1. A method for detecting a document boundary in a captured digital image of a document, the method implemented at least in part by a data processing system and comprising: receiving a color digital image captured using a digital imaging system that depicts a hardcopy document on a background, wherein the color digital image includes image data for a plurality of color channels; for each of the color channels analyzing the image data to determine a corresponding busyness metric, the busyness metric representing a complexity level of the image data, wherein the determination of the busyness metric includes determining a compressibility metric for the image data; selecting the color channel having a lowest busyness level; analyzing the image data for the selected color channel to detect the document boundary of the depicted hardcopy document; and storing an indication of the detected document boundary in a processor-accessible memory.