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from torch import nn
import torch.nn.functional as F
import torch
class AntiAliasInterpolation2d(nn.Module):
"""
Band-limited downsampling, for better preservation of the input signal.
"""
def __init__(self, channels, scale):
super(AntiAliasInterpolation2d, self).__init__()
sigma = (1 / scale - 1) / 2
kernel_size = 2 * round(sigma * 4) + 1
self.ka = kernel_size // 2
self.kb = self.ka - 1 if kernel_size % 2 == 0 else self.ka
kernel_size = [kernel_size, kernel_size]
sigma = [sigma, sigma]
# The gaussian kernel is the product of the
# gaussian function of each dimension.
kernel = 1
meshgrids = torch.meshgrid(
[
torch.arange(size, dtype=torch.float32)
for size in kernel_size
]
)
for size, std, mgrid in zip(kernel_size, sigma, meshgrids):
mean = (size - 1) / 2
kernel *= torch.exp(-(mgrid - mean) ** 2 / (2 * std ** 2))
# Make sure sum of values in gaussian kernel equals 1.
kernel = kernel / torch.sum(kernel)
# Reshape to depthwise convolutional weight
kernel = kernel.view(1, 1, *kernel.size())
kernel = kernel.repeat(channels, *[1] * (kernel.dim() - 1))
self.register_buffer('weight', kernel)
self.groups = channels
self.scale = scale
inv_scale = 1 / scale
self.int_inv_scale = int(inv_scale)
def forward(self, input):
if self.scale == 1.0:
return input
out = F.pad(input, (self.ka, self.kb, self.ka, self.kb))
out = F.conv2d(out, weight=self.weight, groups=self.groups)
out = out[:, :, ::self.int_inv_scale, ::self.int_inv_scale]
return out
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