I am reading and trying to implement a research paper.It uses openGL to define which part of an image to use in order to reconstitute a big image composited of several images.In order to do so, I have to generate several masks and multiply them together for each image.One mask is the focus mask, and they say:We evaluate the per-pixel focusness of the source image using a sharpness operator that is the energy of the image Laplacian applied on a window of 20x20 pixels centered on each evaluated points.My incomprehension is that: isn't Laplacian filter the convolution matrix: 0  -1   0   or    -1  -1  -1-1   4  -1         -1   8  -1 0  -1   0         -1  -1  -1But it's of size 3 x 3 and not 20 x 20.The fact that I'm not a native english speaker maybe fools me on the words and there is something I don't understand in this sentence...EDIT : The PDF with the article is downloadable here for free.=> 4.1 : Focus Mask
Image Laplacian incomprehension
opengl;c++;pixel graphics;filtering