Bit-rate reduction of an image signal, by an algorithm recognising the high degree of correlation ascertainable in specific images. The reproduced image does not replicate the original when viewed in direct comparison, but the losses are not obvious or recognisable under the intended display conditions. The algorithm may apply transform coding, predictive techniques, and other modelling of the image signal, plus some form of entrophy encoding. While the image appears unaltered to normal human vision, it may show losses and artefacts when analysed in other systems (i.e., chroma key, computerised image analysis, etc.). The lost information cannot be regenerated from the compressed bit stream.