Add support for gamma correction #43

README.md

@@ -279,6 +279,16 @@ Possible interpolators, in order of performance, are:
 * `locallyBoundedBicubic`: Use [LBB interpolation](https://github.com/jcupitt/libvips/blob/master/libvips/resample/lbb.cpp#L100), which prevents some "[acutance](http://en.wikipedia.org/wiki/Acutance)" and typically reduces performance by a factor of 2.
 * `nohalo`: Use [Nohalo interpolation](http://eprints.soton.ac.uk/268086/), which prevents acutance and typically reduces performance by a factor of 3.
 
+#### 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`.
+
+`gamma`, if present, is a Number betweem 1 and 3. The default value is `2.2`, a suitable approximation for sRGB images.
+
+This can improve the perceived brightness of a resized image in non-linear colour spaces.
+
+JPEG input images will not take advantage of the shrink-on-load performance optimisation when applying a gamma correction.
+
 ### Output options
 
 #### jpeg()

index.js

@@ -20,6 +20,7 @@ var Sharp = function(input) {
     withoutEnlargement: false,
     sharpen: false,
     interpolator: 'bilinear',
+    gamma: 0,
     progressive: false,
     sequentialRead: false,
     quality: 80,
@@ -164,6 +165,22 @@ Sharp.prototype.nohaloInterpolation = util.deprecate(function() {
   return this.interpolateWith(module.exports.interpolator.nohalo);
 }, 'nohaloInterpolation() is deprecated, use interpolateWith(sharp.interpolator.nohalo) instead');
 
+/*
+  Darken image pre-resize (1/gamma) and brighten post-resize (gamma).
+  Improves brightness of resized image in non-linear colour spaces.
+*/
+Sharp.prototype.gamma = function(gamma) {
+  if (typeof gamma === 'undefined') {
+    // Default gamma correction of 2.2 (sRGB)
+    this.options.gamma = 2.2;
+  } else if (!Number.isNaN(gamma) && gamma >= 1 && gamma <= 3) {
+    this.options.gamma = gamma;
+  } else {
+    throw new Error('Invalid gamma correction (1.0 to 3.0) ' + gamma);
+  }
+  return this;
+};
+
 Sharp.prototype.progressive = function(progressive) {
   this.options.progressive = (typeof progressive === 'boolean') ? progressive : true;
   return this;

src/sharp.cc

@@ -26,6 +26,7 @@ struct resize_baton {
   VipsExtend extend;
   bool sharpen;
   std::string interpolator;
+  double gamma;
   bool progressive;
   bool without_enlargement;
   VipsAccess access_method;
@@ -42,6 +43,7 @@ struct resize_baton {
     gravity(0),
     max(false),
     sharpen(false),
+    gamma(0.0),
     progressive(false),
     without_enlargement(false),
     withMetadata(false) {}
@@ -434,9 +436,9 @@ class ResizeWorker : public NanAsyncWorker {
       }
     }
 
-    // Try to use libjpeg shrink-on-load
+    // Try to use libjpeg shrink-on-load, but not when applying gamma correction
     int shrink_on_load = 1;
-    if (inputImageType == JPEG) {
+    if (inputImageType == JPEG && baton->gamma == 0) {
       if (shrink >= 8) {
         factor = factor / 8;
         shrink_on_load = 8;
@@ -469,6 +471,16 @@ class ResizeWorker : public NanAsyncWorker {
     }
     g_object_unref(in);
 
+    // Gamma encoding (darken)
+    if (baton->gamma >= 1 && baton->gamma <= 3) {
+      VipsImage *gamma_encoded = vips_image_new();
+      if (vips_gamma(shrunk_on_load, &gamma_encoded, "exponent", 1.0 / baton->gamma, NULL)) {
+        return resize_error(baton, shrunk_on_load);
+      }
+      g_object_unref(shrunk_on_load);
+      shrunk_on_load = gamma_encoded;
+    }
+
     VipsImage *shrunk = vips_image_new();
     if (shrink > 1) {
       // Use vips_shrink with the integral reduction
@@ -567,6 +579,16 @@ class ResizeWorker : public NanAsyncWorker {
     }
     g_object_unref(canvased);
 
