You will be shown an excerpt from a computer science scientific research paper, followed by a question about the paper. Please answer the question. Do not include any text in your response other than the answer.

Context: The RoI pooling layer uses max pooling to convert the features inside any valid region of interest into a small feature map with a fixed spatial extent of H\times W (e.g., 7\times 7), where H and W are layer hyper-parameters that are independent of any particular RoI.In this paper, an RoI is a rectangular window into a conv feature map.Each RoI is defined by a four-tuple (r,c,h,w) that specifies its top-left corner (r,c) and its height and width (h,w).

Question: Why does the author use a rectangular shaped RoI?
in order to make a fixed feature map.