This chapter provides a brief overview of modern preprocessing methods for computer vision. In Section 13.1 we introduce methods in which we replace each pixel in the image with a new value. Section 13.2 considers the problem of finding and characterizing edges, corners and interest points in images. In Section 13.3 we discuss visual descriptors; these are low-dimensional vectors that attempt to characterize the interesting aspects of an image region in a compact way. Finally, in Section 13.4 we discuss methods for dimensionality reduction.

Per-pixel transformations

We start our discussion of preprocessing with per-pixel operations: these methods return a single value corresponding to each pixel of the input image. We denote the original 2D array of pixel data as P, where pij is the element at the ith of I rows and the jth of J columns. The element pij is a scalar representing the grayscale intensity. Per-pixel operations return a new 2D array X of the same size as P containing elements xij.

Whitening

The goal of whitening (Figure 13.1) is to provide invariance to fluctuations in the mean intensity level and contrast of the image. Such variation may arise because of a change in ambient lighting intensity, the object reflectance, or the camera gain. To compensate for these factors, the image is transformed so that the resulting pixel values have zero mean and unit variance.