Expand linear operation to allow use of per-channel arrays #3303

docs/api-operation.md

@@ -466,14 +466,32 @@ Returns **Sharp** 
 
 ## linear
 
-Apply the linear formula a \* input + b to the image (levels adjustment)
+Apply the linear formula `a` \* input + `b` to the image to adjust image levels.
+
+When a single number is provided, it will be used for all image channels.
+When an array of numbers is provided, the array length must match the number of channels.
 
 ### Parameters
 
-*   `a` **[number][1]** multiplier (optional, default `1.0`)
-*   `b` **[number][1]** offset (optional, default `0.0`)
+*   `a` **([number][1] | [Array][7]<[number][1]>)** multiplier (optional, default `[]`)
+*   `b` **([number][1] | [Array][7]<[number][1]>)** offset (optional, default `[]`)
 
-<!---->
+### Examples
+
+```javascript
+await sharp(input)
+  .linear(0.5, 2)
+  .toBuffer();
+```
+
+```javascript
+await sharp(rgbInput)
+  .linear(
+    [0.25, 0.5, 0.75],
+    [150, 100, 50]
+  )
+  .toBuffer();
+```
 
 *   Throws **[Error][5]** Invalid parameters
 

lib/constructor.js

@@ -319,8 +319,8 @@ const Sharp = function (input, options) {
     tileId: 'https://example.com/iiif',
     tileBasename: '',
     timeoutSeconds: 0,
-    linearA: 1,
-    linearB: 0,
+    linearA: [],
+    linearB: [],
     // Function to notify of libvips warnings
     debuglog: warning => {
       this.emit('warning', warning);

lib/operation.js

@@ -644,26 +644,55 @@ function boolean (operand, operator, options) {
 }
 
 /**
- * Apply the linear formula a * input + b to the image (levels adjustment)
- * @param {number} [a=1.0] multiplier
- * @param {number} [b=0.0] offset
+ * Apply the linear formula `a` * input + `b` to the image to adjust image levels.
+ *
+ * When a single number is provided, it will be used for all image channels.
+ * When an array of numbers is provided, the array length must match the number of channels.
+ *
+ * @example
+ * await sharp(input)
+ *   .linear(0.5, 2)
+ *   .toBuffer();
+ *
+ * @example
+ * await sharp(rgbInput)
+ *   .linear(
+ *     [0.25, 0.5, 0.75],
+ *     [150, 100, 50]
+ *   )
+ *   .toBuffer();
+ *
+ * @param {(number|number[])} [a=[]] multiplier
+ * @param {(number|number[])} [b=[]] offset
  * @returns {Sharp}
  * @throws {Error} Invalid parameters
  */
 function linear (a, b) {
+  if (!is.defined(a) && is.number(b)) {
+    a = 1.0;
+  } else if (is.number(a) && !is.defined(b)) {
+    b = 0.0;
+  }
   if (!is.defined(a)) {
-    this.options.linearA = 1.0;
+    this.options.linearA = [];
   } else if (is.number(a)) {
+    this.options.linearA = [a];
+  } else if (Array.isArray(a) && a.length && a.every(is.number)) {
     this.options.linearA = a;
   } else {
-    throw is.invalidParameterError('a', 'numeric', a);
+    throw is.invalidParameterError('a', 'number or array of numbers', a);
   }
   if (!is.defined(b)) {
-    this.options.linearB = 0.0;
+    this.options.linearB = [];
   } else if (is.number(b)) {
+    this.options.linearB = [b];
+  } else if (Array.isArray(b) && b.length && b.every(is.number)) {
     this.options.linearB = b;
   } else {
-    throw is.invalidParameterError('b', 'numeric', b);
+    throw is.invalidParameterError('b', 'number or array of numbers', b);
+  }
+  if (this.options.linearA.length !== this.options.linearB.length) {
+    throw new Error('Expected a and b to be arrays of the same length');
   }
   return this;
 }

