
Sampling from DDPM Gradient Correction Step
(a)
(b)
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p✓(xt1|xt)
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q(xt|xt1)
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xT
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xt1
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x0
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zt1⇠p✓(zt1|xt)
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xt1
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xT
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x0
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zt1@g(zt1)
Figure 1: (a) Illustration of the denoising diffusion probabilistic model (DDPM) Markov chain that
relates the distribution of high-quality images with that of the Gaussian noise. (b) DOLPH uses a
pre-trained DDPM for recovering images from amplitude measurements. DOLPH can be viewed as a
modified reverse diffusion process that has a gradient correction step to ensure consistency with the
measured amplitudes.
is also worth noting the connection between use of diffusion models in inverse problems and the
plug-and-play priors (PnP) framework, which also uses image denoisers as image priors [19–21].
In this work, we present
d
iffusion m
o
de
l
s for
ph
ase retrieval (DOLPH) as a new method for using
diffusion models as image priors for solving PR problems of form
(1)
. DOLPH combines the
sampling procedure in diffusion models with the data-consistency updates ensuring that the predicted
amplitudes match the measured ones in
y
. Our numerical results on phase retrieval from coded
diffraction patters (CDP) [22] show that DOLPH can generate realistic looking images from severely
noisy measurements, where the traditional approaches lead to overly smooth images.
2 Proposed Method
DOLPH builds on the sampling process of diffusion models to make it applicable to the problem
in eq.
(1)
. The sampling process uses a denoising convolutional neural network (CNN) pre-
trained using the denoising diffusion probabilistic models (DDPM) framework [13, 15]. DOLPH then
uses the pre-trained CNN denoiser to generate samples consistent with a given set of amplitude
measurements.
2.1 Denoising diffusion probabilistic models
As illustrated in Figure 1(a), DDPM consists of two Markov processes: the fixed forward process and
the learning-based reverse process. The forward process consists of
T≥1
steps, where each step
adds a Gaussian noise with a pre-designated variance, so that the at step
T
the statistical distribution
of data corresponds to the standard Gaussian distribution. Each step of the forward process can be
expressed as
q(xt|xt−1):=N(xt;p1−βtxt−1, βtI),(2)
where
N(x;µ,Σ)
denotes a Gaussian probability density function with the mean vector
µ
and the
covariance matrix
Σ
. The vectors
x1,··· ,xT
are the latent variables that have the same dimensions
2