1. A computer-implemented method comprising: receiving an image; for a first stage: extracting features from the image; identifying, based on the extracted features, non-object regions of the image which do not include an object; rejecting the non-object regions of the image; for one or more subsequent stages: extracting additional features from non-rejected regions of the image; identifying, based on the additional extracted features, additional non-object regions of the image; rejecting the additional non-object regions of the image; for a final stage of the one or more subsequent stages, detecting one or more objects in the image based on features extracted in the final stage.