1
Making a ‘Completely Blind’ Image Quality
Analyzer
Anish Mittal, Rajiv Soundararajan and Alan C. Bovik, Fellow, IEEE
Abstract—An important aim of research on the blind image
quality assessment (IQA) problem is to devise perceptual models
that can predict the quality of distorted images with as little
prior knowledge of the images or their distortions as possible.
Current state-of-the-art ‘general purpose’ no reference (NR) IQA
algorithms require knowledge about anticipated distortions in the
form of training examples and corresponding human opinion
scores. However we have recently derived a blind IQA model
that only makes use of measurable deviations from statistical
regularities observed in natural images, without training on
human-rated distorted images, and, indeed without any expo-
sure to distorted images. Thus, it is ‘completely blind.’ The
new IQA model, which we call the Natural Image Quality
Evaluator (NIQE) is based on the construction of a ‘quality
aware’ collection of statistical features based on a simple and
successful space domain natural scene statistic (NSS) model.
These features are derived from a corpus of natural, undis-
torted images. Experimental results show that the new index
delivers performance comparable to top performing NR IQA
models that require training on large databases of human
opinions of distorted images. A software release is available
at:http://live.ece.utexas.edu/research/quality/niqe
release.zip.
Index Terms—Completely blind, distortion free, no reference,
image quality assessment
I. INTRODUCTION
Americans captured 80 billion digital photographs in 2011
and this number is increasing annually [1]. More than 250
million photographs are being posted daily on facebook. Con-
sumers are drowning in digital visual content and finding ways
to review and control of the quality of digital photographs is
becoming quite challenging.
At the same time, camera manufacturers continue to provide
improvements in photographic quality and resolution. The raw
captured images pass through multiple post processing steps
in the camera pipeline, each requiring parameter tuning. A
problem of great interest is to find ways to automatically
evaluate and control the perceptual quality of the visual content
as a function of these multiple parameters.
Objective image quality assessment refers to automatically
predict the quality of distorted images as would be perceived
by an average human. If a naturalistic reference image is
supplied against which the quality of the distorted image can
be compared, the model is called full reference (FR) [2].
Copyright (c) 2012 I EEE. Personal use of this material is permitted.
However, permission to use this material for any other purposes must be
obtained from the IEEE by sending a request to pubs-permissions@ieee.org.
A. Mittal and A.C. Bovik are with the Laboratory for Image and Video
Engineering (LIVE), The University of Texas at Austin, Texas, USA. R.
Soundararajan was with the University of Texas at Austin while most of this
work was done. He is currently with Qualcomm Research India, Bangalore.
Corresponding authors email address: mittal.anish@gmail.com.
Conversely, NR IQA models assume that only the distorted
image whose quality is being assessed is available. Existing
general purpose NR IQA algorithms are based on models that
can learn to predict human judgments of image quality from
databases of human-rated distorted images [3], [4], [5], [6],
[7]. These kinds of IQA models are necessarily limited, since
they can only assess quality degradations arising from the
distortion types that they have been trained on.
However, it is also possible to contemplate subcategories
of general-purpose NR IQA models having tighter conditions.
A model is ‘opinion-aware’ (OA) if it has been trained on
a database(s) of human rated distorted images and associated
subjective opinion scores. Thus algorithms like DIIVINE [4],
CBIQ [6], LBIQ [7], BLIINDS [5] and BRISQUE [3] are OA
IQA models. Given the impracticality of obtaining collections
of distorted images with co-registered human scores, models
that do not require training on databases of human judgments
of distorted images, and hence are ‘opinion unaware’ (OU), are
of great interest. One such effort was made in this direction by
the authors of [8]. However, their model requires knowledge
of the expected image distortions.
Likewise, among algorithms derived from OU models, dis-
torted images may or may not be available during IQA model
creation or training. For example, in highly unconstrained
environments, such as a photograph upload site, the a priori
nature of distortions may be very difficult to know. Thus
a model may be formulated as ‘distortion aware’ (DA) by
training on (and hence tuning to) specific distortions, or it
may be ‘distortion unaware’ (DU), relying instead only on
exposure to naturalistic source images or image models to
guide the QA process. While this may seem as an extreme
paucity of information to guide design, it is worth observing
that very successful FR IQA models (such as the structural
similarity index (SSIM) [9]) are DU.
