Is there a way to convert opencv object (mat object) to blob in javascript?

vnodrajendran · January 10, 2022, 7:18am

Currently I am figuring out how can we convert directly opencv object to blob without using canvas? Any help here

berak · January 10, 2022, 7:26am

which kind of blob are you talking about ? please explain !

vnodrajendran · January 10, 2022, 7:35am

Image blob. Currently I am writing opencv object into canvas usingimshow and then converting blob using toBlob(). If there is a way in opencv which would directly convert images without using canvas.

berak · January 10, 2022, 8:07am

what do you want to do with it ?
would ImageData do the trick ?

you could look at the imshow() code:

github.com

opencv/opencv/blob/a1143c4ea02afa7c45c2a1e86be431b81a83bcd1/modules/js/src/helpers.js#L109

    
      
                      break;
                  case cv.CV_8UC3:
                      cv.cvtColor(img, img, cv.COLOR_RGB2RGBA);
                      break;
                  case cv.CV_8UC4:
                      break;
                  default:
                      throw new Error('Bad number of channels (Source image must have 1, 3 or 4 channels)');
                      return;
              }
              var imgData = new ImageData(new Uint8ClampedArray(img.data), img.cols, img.rows);
              var ctx = canvas.getContext('2d');
              ctx.clearRect(0, 0, canvas.width, canvas.height);
              canvas.width = imgData.width;
              canvas.height = imgData.height;
              ctx.putImageData(imgData, 0, 0);
              img.delete();
          };
          
          
Module['VideoCapture'] = function(videoSource) {
              var video = null;

ivan-ngchakming · May 25, 2022, 4:37pm

I used cv.blobFromImage to convert opencv Mat object directly to blob, no canvas needed.

const blob = cv.blobFromImage(
    image,  // cv.Mat
    1 / 128,  // scalefactor
    new cv.Size(inputWeight, inputHeight),  // size
    new cv.Scalar(inputMean, inputMean, inputMean),  // mean
    true  // swapRB
);

See OpenCV: Deep Neural Network module for API docs.

berak · May 25, 2022, 5:19pm

nice, but imo you misread it.
question seems not about opencv’s dnn blobs, but about html5 image