Understanding Flow SPTransform

[YoussefFathi]

I would appreciate it if you can clarify for me the following points related to specific parts of the "sp_transforms.py" file :

1. In _transformflow() , what is the purpose of the following block of code ?

   ` # inverse transform coords x0, y0 = self.inverse_transform_coords( width=width, height=height, thetas=theta1)

   x1, y1 = self.inverse_transform_coords(
       width=width, height=height, thetas=theta2, offset_x=u, offset_y=v)
   
   # subtract and create new flow
   u = x1 - x0
   v = y1 - y0
   new_flow = torch.stack([u, v], dim=1)`

2. Why didn't you use only the following part of the code to transform the optical flow just like the _transform_image()_ function ?

    `  # transform coords
   xq, yq = self.transform_coords(width=width, height=height, thetas=theta1)
   
   # interp2
   transformed = self._flow_interp2(new_flow, xq, yq)`

3. What is the difference between the functionality of _inverse_transform_coords() and transform_coords()_ ?

[lliuz]

Since spatial transformations will lead in a change to coordinates, we should sample optical flow on the view of transformed coordinates rather than the original one.

In this code, inverse_transform_coords() transform coordinates: and transform_coords() is its inverse operation.