1. A method for depth mapping, comprising: projecting a pattern of infrared optical radiation onto an object; capturing an infrared image of the pattern on the object using a first image sensor, and processing the infrared image alone to generate pattern-based depth data with respect to the object; capturing a color image of the object using a second image sensor, at a known spacing from the first image sensor, wherein the projected pattern does not appear in the color image, and stereoscopically measuring a local offset between locations in the color image and the infrared image resulting from parallax between the first and second image sensors so as to generate stereoscopic depth data with respect to the object based on the measured offset and the known spacing; and combining the pattern-based depth data from a first area of a scene containing the object with the stereoscopic depth data from a different, second area of the scene in which the projected pattern has a contrast too low to enable reliable detection to create a depth map of the object.