code: protected String nativeCaptureToDataURL(CanvasElement canvas, Element element, String mimeType) {
          VideoElement videoElement = (VideoElement) element;
          int width = videoElement.getVideoWidth();
          int height = videoElement.getVideoHeight();
          if (Double.isNaN(width) || Double.isNaN(height)) {
              width = videoElement.getClientWidth();
              height = videoElement.getClientHeight();
          }
          canvas.setWidth(width);
          canvas.setHeight(height);
          Context2d context = canvas.getContext2d();
          context.drawImage(videoElement, 0, 0, width, height);
          return canvas.toDataUrl(mimeType);
      }
docs: Native call to capture the frame of the video stream.