--- # Appendix Thanks to: [@mbostock](https://observablehq.com/@mbostock) for the help making the elegant code. https://observablehq.com/@mbostock/hello-canny-edge-detection [@mootari](https://observablehq.com/@mootari) for getting openCV working inside observable. https://observablehq.com/@mootari/opencv-js ### openCV functions

Canny()
cv.[Canny](https://docs.opencv.org/3.4/dd/d1a/group__imgproc__feature.html#ga04723e007ed888ddf11d9ba04e2232de)( input, output, threshold1, threshold2[, edges[, apertureSize[, L2gradient]]] )

The [Canny edge detection](http://justin-liang.com/tutorials/canny/).

https://docs.opencv.org/3.4/da/d5c/tutorial_canny_detector.html

Canny	(
  InputArray  input,              // image	8-bit input image.
  OutputArray output,              // edges	output edge map; single channels 8-bit image, which has the same size as image .
  double      threshold1,         // threshold1	first threshold for the hysteresis procedure.
  double      threshold2,         // threshold2	second threshold for the hysteresis procedure.
  int         apertureSize = 3,   // apertureSize	aperture size for the Sobel operator.
  bool        L2gradient = false 
)

resize()
[cv.resize](https://docs.opencv.org/4.x/dd/d52/tutorial_js_geometric_transformations.html)(src, dst, dsize, fx = 0, fy = 0, interpolation = cv.INTER_LINEAR)

resize(
  src	  input image
  dst	  output image; it has the size dsize (when it is non-zero) or the size computed from src.size(), fx, and fy; the type of dst is the same as of src.
  dsize	output image size; if it equals zero, it is computed as:  dsize=Size(round(fx∗src.cols),round(fy∗src.rows)) Either dsize or both fx and fy must be non-zero.
  fx	   scale factor along the horizontal axis; when it equals 0, it is computed as (double)dsize.width/src.cols
  fy	   scale factor along the vertical axis;   when it equals 0, it is computed as (double)dsize.height/src.rows
  interpolation	interpolation method(see cv.InterpolationFlags)

Get a [pixel colour](https://docs.opencv.org/4.x/de/d06/tutorial_js_basic_ops.html#:~:text=3.-,ptr,-Mat%20Type).

let row = 3, col = 4;
let src = cv.imread("canvasInput");
let pixel = src.ucharPtr(row, col);
let R = pixel[0];
let G = pixel[1];
let B = pixel[2];
let A = pixel[3];

addWeighted ( src1, alpha, src2, beta, gamma, dst, dtype = -1)

InputArray src1, double alpha, InputArray src2, double beta, double gamma, OutputArray dst, int dtype = -1 )

let scalar = new cv.Scalar(R, G, B, Alpha);