src/operations.cc

@@ -306,10 +306,14 @@ namespace sharp {
   /*
    * Calculate (a * in + b)
    */
-  VImage Linear(VImage image, double const a, double const b) {
-    if (HasAlpha(image)) {
+  VImage Linear(VImage image, std::vector<double> const a, std::vector<double> const b) {
+    size_t const bands = static_cast<size_t>(image.bands());
+    if (a.size() > bands) {
+      throw VError("Band expansion using linear is unsupported");
+    }
+    if (HasAlpha(image) && a.size() != bands && (a.size() == 1 || a.size() == bands - 1 || bands - 1 == 1)) {
       // Separate alpha channel
-      VImage alpha = image[image.bands() - 1];
+      VImage alpha = image[bands - 1];
       return RemoveAlpha(image).linear(a, b).bandjoin(alpha);
     } else {
       return image.linear(a, b);

src/operations.h

@@ -90,7 +90,7 @@ namespace sharp {
   /*
    * Linear adjustment (a * in + b)
    */
-  VImage Linear(VImage image, double const a, double const b);
+  VImage Linear(VImage image, std::vector<double> const a,  std::vector<double> const b);
 
   /*
    * Recomb with a Matrix of the given bands/channel size.

src/pipeline.cc

@@ -688,7 +688,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       }
 
       // Linear adjustment (a * in + b)
-      if (baton->linearA != 1.0 || baton->linearB != 0.0) {
+      if (!baton->linearA.empty()) {
         image = sharp::Linear(image, baton->linearA, baton->linearB);
       }
 
@@ -1454,8 +1454,8 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->trimThreshold = sharp::AttrAsDouble(options, "trimThreshold");
   baton->gamma = sharp::AttrAsDouble(options, "gamma");
   baton->gammaOut = sharp::AttrAsDouble(options, "gammaOut");
-  baton->linearA = sharp::AttrAsDouble(options, "linearA");
-  baton->linearB = sharp::AttrAsDouble(options, "linearB");
+  baton->linearA = sharp::AttrAsVectorOfDouble(options, "linearA");
+  baton->linearB = sharp::AttrAsVectorOfDouble(options, "linearB");
   baton->greyscale = sharp::AttrAsBool(options, "greyscale");
   baton->normalise = sharp::AttrAsBool(options, "normalise");
   baton->claheWidth = sharp::AttrAsUint32(options, "claheWidth");

src/pipeline.h

@@ -100,8 +100,8 @@ struct PipelineBaton {
   double trimThreshold;
   int trimOffsetLeft;
   int trimOffsetTop;
-  double linearA;
-  double linearB;
+  std::vector<double> linearA;
+  std::vector<double> linearB;
   double gamma;
   double gammaOut;
   bool greyscale;
@@ -251,8 +251,8 @@ struct PipelineBaton {
     trimThreshold(0.0),
     trimOffsetLeft(0),
     trimOffsetTop(0),
-    linearA(1.0),
-    linearB(0.0),
+    linearA{},
+    linearB{},
     gamma(0.0),
     greyscale(false),
     normalise(false),

test/unit/linear.js

@@ -65,15 +65,34 @@ describe('Linear adjustment', function () {
       });
   });
 
-  it('Invalid linear arguments', function () {
-    assert.throws(function () {
-      sharp(fixtures.inputPngOverlayLayer1)
-        .linear('foo');
+  it('per channel level adjustment', function (done) {
+    sharp(fixtures.inputWebP)
+      .linear([0.25, 0.5, 0.75], [150, 100, 50]).toBuffer(function (err, data, info) {
+        if (err) throw err;
+        fixtures.assertSimilar(fixtures.expected('linear-per-channel.jpg'), data, done);
+      });
   });
 
-    assert.throws(function () {
-      sharp(fixtures.inputPngOverlayLayer1)
-        .linear(undefined, { bar: 'baz' });
-    });
+  it('Invalid linear arguments', function () {
+    assert.throws(
+      () => sharp().linear('foo'),
+      /Expected number or array of numbers for a but received foo of type string/
+    );
+    assert.throws(
+      () => sharp().linear(undefined, { bar: 'baz' }),
+      /Expected number or array of numbers for b but received \[object Object\] of type object/
+    );
+    assert.throws(
+      () => sharp().linear([], [1]),
+      /Expected number or array of numbers for a but received {2}of type object/
+    );
+    assert.throws(
+      () => sharp().linear([1, 2], [1]),
+      /Expected a and b to be arrays of the same length/
+    );
+    assert.throws(
+      () => sharp().linear([1]),
+      /Expected a and b to be arrays of the same length/
+    );
   });
 });