace(VIPS_INTERPRETATION_B_W)[0];
         // Estimate entropy via histogram of greyscale value frequency
-        baton->entropy = std::abs(image.colourspace(VIPS_INTERPRETATION_B_W)[0].hist_find().hist_entropy());
+        baton->entropy = std::abs(greyscale.hist_find().hist_entropy());
+        // Estimate sharpness via standard deviation of greyscale laplacian
+        VImage laplacian = VImage::new_matrixv(3, 3,
+          0.0,  1.0, 0.0,
+          1.0, -4.0, 1.0,
+          0.0,  1.0, 0.0);
+        laplacian.set("scale", 9.0);
+        baton->sharpness = greyscale.conv(laplacian).deviate();
       } catch (vips::VError const &err) {
         (baton->err).append(err.what());
       }
@@ -123,6 +132,7 @@ class StatsWorker : public Napi::AsyncWorker {
       info.Set("channels", channels);
       info.Set("isOpaque", baton->isOpaque);
       info.Set("entropy", baton->entropy);
+      info.Set("sharpness", baton->sharpness);
       Callback().MakeCallback(Receiver().Value(), { env.Null(), info });
     } else {
       Callback().MakeCallback(Receiver().Value(), { Napi::Error::New(env, baton->err).Value() });

src/stats.h

@@ -47,13 +47,15 @@ struct StatsBaton {
   std::vector<ChannelStats> channelStats;
   bool isOpaque;
   double entropy;
+  double sharpness;
 
   std::string err;
 
   StatsBaton():
     input(nullptr),
     isOpaque(true),
-    entropy(0.0)
+    entropy(0.0),
+    sharpness(0.0)
     {}
 };
 

test/unit/stats.js

@@ -25,6 +25,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 0.7883011147075762));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -84,6 +85,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0.3409031108021736));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 9.111356137722868));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -110,6 +112,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(false, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0.06778064835816622));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 2.522916068931278));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -185,6 +188,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(false, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 0));
 
       // alpha channel
       assert.strictEqual(0, stats.channels[3].min);
@@ -212,6 +216,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 0.3851250782608986));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 10.312521863719589));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -239,6 +244,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.51758075132966));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 9.959951636662941));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -298,6 +304,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 6.087309412541799));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 2.9250574456255682));
 
       // red channel
       assert.strictEqual(35, stats.channels[0].min);
@@ -357,6 +364,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(false, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 1));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 15.870619016486861));
 
       // gray channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -410,6 +418,7 @@ describe('Image Stats', function () {
 
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 0.788301114707569));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -472,6 +481,7 @@ describe('Image Stats', function () {
     return pipeline.stats().then(function (stats) {
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 0.788301114707569));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -529,6 +539,7 @@ describe('Image Stats', function () {
     return sharp(fixtures.inputJpg).stats().then(function (stats) {
       assert.strictEqual(true, stats.isOpaque);
       assert.strictEqual(true, isInAcceptableRange(stats.entropy, 7.319914765248541));
+      assert.strictEqual(true, isInAcceptableRange(stats.sharpness, 0.788301114707569));
 
       // red channel
       assert.strictEqual(0, stats.channels[0].min);
@@ -582,6 +593,18 @@ describe('Image Stats', function () {
     });
   });
 
+  it('Blurred image has lower sharpness than original', () => {
+    const original = sharp(fixtures.inputJpg).stats();
+    const blurred = sharp(fixtures.inputJpg).blur().toBuffer().then(blur => sharp(blur).stats());
+
+    return Promise
+      .all([original, blurred])
+      .then(([original, blurred]) => {
+        assert.strictEqual(true, isInAcceptableRange(original.sharpness, 0.7883011147075476));
+        assert.strictEqual(true, isInAcceptableRange(blurred.sharpness, 0.4791559805997398));
+      });
+  });
+
   it('File input with corrupt header fails gracefully', function (done) {
     sharp(fixtures.inputJpgWithCorruptHeader)
       .stats(function (err) {