Question on alignment.py

[codeslake]

Hi,

I have a question on alignment.py in which affine transformation is patch-wisely applied. I am not an expert on deformable convolution, which I believe the code is based on, so please be patient with my question ;)

1. In def _get_p_n(...), why do you subtract 0.5 and add 0.6 to the range? Is it for the even-size kernel?

2. In def _get_p(...) Why do you add (self.kernel_size-1)//2+0.5 to affined transformed coordinates? Shouldn't it be (self.kernel_size-1)//2-0.5 for making coordinate range between 0~h-1?

[Tengfei-Wang]

It is for even-size kernels. Currently, the alignment only supports ever-sized kerkers. For example, when kernel size is 6, this gives a local mesh grid of [-2.5, -1.5, -0.5, 0.5, 1.5, 2.5]^2.
In this case, add (self.kernel_size-1)//2+0.5=2.5 transform the local mesh grid to 0~5. We then add it on p_0 to obtain global mesh coordinates.

[codeslake]

It is for even-size kernels. Currently, the alignment only supports ever-sized kerkers. For example, when kernel size is 6, this gives a local mesh grid of [-2.5, -1.5, -0.5, 0.5, 1.5, 2.5]^2. In this case, add (self.kernel_size-1)//2+0.5=2.5 transform the local mesh grid to 0~5. We then add it on p_0 to obtain global mesh coordinates.

But if you add (self.kernel_size-1)//2+0.5, the range of global mesh coordinate becomes 1~h. It lies between 0~h-1 if we add (self.kernel_size-1)//2-0.5. Isn't it the global mesh coordinate that should be in range 0~h-1?

[Tengfei-Wang]

It is for even-size kernels. Currently, the alignment only supports ever-sized kerkers. For example, when kernel size is 6, this gives a local mesh grid of [-2.5, -1.5, -0.5, 0.5, 1.5, 2.5]^2. In this case, add (self.kernel_size-1)//2+0.5=2.5 transform the local mesh grid to 0~5. We then add it on p_0 to obtain global mesh coordinates.

But if you add (self.kernel_size-1)//2+0.5, the range of global mesh coordinate becomes 1h. It lies between 0h if we add (self.kernel_size-1)//2-0.5. Isn't it the global mesh coordinate that should be in range 0~h-1?

Yes, or you may also modify the range of p_0 to [0, h-1].

[codeslake]

Ok, Thanks for the answer!