A method and system for estimating the background signal over an arbitrarily-sized region of a scanned image of a molecular array, including a background region surrounding the ROI corresponding to the feature. A bit mask is generated, based on a molecular-array, feature-based data set that includes pixel-based intensities and a list of features, including feature coordinates and feature ROI radii, to indicate those pixels in the scanned image of the molecular array corresponding to background, and those pixels in the scanned image of the molecular array corresponding to features and ROIs. An integrated intensity for a background region of arbitrary size and shape can be efficiently determined by selecting pixels within the background region that are indicated to be background pixels in the bit mask. By selecting background anuli of sufficient size to overlap with the background anuli of neighboring features, a relatively continuous function of background-signal-verusus-position can be obtained across the surface of a molecular array in order to examine non-local, background-signal-related phenomena.