Improve tint luminance with weighting function

docs/api-colour.md

@@ -1,7 +1,7 @@
 ## tint
-> tint(rgb) ⇒ <code>Sharp</code>
+> tint(tint) ⇒ <code>Sharp</code>
 
-Tint the image using the provided chroma while preserving the image luminance.
+Tint the image using the provided colour.
 An alpha channel may be present and will be unchanged by the operation.
 
 
@@ -12,7 +12,7 @@ An alpha channel may be present and will be unchanged by the operation.
 
 | Param | Type | Description |
 | --- | --- | --- |
-| rgb | <code>string</code> \| <code>Object</code> | parsed by the [color](https://www.npmjs.org/package/color) module to extract chroma values. |
+| tint | <code>String</code> \| <code>Object</code> | Parsed by the [color](https://www.npmjs.org/package/color) module. |
 
 **Example**  
 ```js

docs/changelog.md

@@ -20,6 +20,10 @@ Requires libvips v8.15.0
 * Options for `trim` operation must be an Object, add new `lineArt` option.
   [#2363](https://github.com/lovell/sharp/issues/2363)
 
+* Improve luminance of `tint` operation with weighting function.
+  [#3338](https://github.com/lovell/sharp/issues/3338)
+  [@jcupitt](https://github.com/jcupitt)
+
 * Ensure all `Error` objects contain a `stack` property.
   [#3653](https://github.com/lovell/sharp/issues/3653)
 

lib/colour.js

@@ -19,22 +19,20 @@ const colourspace = {
 };
 
 /**
- * Tint the image using the provided chroma while preserving the image luminance.
+ * Tint the image using the provided colour.
  * An alpha channel may be present and will be unchanged by the operation.
  *
  * @example
  * const output = await sharp(input)
  *   .tint({ r: 255, g: 240, b: 16 })
  *   .toBuffer();
  *
- * @param {string|Object} rgb - parsed by the [color](https://www.npmjs.org/package/color) module to extract chroma values.
+ * @param {string|Object} tint - Parsed by the [color](https://www.npmjs.org/package/color) module.
  * @returns {Sharp}
  * @throws {Error} Invalid parameter
  */
-function tint (rgb) {
-  const colour = color(rgb);
-  this.options.tintA = colour.a();
-  this.options.tintB = colour.b();
+function tint (tint) {
+  this._setBackgroundColourOption('tint', tint);
   return this;
 }
 

lib/constructor.js

@@ -212,8 +212,7 @@ const Sharp = function (input, options) {
     kernel: 'lanczos3',
     fastShrinkOnLoad: true,
     // operations
-    tintA: 128,
-    tintB: 128,
+    tint: [-1, 0, 0, 0],
     flatten: false,
     flattenBackground: [0, 0, 0],
     unflatten: false,

lib/index.d.ts

@@ -239,12 +239,12 @@ declare namespace sharp {
         //#region Color functions
 
         /**
-         * Tint the image using the provided chroma while preserving the image luminance.
+         * Tint the image using the provided colour.
          * An alpha channel may be present and will be unchanged by the operation.
-         * @param rgb Parsed by the color module to extract chroma values.
+         * @param tint Parsed by the color module.
          * @returns A sharp instance that can be used to chain operations
          */
-        tint(rgb: Color): Sharp;
+        tint(tint: Color): Sharp;
 
         /**
          * Convert to 8-bit greyscale; 256 shades of grey.

src/operations.cc

@@ -16,30 +16,44 @@ using vips::VError;
 
 namespace sharp {
   /*
-   * Tint an image using the specified chroma, preserving the original image luminance
+   * Tint an image using the provided RGB.
    */
-  VImage Tint(VImage image, double const a, double const b) {
-    // Get original colourspace
+  VImage Tint(VImage image, std::vector<double> const tint) {
+    std::vector<double> const tintLab = (VImage::black(1, 1) + tint)
+      .colourspace(VIPS_INTERPRETATION_LAB, VImage::option()->set("source_space", VIPS_INTERPRETATION_sRGB))
+      .getpoint(0, 0);
+    // LAB identity function
+    VImage identityLab = VImage::identity(VImage::option()->set("bands", 3))
