Mastering OpenCV 3 - Second Edition

Shrinking the camera image

We can use the resize() function to shrink an image to a certain size or scale factor. Face detection usually works quite well for any image size greater than 240x240 pixels (unless you need to detect faces that are far away from the camera), because it will look for any faces larger than the minFeatureSize (typically 20x20 pixels). So let's shrink the camera image to be 320 pixels wide; it doesn't matter if the input is a VGA webcam or a five mega pixel HD camera. It is also important to remember and enlarge the detection results, because if you detect faces in a shrunk image then the results will also be shrunk. Note that instead of shrinking the input image, you could use a large value for the minFeatureSize ...