aspect
By default, aspect ratio of the straightened document is automatically determined based on the detected document corners. If the document is significantly deformed, the estimated aspect ratio may be inaccurate. In such cases, providing a list of expected aspect ratios may help improve the accuracy of the straightening. The closest matching aspect ratio from the list will be used. Note, that if for a given paper format you want to support both portrait and landscape orientations, you need to provide both aspect ratios (e.g. 1:√2 and √2:1 for A series papers).