How to return cv::Mat in nodejs addon

Question

I've written nodejs addon using V8. I'm stuck at a point where I'm trying to return Mat but all I'm getting is corrupted image with size of 2mb (for particular image). Am I doing something wrong? How can I do this using V8?

CPP code snippet

cv::Mat image = ...
std::string my_cv_mat(image.begin<unsigned char>(), image.end<unsigned char>());
args.GetReturnValue().Set(v8::String::NewFromUtf8(isolate, my_cv_mat.c_str()).ToLocalChecked());

Nodejs code snippet

// body is express's req.body
let bodyBuffer = new Buffer.from(body, "binary");
let detectedFaces = faceDetect.detect(bodyBuffer)  //here I'm reading above Mat image into char string
const out_file = path.basename("output.png")
fs.writeFileSync(out_file, new Buffer.from(detectedFaces, "binary"))

--- UPDATE ---

I've updated the code but output is still not an image and its size is now 23mb while it should be 700kb.

CPP updated code snippet

  unsigned int img_size = image.total() * image.elemSize();
  char* image_char = reinterpret_cast<char*>(image.data);
  Nan::MaybeLocal<v8::Object> imageBuffer = Nan::CopyBuffer(image_char, img_size);
  args.GetReturnValue().Set(imageBuffer.ToLocalChecked());

nodejs code snippet

    let detectedFaces = faceDetect.detect(bodyBuffer)
    const out_file = path.basename("output.png")
    fs.writeFileSync(out_file, new Buffer.from(detectedFaces, "binary"))

jmrk · Accepted Answer · 2021-03-11 19:08:59Z

1

I don't think a UTF-8 encoded string (or, more accurately: constructing a JavaScript string from raw image data that you're telling the String constructor to treat as UTF-8 and decode accordingly) is the right tool for the job.

Try creating a Buffer directly in your addon, and returning that as the call's result (instead of a v8::String).

answered Mar 11, 2021 at 19:08

jmrk

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6 Comments

Atul Singh Over a year ago

Can you please show me an example. I didn't see anything related to returning buffer directly.

jmrk Over a year ago

See examples e.g. here: stackoverflow.com/questions/41766056/… and here: stackoverflow.com/questions/48272168/… ; documentation: nodejs.org/api/n-api.html#n_api_napi_create_buffer and github.com/nodejs/node-addon-api/blob/main/doc/buffer.md

Atul Singh Over a year ago

I've updated the question. I did the same as saw in the example but still no luck.

jmrk Over a year ago

What is the data format that's stored in your cv::Mat? Is it plain bitmap data, or some other format? It sounds like it's not in PNG format yet, so encoding it as (compressed?) .png would be a separate conversion step. "How to encode raw image data as PNG?" would be a separate and different question though (unrelated to Node.js, Node Addons, or V8). A node::Buffer transports the data you have, with the size that it has.

Atul Singh Over a year ago

Yes, I'm [passing jpeg image as input. I'll try the way you said. @jmrk

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