Improve tint luminance with weighting function (#3859)

Co-authored-by: John Cupitt <jcupitt@gmail.com>

lib/colour.js

@@ -19,7 +19,7 @@ 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
@@ -27,14 +27,12 @@ const colourspace = {
  *   .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.