+      .colourspace(VIPS_INTERPRETATION_LAB, VImage::option()->set("source_space", VIPS_INTERPRETATION_sRGB));
+    // Scale luminance range, 0.0 to 1.0
+    VImage l = identityLab[0] / 100;
+    // Weighting functions
+    VImage weightL = 1.0 - 4.0 * ((l - 0.5) * (l - 0.5));
+    VImage weightAB = (weightL * tintLab).extract_band(1, VImage::option()->set("n", 2));
+    identityLab = identityLab[0].bandjoin(weightAB);
+    // Convert lookup table to sRGB
+    VImage lut = identityLab.colourspace(VIPS_INTERPRETATION_sRGB,
+      VImage::option()->set("source_space", VIPS_INTERPRETATION_LAB));
+    // Original colourspace
     VipsInterpretation typeBeforeTint = image.interpretation();
     if (typeBeforeTint == VIPS_INTERPRETATION_RGB) {
       typeBeforeTint = VIPS_INTERPRETATION_sRGB;
     }
-    // Extract luminance
-    VImage luminance = image.colourspace(VIPS_INTERPRETATION_LAB)[0];
-    // Create the tinted version by combining the L from the original and the chroma from the tint
-    std::vector<double> chroma {a, b};
-    VImage tinted = luminance
-      .bandjoin(chroma)
-      .copy(VImage::option()->set("interpretation", VIPS_INTERPRETATION_LAB))
-      .colourspace(typeBeforeTint);
-    // Attach original alpha channel, if any
+    // Apply lookup table
     if (HasAlpha(image)) {
-      // Extract original alpha channel
       VImage alpha = image[image.bands() - 1];
-      // Join alpha channel to normalised image
-      tinted = tinted.bandjoin(alpha);
+      image = RemoveAlpha(image)
+        .colourspace(VIPS_INTERPRETATION_B_W)
+        .maplut(lut)
+        .colourspace(typeBeforeTint)
+        .bandjoin(alpha);
+    } else {
+      image = image
+        .colourspace(VIPS_INTERPRETATION_B_W)
+        .maplut(lut)
+        .colourspace(typeBeforeTint);
     }
-    return tinted;
+    return image;
   }
 
   /*

src/operations.h

@@ -15,9 +15,9 @@ using vips::VImage;
 namespace sharp {
 
   /*
-   * Tint an image using the specified chroma, preserving the original image luminance
+   * Tint an image using the provided RGB.
    */
-  VImage Tint(VImage image, double const a, double const b);
+  VImage Tint(VImage image, std::vector<double> const tint);
 
   /*
    * Stretch luminance to cover full dynamic range.

src/pipeline.cc

@@ -736,8 +736,8 @@ class PipelineWorker : public Napi::AsyncWorker {
       }
 
       // Tint the image
-      if (baton->tintA < 128.0 || baton->tintB < 128.0) {
-        image = sharp::Tint(image, baton->tintA, baton->tintB);
+      if (baton->tint[0] >= 0.0) {
+        image = sharp::Tint(image, baton->tint);
       }
 
       // Remove alpha channel, if any
@@ -1527,8 +1527,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->normalise = sharp::AttrAsBool(options, "normalise");
   baton->normaliseLower = sharp::AttrAsUint32(options, "normaliseLower");
   baton->normaliseUpper = sharp::AttrAsUint32(options, "normaliseUpper");
-  baton->tintA = sharp::AttrAsDouble(options, "tintA");
-  baton->tintB = sharp::AttrAsDouble(options, "tintB");
+  baton->tint = sharp::AttrAsVectorOfDouble(options, "tint");
   baton->claheWidth = sharp::AttrAsUint32(options, "claheWidth");
   baton->claheHeight = sharp::AttrAsUint32(options, "claheHeight");
   baton->claheMaxSlope = sharp::AttrAsUint32(options, "claheMaxSlope");

src/pipeline.h

@@ -69,8 +69,7 @@ struct PipelineBaton {
   bool premultiplied;
   bool tileCentre;
   bool fastShrinkOnLoad;
-  double tintA;
-  double tintB;
+  std::vector<double> tint;
   bool flatten;
   std::vector<double> flattenBackground;
   bool unflatten;
@@ -239,8 +238,7 @@ struct PipelineBaton {
     attentionX(0),
     attentionY(0),
     premultiplied(false),
-    tintA(128.0),
-    tintB(128.0),
+    tint{ -1.0, 0.0, 0.0, 0.0 },
     flatten(false),
     flattenBackground{ 0.0, 0.0, 0.0 },
     unflatten(false),

