Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for rendering an image. One of the methods includes, for each of a plurality of pixels of the image: maintaining current color data for each of a plurality of render elements generated by sampling color contribution values for the render element for the pixel; determining, for each of the plurality of render elements, whether or not to sample another color contribution value for the render element from the current color data for the render element for the pixel and the current color data for the render element for one or more pixels that neighbor the pixel in the image; and sampling a new color contribution value only for the render elements for which it was determined to sample another contribution value.