Algorithm suggestions for 2d and 3d volumetric combination of voxels?

I built an image to voxel converter for exporting to the game Avorion over the weekend. I am using a naive approach and treating each pixel as a box shape. I need to learn how to write an algorithm for combining voxels with the same color into rectangular cuboids. This will reduce the imported shapes. As is the game has issues with 64x64x1 images on import. It would be good to reduce the object count by creating cuboids with the pixels that can be combined. I would like to learn to write the algorithm for both 2d and 3d.