# Laplace Rule In Imaging

1. Feb 16, 2005

### richnfg

Reading my physics book here, I've found something I'm not too sure on. It's called the Laplace rule and it used for finding edges in a image (digital image made from pixels and numbers). It says the rule but doesn't explain it at all:

subtract the N,S,E and W neighbours from 4 times the value of each pixel.

N being north etc..

Anyone care to explain this to me?

2. Feb 16, 2005

### Moose352

Take a pixel. Multiply its (color) values by 4. Now, subtract from this the values of the pixel above it (north), the pixel below (south), the pixel to the right (east), and the pixel to the left (west). The result is the new value for that pixel. If you do this operation to every pixel in the image, the resulting image will show the edges of the original image.

3. Feb 17, 2005

### Alkatran

Why it works:

Think about it, if the pixel you're working on is the same color as the ones around it, what will it's final color be?

4*x - x - x - x - x = 0 the pixel with similar pixels around it become black.

If the colors are different it doesn't turn black. Simple as that.