Panels a–c: effective convolution function for samples at different positions within the sub-grid; panel a: sample at the center ofthe sub-grid; panel b: sample at the leftmost position within the sub-grid before the main lobe wraps around; panel c: sample at the rightmost position within the sub-grid before the main lobe wraps around. Panel d: gridding error as a function of position of the sample within the sub-grid. Close to the center of the sub-grid the error changes little with position. The error increases quickly with distance from the center immediately before the maximum distance is reached.