Diagonal discrete cosine transforms for image coding

A new block-based DCT framework has been developed recently in[1] in which the first transform may choose to follow a direction other than the vertical or horizontal one - the default direction in the conventional DCT. In this paper, we focus on two diagonal directions because they are visually more important than other directions in an image block (except the vertical and horizontal ones). Specifically, we re-formulate the framework of two diagonal DCTs and use them in combination with the conventional DCT. We will discuss issues such as the directional mode selection and the cross-check of directional modes. Some experimental results are provided to demonstrate the effectiveness of our proposed diagonal DCT's in image coding applications.