A conversion between two texture compression formats comprises calculations performed at index-level for reducing handling of values with color bit-length and an amount of calculations with color values. Format conversion can thus be performed in real time upon displaying an image using the compressed texture data, without significant slowing down of a display rate of the images. It may be implemented in particular for conversion from DXT1—to ETC1 compression format, and a non-flipped or flipped orientation of an ETC1—compressed texture data block can thus be determined from said texture data block as initially compressed in DXT1 format.