Our contribution in this direction is the development of a
NSS-based modeling framework for OU-DU NR IQA design,
resulting in a first of a kind NSS-driven blind OU-DU IQA
model which does not require exposure to distorted images
a priori, nor any training on human opinion scores. The
new NR OU-DU IQA quality index performs better than the
popular FR peak signal-to-noise-ratio (PSNR) and structural
similarity (SSIM) index and delivers performance at par with
top performing NR OA-DA IQA approaches.
II. NO REFERENCE OPINION-UNAWARE
DISTORTION-UNAWARE IQA MODEL
Our new NR OU-DU IQA model is based on constructing
a collection of ‘quality aware’ features and fitting them to
2
(a) (b)
Fig. 1. The marked blocks in the images (a) and (b) depict instances of
natural image patches selected using a local sharpness measure.
a multivariate Gaussian (MVG) model. The quality aware
features are derived from a simple but highly regular natural
scene statistic (NSS) model.The quality of a given test image is
then expressed as the distance between a multivariate Gaussian
(MVG) fit of the NSS features extracted from the t est image,
and a MVG model of the quality aware features extracted from
the corpus of natural images.
A. Spatial Domain NSS
Our ‘completely blind’ IQA model is founded on percep-
tually relevant spatial domain NSS features extracted from
local image patches that effectively capture the essential low-
order statistics of natural images. The classical spatial NSS
model [10] that we use begins by preprocessing the i mage by
processes of local mean removal and divisive normalization:
ˆ
I(i, j) =
I(i, j) − µ(i, j)
σ(i, j) + 1
(1)
where i ∈ {1, 2 . . . M }, j ∈ {1, 2 . . . N} are spatial indices,
M and N are the image dimensions, and
µ(i, j) =
K
X
k=−K
L
X
l=−L
w
k,l
I(i + k, j + l) (2)
σ(i, j) =
v
u
u
t
K
X
k=−K
L
X
l=−L
w
k,l
[I(i + k, j + l) − µ(i, j)]
2
(3)
estimate the local mean and contrast, respectively, where
w = {w
k,l
|k = −K, . . . , K, l = − L, . . . L} is a 2D circularly-
symmetric Gaussian weighting function sampled out t o 3
standard deviations (K = L = 3) and rescaled to unit volume.
The coefficients (1) have been observed to reliably follow a
Gaussian distribution when computed from natural images that
have suffered little or no apparent distortion [10]. This ideal
model, however, is violated when the images do not derive
from a natural source (e.g. computer graphics) or when natural
images are subjected to unnatural distortions. The degree of
modification can be indicative of perceptual distortion severity.
The NSS features used in the NIQE index are similar to
those used in a prior OA-DA IQA model called BRISQUE
[3]. However, NIQE only uses the NSS features from a corpus
of natural images while BRISQUE is trained on features
obtained from both natural and distorted images and also on
human judgments of the quality of these images. Therefore,
BRISQUE is limited to the types of distortions it has been
tuned to. By comparison, the NIQE Index is not tied to
any specific distortion type, yet, as will be shown, delivers
nearly comparable predictive power on the same distortions
the BRISQUE index has been trained on, with a similar low
complexity.
B. Patch Selection
Once the image coefficients (1) are computed, the image is
partitioned into P ×P patches. Specific NSS features are then
computed from the coefficients of each patch. However, only
a subset of the patches are used for the following reason.
Every image is subject to some kind of limiting distortion
[11]. For instance, there is a loss of resolution due to defocus
blur in parts of most images due to the limited depth of field
(DOF) of any single-lens camera. Since humans appear to
more heavily weight their judgments of image quality from the
sharp image regions [12], more salient quality measurements
can be made from sharp patches. Setting aside the question of
the aesthetic appeal of having some parts of an image sharper
than others, any defocus blur represents a potential loss of
visual information.
We use a simple device to preferentially select from amongst
a collection of natural patches those that are richest in infor-
mation and less likely to have been subjected to a limiting
distortion. This subset of patches is then used to construct a
model of the statistics of natural image patches.
The variance field (3) has been largely ignored in the past in
NSS based image analysis, but it is a rich source of structural
image information that can be used to quantify local image
sharpness. Letting the P × P sized patches be indexed b =
1, 2, .., B, a direct approach is to compute the average local
deviation field of each patch indexed b:
δ(b) =
XX
(i,j)∈patchb
σ(i, j) (4)
where δ denotes local activity/sharpness.
