Image predictive decoding method, image predictive decoding apparatus, image predictive coding method, image predictive coding apparatus, and data storage media

An image predictive coding method for coding a target image, which is a target of coding, including an arbitrarily shaped object whose size is variable, to obtain a coded object image, comprising the steps of

• generating prediction object image by using, as a reference object image, at least one reproduced object image which includes non-zero image data to be referred to and which is reproduced before said target image;
• and
• compressively coding a difference between said prediction object image and said target image;
• wherein
• said coded object image has a flag which indicates whether said target image includes non-zero image data which is to be referred to when said target image is used as a reference object image for another target image,
• said prediction object image is generated using only a backward reference object image when it is determined that the size of a forward reference object image is zero and the size of the backward reference object image is not zero,
• said prediction object is generated using only the forward reference object image when it is determined that the size of the forward reference object image is not zero and the size of the backward reference object image is zero, and
• said prediction object image is generated using both the forward and backward reference object images when it is determined that the size of both the forward and backward reference object images are not zero.