Transform coding of images using directionally adaptive vector quantization

An image coding technique is described that compresses monochrome digital TV images from 8 to about 1 bit/pixel while maintaining high quality. First, for subblocks of 8×8 pixels the discrete cosine transform is calculated. Then, the resulting block of 8×8 transform coefficients is divided into a number of subvectors, each of which is normalized and quantized using vector quantization. The subvector construction and the vector quantization are performed adaptively to the `direction´ of spatial activity in the pixel subblock. The quantization also adapts to the energy of the subvector