Add tint operation to set image chroma

src/operations.cc

@@ -152,6 +152,32 @@ namespace sharp {
     return dst.bandjoin(mask.cast(dst.format()));
   }
 
+  /*
+   * Tint an image using the specified chroma, preserving the original image luminance
+   */
+  VImage Tint(VImage image, double const a, double const b) {
+    // Get original colourspace
+    VipsInterpretation typeBeforeTint = image.interpretation();
+    if (typeBeforeTint == VIPS_INTERPRETATION_RGB) {
+      typeBeforeTint = VIPS_INTERPRETATION_sRGB;
+    }
+    // Create 2 band image with every pixel set to the tint chroma
+    std::vector<double> chromaPixel {a, b};
+    VImage chroma = image.new_from_image(chromaPixel);
+    // 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
+    VImage tinted = luminance.bandjoin(chroma).colourspace(typeBeforeTint);
+    // Attach original alpha channel, if any
+    if (HasAlpha(image)) {
+      // Extract original alpha channel
+      VImage alpha = image[image.bands() - 1];
+      // Join alpha channel to normalised image
+      tinted = tinted.bandjoin(alpha);
+    }
+    return tinted;
+  }
+
   /*
    * Stretch luminance to cover full dynamic range.
    */

src/operations.h

@@ -46,6 +46,11 @@ namespace sharp {
   */
   VImage Cutout(VImage src, VImage dst, const int gravity);
 
+  /*
+   * Tint an image using the specified chroma, preserving the original image luminance
+   */
+  VImage Tint(VImage image, double const a, double const b);
+
   /*
    * Stretch luminance to cover full dynamic range.
    */

src/pipeline.cc

@@ -682,6 +682,11 @@ class PipelineWorker : public Nan::AsyncWorker {
         image = sharp::Bandbool(image, baton->bandBoolOp);
       }
 
+      // Tint the image
+      if (baton->tintA > 0 || baton->tintB > 0) {
+        image = sharp::Tint(image, baton->tintA, baton->tintB);
+      }
+
       // Extract an image channel (aka vips band)
       if (baton->extractChannel > -1) {
         if (baton->extractChannel >= image.bands()) {
@@ -1167,6 +1172,9 @@ NAN_METHOD(pipeline) {
   for (unsigned int i = 0; i < 4; i++) {
     baton->background[i] = AttrTo<double>(background, i);
   }
+  // Tint chroma
+  baton->tintA = AttrTo<double>(options, "tintA");
+  baton->tintB = AttrTo<double>(options, "tintB");
   // Overlay options
   if (HasAttr(options, "overlay")) {
     baton->overlay = CreateInputDescriptor(AttrAs<v8::Object>(options, "overlay"), buffersToPersist);

src/pipeline.h

@@ -70,6 +70,8 @@ struct PipelineBaton {
   std::string kernel;
   bool fastShrinkOnLoad;
   double background[4];
+  double tintA;
+  double tintB;
   bool flatten;
   bool negate;
   double blurSigma;
@@ -155,6 +157,8 @@ struct PipelineBaton {
     cropOffsetLeft(0),
     cropOffsetTop(0),
     premultiplied(false),
+    tintA(0.0),
+    tintB(0.0),
     flatten(false),
     negate(false),
     blurSigma(0.0),