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mixture.py
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mixture.py
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# coding: utf-8
# Code is adapted from:
# https://github.com/pclucas14/pixel-cnn-pp
# https://github.com/openai/pixel-cnn
from __future__ import with_statement, print_function, absolute_import
import math
import numpy as np
import torch
from torch import nn
from torch.autograd import Variable
from torch.nn import functional as F
def log_sum_exp(x):
""" numerically stable log_sum_exp implementation that prevents overflow """
# TF ordering
axis = len(x.size()) - 1
m, _ = torch.max(x, dim=axis)
m2, _ = torch.max(x, dim=axis, keepdim=True)
return m + torch.log(torch.sum(torch.exp(x - m2), dim=axis))
def discretized_mix_logistic_loss(y_hat, y, num_classes=256,
log_scale_min=-7.0, reduce=True):
"""Discretized mixture of logistic distributions loss
Note that it is assumed that input is scaled to [-1, 1].
Args:
y_hat (Variable): Predicted output (B x C x T)
y (Variable): Target (B x T x 1).
num_classes (int): Number of classes
log_scale_min (float): Log scale minimum value
reduce (bool): If True, the losses are averaged or summed for each
minibatch.
Returns
Variable: loss
"""
assert y_hat.dim() == 3
assert y_hat.size(1) % 3 == 0
nr_mix = y_hat.size(1) // 3
# (B x T x C)
y_hat = y_hat.transpose(1, 2)
# unpack parameters. (B, T, num_mixtures) x 3
logit_probs = y_hat[:, :, :nr_mix]
means = y_hat[:, :, nr_mix:2 * nr_mix]
log_scales = torch.clamp(y_hat[:, :, 2 * nr_mix:3 * nr_mix], min=log_scale_min)
# B x T x 1 -> B x T x num_mixtures
y = y.expand_as(means)
centered_y = y - means
inv_stdv = torch.exp(-log_scales)
plus_in = inv_stdv * (centered_y + 1. / (num_classes - 1))
cdf_plus = F.sigmoid(plus_in)
min_in = inv_stdv * (centered_y - 1. / (num_classes - 1))
cdf_min = F.sigmoid(min_in)
# log probability for edge case of 0 (before scaling)
# equivalent: torch.log(F.sigmoid(plus_in))
log_cdf_plus = plus_in - F.softplus(plus_in)
# log probability for edge case of 255 (before scaling)
# equivalent: (1 - F.sigmoid(min_in)).log()
log_one_minus_cdf_min = -F.softplus(min_in)
# probability for all other cases
cdf_delta = cdf_plus - cdf_min
mid_in = inv_stdv * centered_y
# log probability in the center of the bin, to be used in extreme cases
# (not actually used in our code)
log_pdf_mid = mid_in - log_scales - 2. * F.softplus(mid_in)
# tf equivalent
"""
log_probs = tf.where(x < -0.999, log_cdf_plus,
tf.where(x > 0.999, log_one_minus_cdf_min,
tf.where(cdf_delta > 1e-5,
tf.log(tf.maximum(cdf_delta, 1e-12)),
log_pdf_mid - np.log(127.5))))
"""
# TODO: cdf_delta <= 1e-5 actually can happen. How can we choose the value
# for num_classes=65536 case? 1e-7? not sure..
inner_inner_cond = (cdf_delta > 1e-5).float()
inner_inner_out = inner_inner_cond * \
torch.log(torch.clamp(cdf_delta, min=1e-12)) + \
(1. - inner_inner_cond) * (log_pdf_mid - np.log((num_classes - 1) / 2))
inner_cond = (y > 0.999).float()
inner_out = inner_cond * log_one_minus_cdf_min + (1. - inner_cond) * inner_inner_out
cond = (y < -0.999).float()
log_probs = cond * log_cdf_plus + (1. - cond) * inner_out
log_probs = torch.sum(log_probs, dim=-1, keepdim=True) + F.log_softmax(logit_probs, -1)
if reduce:
return -torch.sum(log_sum_exp(log_probs))
else:
return -log_sum_exp(log_probs).unsqueeze(-1)
def to_one_hot(tensor, n, fill_with=1.):
# we perform one hot encore with respect to the last axis
one_hot = torch.FloatTensor(tensor.size() + (n,)).zero_()
if tensor.is_cuda:
one_hot = one_hot.cuda()
one_hot.scatter_(len(tensor.size()), tensor.unsqueeze(-1), fill_with)
return Variable(one_hot)
def sample_from_discretized_mix_logistic(y, log_scale_min=-7.0):
"""
Sample from discretized mixture of logistic distributions
Args:
y (Variable): B x C x T
log_scale_min (float): Log scale minimum value
Returns:
Variable: sample in range of [-1, 1].
"""
assert y.size(1) % 3 == 0
nr_mix = y.size(1) // 3
# B x T x C
y = y.transpose(1, 2)
logit_probs = y[:, :, :nr_mix]
# sample mixture indicator from softmax
temp = logit_probs.data.new(logit_probs.size()).uniform_(1e-5, 1.0 - 1e-5)
temp = logit_probs.data - torch.log(- torch.log(temp))
_, argmax = temp.max(dim=-1)
# (B, T) -> (B, T, nr_mix)
one_hot = to_one_hot(argmax, nr_mix)
# select logistic parameters
means = torch.sum(y[:, :, nr_mix:2 * nr_mix] * one_hot, dim=-1)
log_scales = torch.clamp(torch.sum(
y[:, :, 2 * nr_mix:3 * nr_mix] * one_hot, dim=-1), min=log_scale_min)
# sample from logistic & clip to interval
# we don't actually round to the nearest 8bit value when sampling
u = Variable(means.data.new(means.size()).uniform_(1e-5, 1.0 - 1e-5))
x = means + torch.exp(log_scales) * (torch.log(u) - torch.log(1. - u))
x = torch.clamp(torch.clamp(x, min=-1.), max=1.)
return x