test/unit/tint.js

@@ -8,97 +8,100 @@ const assert = require('assert');
 const sharp = require('../../');
 const fixtures = require('../fixtures');
 
+// Allow for small rounding differences between platforms
+const maxDistance = 6;
+
 describe('Tint', function () {
   it('tints rgb image red', function (done) {
     const output = fixtures.path('output.tint-red.jpg');
     sharp(fixtures.inputJpg)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint('#FF0000')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(true, info.size > 0);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-red.jpg'), 18);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-red.jpg'), maxDistance);
         done();
       });
   });
 
   it('tints rgb image green', function (done) {
     const output = fixtures.path('output.tint-green.jpg');
     sharp(fixtures.inputJpg)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint('#00FF00')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(true, info.size > 0);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-green.jpg'), 27);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-green.jpg'), maxDistance);
         done();
       });
   });
 
   it('tints rgb image blue', function (done) {
     const output = fixtures.path('output.tint-blue.jpg');
     sharp(fixtures.inputJpg)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint('#0000FF')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(true, info.size > 0);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-blue.jpg'), 14);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-blue.jpg'), maxDistance);
         done();
       });
   });
 
   it('tints rgb image with sepia tone', function (done) {
     const output = fixtures.path('output.tint-sepia-hex.jpg');
     sharp(fixtures.inputJpg)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint('#704214')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(320, info.width);
         assert.strictEqual(240, info.height);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-sepia.jpg'), 10);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-sepia.jpg'), maxDistance);
         done();
       });
   });
 
   it('tints rgb image with sepia tone with rgb colour', function (done) {
     const output = fixtures.path('output.tint-sepia-rgb.jpg');
     sharp(fixtures.inputJpg)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint([112, 66, 20])
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(320, info.width);
         assert.strictEqual(240, info.height);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-sepia.jpg'), 10);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-sepia.jpg'), maxDistance);
         done();
       });
   });
 
   it('tints rgb image with alpha channel', function (done) {
     const output = fixtures.path('output.tint-alpha.png');
     sharp(fixtures.inputPngRGBWithAlpha)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint('#704214')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(320, info.width);
         assert.strictEqual(240, info.height);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-alpha.png'), 10);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-alpha.png'), maxDistance);
         done();
       });
   });
 
   it('tints cmyk image red', function (done) {
     const output = fixtures.path('output.tint-cmyk.jpg');
     sharp(fixtures.inputJpgWithCmykProfile)
-      .resize(320, 240, { fastShrinkOnLoad: false })
+      .resize(320, 240)
       .tint('#FF0000')
       .toFile(output, function (err, info) {
         if (err) throw err;
         assert.strictEqual(true, info.size > 0);
-        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-cmyk.jpg'), 15);
+        fixtures.assertMaxColourDistance(output, fixtures.expected('tint-cmyk.jpg'), maxDistance);
         done();
       });
   });