Where to get negative sample images for Haar train

Question

I need a collection of sample images to train a Haar-based classifier for face detection. I read that a ratio of 2 negative examples for each positive example is acceptable. I searched around the web and found many databases containing positive examples to train my classifier (that is, images that contain faces), however, I can't any database with negative examples. Right now, I have over 18000 positive examples. Where can I find 2000 or more negative examples?

Answer 1

use http://tutorial-haartraining.googlecode.com/svn/trunk/data/negatives/

or any other image set that has no objects you need to recognize

NOTE: the number of samples you mention is too big, you don't need so much to obtain high accuracy