# Image Laplacian incomprehension

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  -1


But 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

• I think "window of 20x20 pixels" applies to "energy" rather than to "image Laplacian", i.e. you compute the Laplacian as usual, the compute its total energy (i.e. sum of squared values) on a window of 20x20 pixels. But you should add a link or reference to the research paper to be sure.
– user106
Jun 8 '17 at 15:36
• That would make sense, maybe this is it. I can't tell. But that's a good idea and I will try to implement it to see. You're right for the link, I will add it in the post. Jun 9 '17 at 9:53