Expose linear transform feature of libvips (#1024)

src/operations.cc

@@ -341,4 +341,18 @@ namespace sharp {
     return image.extract_area(left, top, width, height);
   }
 
+  /*
+   * Calculate (a * in + b)
+   */
+  VImage Linear(VImage image, double const a, double const b) {
+    if (HasAlpha(image)) {
+      // Separate alpha channel
+      VImage imageWithoutAlpha = image.extract_band(0,
+        VImage::option()->set("n", image.bands() - 1));
+      VImage alpha = image[image.bands() - 1];
+      return imageWithoutAlpha.linear(a, b).bandjoin(alpha);
+    } else {
+      return image.linear(a, b);
+    }
+  }
 }  // namespace sharp

src/operations.h

@@ -92,6 +92,11 @@ namespace sharp {
   */
   VImage Trim(VImage image, int const tolerance);
 
+  /*
+   * Linear adjustment (a * in + b)
+   */
+  VImage Linear(VImage image, double const a, double const b);
+
 }  // namespace sharp
 
 #endif  // SRC_OPERATIONS_H_

src/pipeline.cc

@@ -644,6 +644,11 @@ class PipelineWorker : public Nan::AsyncWorker {
         image = sharp::Gamma(image, baton->gamma);
       }
 
+      // Linear adjustment (a * in + b)
+      if (baton->linearA != 1.0 || baton->linearB != 0.0) {
+        image = sharp::Linear(image, baton->linearA, baton->linearB);
+      }
+
       // Apply normalisation - stretch luminance to cover full dynamic range
       if (baton->normalise) {
         image = sharp::Normalise(image);
@@ -1185,6 +1190,8 @@ NAN_METHOD(pipeline) {
   baton->thresholdGrayscale = AttrTo<bool>(options, "thresholdGrayscale");
   baton->trimTolerance = AttrTo<int32_t>(options, "trimTolerance");
   baton->gamma = AttrTo<double>(options, "gamma");
+  baton->linearA = AttrTo<double>(options, "linearA");
+  baton->linearB = AttrTo<double>(options, "linearB");
   baton->greyscale = AttrTo<bool>(options, "greyscale");
   baton->normalise = AttrTo<bool>(options, "normalise");
   baton->useExifOrientation = AttrTo<bool>(options, "useExifOrientation");

src/pipeline.h

@@ -79,6 +79,8 @@ struct PipelineBaton {
   int threshold;
   bool thresholdGrayscale;
   int trimTolerance;
+  double linearA;
+  double linearB;
   double gamma;
   bool greyscale;
   bool normalise;
@@ -160,6 +162,8 @@ struct PipelineBaton {
     threshold(0),
     thresholdGrayscale(true),
     trimTolerance(0),
+    linearA(1.0),
+    linearB(0.0),
     gamma(0.0),
     greyscale(false),
     normalise(false),