An example embodiment may involve obtaining an a×b pixel macro-cell from an input image. The a×b pixel macro-cell may contain 4 non-overlapping m×n pixel cells. The a×b pixels in the a×b pixel macro-cell may have respective color values and may be associated with respective object type tags. The example embodiment may also include selecting a compression technique to either (i) compress the a×b pixel macro-cell as a whole, or (ii) compress the a×b pixel macro-cell by compressing each of the 4 non-overlapping m×n pixel cells separately. The example embodiment may further include compressing the a×b pixel macro-cell according to the selected compression technique, and writing a representation of the compressed a×b pixel macro-cell to a computer-readable output medium.