Detecting change in two consecutive pictures

Question

As a toy project i want to write an application that will take pictures from a web cam and then detect changes in them. I want to detect if someone walked in front of the webcam.

Can you guys, show me some pointers on how can i detect a major change in two consecutive shots?

Answer 1

Subtract the per pixel lightness values from one image to the other.

This will give you high values per pixel where something is moved, and low values for where things are the same. Total these values and check to see if it's above a certain threshold number to see if a major change has occured.

I took a photo of my hand from my laptop to show this:

Answer 2

I would start by scaling down the image to something like 30K to 60K pixels. This not only will speed up calculation, but also get rid of minute irrelevant changes. For instance you don't want to detect a curtain near an open window moved by the wind.

Next, sum the squared differences of the pixel values. If you want to do it thoroughly, you can do it for R, G and B separately:

Delta := 0;
for y := 0 to Im1.Height do begin
  for x := 0 to Im1.Width do begin
    DeltaR := ( Im1[x,y].Red   - Im2[x,y].Red ) ^ 2;
    DeltaG := ( Im1[x,y].Green - Im2[x,y].Green ) ^ 2;
    DeltaB := ( Im1[x,y].Blue  - Im2[x,y].Blue ) ^ 2;
    Delta := Delta + DeltaR + DeltaG + DeltaB;
  end;
end;

if (Delta > Treshold) then
  Say( 'Hi Bob!' );

(this is just pseudo-code and as code would process rather slowly, google for "scanline" if you want to process all pixels in an image quickly)

You may want to define the treshold value empirically: walk slowly by the camera, preferably wearing clothes matching the background color and see what delta's you get.
Skipping frames should increase sensitivity.