Example of using a convolution kernel to implement a Laplacian filter in OpenCV for Processing. Example output: https://www.flickr.com/photos/unavoidablegrain/16342382412/

opencv_for_processing_laplacian.pde

import gab.opencv.*;
import org.opencv.imgproc.Imgproc;
import org.opencv.core.Mat;
import org.opencv.core.CvType;

OpenCV opencv;
PImage img;

void setup() {
  img = loadImage("arch_sunset.jpg");
  size(img.width*2, img.height);
  opencv = new OpenCV(this, img);
  int kernelSize = 9;

  Mat kernel = new Mat(kernelSize, kernelSize, CvType.CV_32F);

  kernel.put(0, 0, 0);
  kernel.put(0, 1, -1);
  kernel.put(0, 2, 0);

  kernel.put(1, 0, -1);
  kernel.put(1, 1, 4);
  kernel.put(1, 2, -1);

  kernel.put(2, 0, 0);
  kernel.put(2, 1, -1);
  kernel.put(2, 2, 0);
  
  Imgproc.filter2D(opencv.getGray(), opencv.getGray(), -1, kernel);
  noLoop();
}

void draw() {
  image(img, 0, 0);
  image(opencv.getOutput(), img.width, 0);
}

I hit this accidentally, by the way...

there is an error at new Mat, because you are using a 3x3 laplacian, but creating 9x9.

kernelSize is in rows x cols, before multiply ;)