+    // Gamma decoding (brighten)
+    if (baton->gamma >= 1 && baton->gamma <= 3) {
+      VipsImage *gamma_decoded = vips_image_new();
+      if (vips_gamma(sharpened, &gamma_decoded, "exponent", baton->gamma, NULL)) {
+        return resize_error(baton, sharpened);
+      }
+      g_object_unref(sharpened);
+      sharpened = gamma_decoded;
+    }
+
     // Always convert to sRGB colour space
     VipsImage *colourspaced = vips_image_new();
     vips_colourspace(sharpened, &colourspaced, VIPS_INTERPRETATION_sRGB, NULL);
@@ -702,6 +724,7 @@ NAN_METHOD(resize) {
   baton->gravity = options->Get(NanNew<String>("gravity"))->Int32Value();
   baton->sharpen = options->Get(NanNew<String>("sharpen"))->BooleanValue();
   baton->interpolator = *String::Utf8Value(options->Get(NanNew<String>("interpolator"))->ToString());
+  baton->gamma = options->Get(NanNew<String>("gamma"))->NumberValue();
   baton->progressive = options->Get(NanNew<String>("progressive"))->BooleanValue();
   baton->without_enlargement = options->Get(NanNew<String>("withoutEnlargement"))->BooleanValue();
   baton->access_method = options->Get(NanNew<String>("sequentialRead"))->BooleanValue() ? VIPS_ACCESS_SEQUENTIAL : VIPS_ACCESS_RANDOM;

tests/perf.js

@@ -245,6 +245,18 @@ async.series({
           }
         });
       }
+    }).add("sharp-file-buffer-gamma", {
+      defer: true,
+      fn: function(deferred) {
+        sharp(inputJpg).resize(width, height).gamma().toBuffer(function(err, buffer) {
+          if (err) {
+            throw err;
+          } else {
+            assert.notStrictEqual(null, buffer);
+            deferred.resolve();
+          }
+        });
+      }
     }).add("sharp-file-buffer-progressive", {
       defer: true,
       fn: function(deferred) {

tests/unit.js

@@ -18,6 +18,8 @@ var outputTiff = path.join(fixturesPath, "output.tiff");
 
 var inputJpgWithExif = path.join(fixturesPath, "Landscape_8.jpg"); // https://github.com/recurser/exif-orientation-examples/blob/master/Landscape_8.jpg
 
+var inputJpgWithGammaHoliness = path.join(fixturesPath, "gamma_dalai_lama_gray.jpg"); // http://www.4p8.com/eric.brasseur/gamma.html
+
 var inputPng = path.join(fixturesPath, "50020484-00001.png"); // http://c.searspartsdirect.com/lis_png/PLDM/50020484-00001.png
 var inputWebp = path.join(fixturesPath, "4.webp"); // http://www.gstatic.com/webp/gallery/4.webp
 var inputGif = path.join(fixturesPath, "Crash_test.gif"); // http://upload.wikimedia.org/wikipedia/commons/e/e3/Crash_test.gif
@@ -571,6 +573,25 @@ async.series([
       });
     });
   },
+  // Gamma correction
+  function(done) {
+    sharp(inputJpgWithGammaHoliness).resize(129, 111).toFile(path.join(fixturesPath, 'output.gamma-0.0.jpg'), function(err) {
+      if (err) throw err;
+      done();
+    });
+  },
+  function(done) {
+    sharp(inputJpgWithGammaHoliness).resize(129, 111).gamma().toFile(path.join(fixturesPath, 'output.gamma-2.2.jpg'), function(err) {
+      if (err) throw err;
+      done();
+    });
+  },
+  function(done) {
+    sharp(inputJpgWithGammaHoliness).resize(129, 111).gamma(3).toFile(path.join(fixturesPath, 'output.gamma-3.0.jpg'), function(err) {
+      if (err) throw err;
+      done();
+    });
+  },
   // Verify internal counters
   function(done) {
     var counters = sharp.counters();