Described is a technology by which a large image is processed into a seamless, radiometric mean image containing meaningful image objects. A large source image is tiled into non-overlapping tiles, which are each segmented into regions and merged via their regions. Source image pixels are separated into tiles comprising rows and columns. Once tiled, the tiles are segmented into regions, with each region having an identifier and other associated information such as size data, boundary data and neighbor data. The tiles are merged by merging columns or rows of tiles into row blocks or column blocks, respectively. The row blocks or column blocks are vertically or horizontally merged, respectively. Merging may proceed by selecting a pair of regions based on merging cost, merging that pair, updating its associated data and data of neighbor regions based on the merge, and repeating for other pairs until merging stop criteria are met.