Expose linear transform feature of libvips (#1024)

lib/operation.js

@@ -406,6 +406,33 @@ function boolean (operand, operator, options) {
   return this;
 }
 
+/**
+ * Apply the linear formula a * input + b to the image (levels adjustment)
+ * @param {Number} [a=1.0] multiplier
+ * @param {Number} [b=0.0] offset
+ * @returns {Sharp}
+ * @throws {Error} Invalid parameters
+ */
+function linear (a, b) {
+  if (!is.defined(a)) {
+    this.options.linearA = 1.0;
+  } else if (is.number(a)) {
+    this.options.linearA = a;
+  } else {
+    throw new Error('Invalid linear transform multiplier ' + a);
+  }
+
+  if (!is.defined(b)) {
+    this.options.linearB = 0.0;
+  } else if (is.number(b)) {
+    this.options.linearB = b;
+  } else {
+    throw new Error('Invalid linear transform offset ' + b);
+  }
+
+  return this;
+}
+
 /**
  * Decorate the Sharp prototype with operation-related functions.
  * @private
@@ -427,7 +454,8 @@ module.exports = function (Sharp) {
     normalize,
     convolve,
     threshold,
-    boolean
+    boolean,
+    linear
   ].forEach(function (f) {
     Sharp.prototype[f.name] = f;
   });