I want to use a C-Function from Node.js by using N-API with node-addon-api module wrapper. This is the first time with N-API for me and I'm also a beginner with Node and C++. I have a experience in C programming of embedded systems but this Node.jS / N-API thing I don't understand completely yet...

What I wan't to do is to call a C-Function with this prototype from Node.js:

unsigned char *MyFunction(unsigned char *data, size_t size, size_t min, size_t max)

*data is a pointer to an array containing RGB image data [R0, G0, B0, R1, G1, B1, ...] with size size which should be processed in MyFunction (extracting RGB channels, inverting, ...).

What I have so far is this c++ code:

#include <napi.h>

using namespace Napi;

Napi::Value Method(const Napi::CallbackInfo& info) {
  Napi::Env env = info.Env();

  if (info.Length() != 3) {
    Napi::TypeError::New(env, "Wrong arguments").ThrowAsJavaScriptException();
    return env.Null();    
  }
  else {
    const Napi::Array inputArray = info[0].As<Napi::Array>();
    const float smin = info[1].As<Napi::Number>().FloatValue();
    const float smax = info[2].As<Napi::Number>().FloatValue();
    const unsigned int length = inputArray.Length();
    unsigned int i;
    Napi::Array outputArray = Napi::Array::New(env, length);
    Napi::Array redArray = Napi::Array::New(env, length / 3);
    Napi::Array greenArray = Napi::Array::New(env, length / 3);
    Napi::Array blueArray = Napi::Array::New(env, length / 3);

    // Extract Channels
    for (i = 0; i < (length / 3); i++) {
      redArray[i] = inputArray[i * 3];
      greenArray[i] = inputArray[i * 3 + 1];
      blueArray[i] = inputArray[i * 3 + 2];
    }

    // Apply Simple Color Balance to each channel
    for (i = 0; i < (length / 3); i++) {
      outputArray[i] = redArray[i];
    }
    return redArray;    // Test => this works
  }
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {
  exports.Set(Napi::String::New(env, "Test"),
              Napi::Function::New(env, Method));
  return exports;
}

NODE_API_MODULE(module, Init)

and this is the node part:

const  sharp  = require('sharp');
sharp('testPic.jpg')
  .resize(800, 600)
  .toFile('SharpOut.jpg')
  .then( data => { 
    console.log('Resize & Negate => OK')
    // Extract Channel data
    sharp('SharpOut.jpg')
    .raw()
    .toBuffer()
    .then(data => {
      var smin = 0.0;
      var smax = 0.0;
      var testArr = [];
      for (let n = 0; n < data.length; n++) {
        testArr[n] = data[n];
      }      
      const HelloWorld = require('./build/Release/module.node');
      const result =  HelloWorld.Test(testArr, smin, smax);
    })
    .catch(err => {
      console.log('ERROR during extraction of RED channel. ERR:');
      console.log(err);
    });
  })
  .catch( err => { 
    console.log('ERROR');
    console.log(err);
  });

My problems

1. Sharp outputs a buffer and not an array but with ArrayBuffer instead of Array I was not able to get working code. Compiling is ok but when I execute it in node I'm getting

Error: Invalid argument at D:\temp\testSCB\index.js:30:34

which is this line of code const result = HelloWorld.Test(testArr, smin, smax);)

2. If I change redArray[i] = inputArray[i * 3]; to redArray[i] = ~(inputArray[i * 3]); to invert the color I'm getting two errors:

error C2675: unary '~': 'Napi::Value' does not define this operator or a conversion to a type acceptable to the predefined operator

and

error C2088: '~': illegal for class

My Question

What is the correct way to implement my c-Function to work from node.js?

Thanks for any help!