Methods, systems and apparatus, including computer program products, for processing electronic images. An electronic image is transformed to generate a transformed image according to a first coordinate transformation. The transformed image is filtered, and a second coordinate transformation is applied to the filtered image to align the filtered image with an accumulation array that includes multiple accumulation elements. Each accumulation element is updated based on a respective value in the aligned image. The operations of generating a transformed image according to a first transformation, filtering the transformed image, applying a second transformation and updating each accumulation element are iterated for one or more further first coordinate transformations. These techniques can be implemented to detect features in the image using a graphics processing unit.