Add contrast limiting adaptive histogram equalization (CLAHE) operator (#2726)

src/operations.cc

@@ -92,6 +92,13 @@ namespace sharp {
     return image;
   }
 
+  /*
+   * Contrast limiting adapative histogram equalization (CLAHE)
+   */
+  VImage Clahe(VImage image, int const width, int const height, int const maxSlope) {
+    return image.hist_local(width, height, VImage::option()->set("max_slope", maxSlope));
+  }
+
   /*
    * Gamma encoding/decoding
    */

src/operations.h

@@ -35,6 +35,11 @@ namespace sharp {
    */
   VImage Normalise(VImage image);
 
+  /*
+   * Contrast limiting adapative histogram equalization (CLAHE)
+   */
+  VImage Clahe(VImage image, int const width, int const height, int const maxSlope);
+
   /*
    * Gamma encoding/decoding
    */

src/pipeline.cc

@@ -345,6 +345,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       bool const shouldApplyMedian = baton->medianSize > 0;
       bool const shouldComposite = !baton->composite.empty();
       bool const shouldModulate = baton->brightness != 1.0 || baton->saturation != 1.0 || baton->hue != 0.0;
+      bool const shouldApplyClahe = baton->claheWidth != 0 && baton->claheHeight != 0;
 
       if (shouldComposite && !sharp::HasAlpha(image)) {
         image = sharp::EnsureAlpha(image, 1);
@@ -650,6 +651,11 @@ class PipelineWorker : public Napi::AsyncWorker {
         image = sharp::Normalise(image);
       }
 
+      // Apply contrast limiting adaptive histogram equalization (CLAHE)
+      if (shouldApplyClahe) {
+        image = sharp::Clahe(image, baton->claheWidth, baton->claheHeight, baton->claheMaxSlope);
+      }
+
       // Apply bitwise boolean operation between images
       if (baton->boolean != nullptr) {
         VImage booleanImage;
@@ -1330,6 +1336,9 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->linearB = sharp::AttrAsDouble(options, "linearB");
   baton->greyscale = sharp::AttrAsBool(options, "greyscale");
   baton->normalise = sharp::AttrAsBool(options, "normalise");
+  baton->claheWidth = sharp::AttrAsUint32(options, "claheWidth");
+  baton->claheHeight = sharp::AttrAsUint32(options, "claheHeight");
+  baton->claheMaxSlope = sharp::AttrAsUint32(options, "claheMaxSlope");
   baton->useExifOrientation = sharp::AttrAsBool(options, "useExifOrientation");
   baton->angle = sharp::AttrAsInt32(options, "angle");
   baton->rotationAngle = sharp::AttrAsDouble(options, "rotationAngle");

src/pipeline.h

@@ -109,6 +109,9 @@ struct PipelineBaton {
   double gammaOut;
   bool greyscale;
   bool normalise;
+  int claheWidth;
+  int claheHeight;
+  int claheMaxSlope;
   bool useExifOrientation;
   int angle;
   double rotationAngle;
@@ -234,6 +237,9 @@ struct PipelineBaton {
     gamma(0.0),
     greyscale(false),
     normalise(false),
+    claheWidth(0),
+    claheHeight(0),
+    claheMaxSlope(3),
     useExifOrientation(false),
     angle(0),
     rotationAngle(0.0),