Block distortion standards are obtained by considering the non-linearity of the visual sensation on stimulus, the non-linearity of illumination brightness versus voltage in an image display apparatus, the measurement of errors of lightness which is sensed on brightness stimulus by human beings, the discrimination threshold characteristic on lightness of the visual sensation, and brightness ruggedness. The brightness ruggedness is made proximate by brightness dispersion, so that a calculation cost is lowered. Formulas are given for the various kinds of distortion and, in particular, for the distortion at the boundaries between adjacent blocks. The quantization step is controlled in accordance with the desired block distortion. <IMAGE>
Method for compressing and extending an image by transforming orthogonally and encoding the image