Method and apparatus for coding a transparency signal

A transparency coding apparatus for coding a transparency signal corresponding to an object constituting an image, which transparency signal indicates a composition ratio used when compcsiting an image signal of the object with another image signal. This apparatus comprises a scaling value detection unit for receiving a transparency signal whose signal value has been changed with a fixed scale factor, and detecting a specific signal value corresponding to the fixed scale factor as a scaling value; a scaling unit for performing scaling on the transparency signal whose signal value has been changed, by using the detected scaling value, and outputting a scaled transparency signal whose signal value is restored to the original signal value before the change; a first coding unit for coding the scaling value to output coded scaling value data; a blocking unit for dividing the scaled transparency signal into signals corresponding to plural blocks each comprising a predetermined number of pixels, and outputting a transparency signal corresponding to each block; and a second coding unit for coding the transparency signal corresponding to each block to output a coded transparency signal. Therefore, the process of uniformly changing the transparency values of the transparency signal in VOP units, can be performed easily and independently of coding of the transparency signal, while improving the coding efficiency of the transparency signal.