Question on flow normalize

[ghost]

Hello, Could I ask why you normalize the flow by vgrid[:,0,:,:] = 2.0*vgrid[:,0,:,:].clone() / max(W-1,1)-1.0 in the warping function?

Due to minus value in flow, the vgrid could also have minus and then the result of that line could be less then -1. Isnt that out of purposed vgrid range [-1, 1]?

Can we assume that vgrid == sum(grid, flow) always stay larger than 0?