A system for merging a plurality of image sources into a single image stream defines a predistortion image mask for each source. Each mask defines a distorted image space to account for distortion in the display optics, and indicates a region or regions in the desired output image where one merge method should be used versus another (e.g., overlay vs. replace). Each source mask may have a separate level of granularity according to the requirements of the included image and corresponding distortion.