1. An image processor ( 10 ) for detection of an object in an image or sequence of images, each image being formed of pixels, and the image processor comprising: a window selector for choosing a detection window within the image, a self-similarity computation part ( 40 ) configured to determine similarity information for a group of the pixels in any part of the detection window, to represent an amount of self-similarity of that group to other groups in any other part of the detection window, and for repeating the determination for groups in all parts of the detection window, to generate a global self-similarity descriptor for the chosen detection window, wherein the self-similarity computation part is configured to determine a plurality of similarity values, each similarity value being based on a comparison of self-similarity information for a first group of the pixels with a second group of the pixels, and wherein the plurality of similarity values are comprised in the global self-similarity descriptor; and a classifier ( 50 ) configured to classify whether the object is present in the detection window of the image from the global self-similarity descriptor for that detection window.