Once the sharpness of each patch is found, those having a
suprathreshold sharpness δ > T are selected. The threshold T
is picked to be a fraction p of the peak patch sharpness over
the image. In our experiments, we used the nominal value p
= 0.75. Examples of this kind of patch selection are shown in
Fig. 1. We have observed only small variations in performance
when p is varied in the range [0.6, 0.9].
C. Characterizing Image Patches
Given a collection of natural image patches selected as
above, their statistics are characterized by ‘quality aware’ NSS
features computed from each selected patch [3]. Prior studies
of NSS based image quality have shown that the generalized
Gaussian distribution effectively captures the behavior of the
coefficients (1) of natural and distorted versions of them [13].
The generalized Gaussian distribution (GGD) with zero
mean is given by:
f(x; α, β) =
α
2βΓ(1/α)
exp
−
|x|
β
α
(5)
3
where Γ(·) is the gamma function:
Γ(a) =
Z
∞
0
t
a−1
e
−t
dt a > 0. (6)
The parameters of the GGD (α, β), can be reliably estimated
using the moment-matching based approach proposed in [14].
The signs of the transformed image coefficients (1) have
been observed to follow a fairly regular structure. However,
distortions disturb this correlation structure [3]. This deviation
can be captured by analyzing the sample distribution of the
products of pairs of adjacent coefficients computed along
horizontal, vertical and diagonal orientations:
ˆ
I(i, j)
ˆ
I(i, j+1),
ˆ
I(i, j)
ˆ
I(i+1, j),
ˆ
I(i, j)
ˆ
I(i+1, j +1) and
ˆ
I(i, j)
ˆ
I(i+1, j −1)
for i ∈ {1, 2 . . . M} and j ∈ {1, 2 . . . N } [3].
The products of neighboring coefficients are well-modeled
as following a zero mode asymmetric generalized Gaussian
distribution (AGGD) [15]:
f(x; γ, β
l
, β
r
) =
γ
(β
l
+β
r
)Γ
(
1
γ
)
exp
−
−x
β
l
γ
∀x ≤ 0
γ
(β
l
+β
r
)Γ
(
1
γ
)
exp
−
x
β
r
γ
∀x ≥ 0.
(7)
The parameters of the AGGD (γ, β
l
, β
r
) can be efficiently
estimated using the moment-matching based approach in [15].
The mean of the distribution is also useful:
η = (β
r
− β
l
)
Γ(
2
γ
)
Γ(
1
γ
)
. (8)
By extracting estimates along the four orientations, 16 pa-
rameters are arrived at yielding 18 overall. All features are
computed at two scales to capture multiscale behavior, by low
pass filtering and downsampling by a factor of 2, yielding a
set of 36 features.
D. Multivariate Gaussian Model
A simple model of the NSS features computed from natural
image patches can be obtained by fitting them with an MVG
density, providing a ri ch representation of them:
f
X
(x
1
, . . . , x
k
) =
1
(2π)
k/2
|Σ|
1/2
exp(−
1
2
(x−ν)
T
Σ
−1
(x−ν))
(9)
where (x
1
, . . . , x
k
) are the NSS features computed in (5)-
(8), and ν and Σ denote the mean and covariance matrix
of the MVG model, which are estimated using a stan-
dard maximum likelihood estimation procedure [16]. We se-
lected a varied set of 125 natural images with si zes rang-
ing from 480 × 320 to 1280 × 720 to obtain the multi-
variate Gaussian model. Images were s elected from copy-
right free Flickr data and from the Berkeley image seg-
mentation database [17] making sure that no overlap occurs
with the test image content. The images may be viewed at
http://live.ece.utexas.edu/research/quality/pristinedata.zip.
TABLE I
MEDIAN SPEARMAN RANK ORDERED CORRELATION COEFFICIENT
(SROCC) A CROSS 1000 TRAIN-TEST COMBINATIONS ON THE LIVE IQA
DATABASE. Italics INDICATE (OA/OU)-DA NO-REFERENCE ALGORITHMS
AND bold face INDICAT ES THE NEW OU-DU MODEL ALGORITHM.
JP2K JPEG WN Blur FF All
PSNR 0.8646 0.8831 0.9410 0.7515 0.8736 0.8636
SSIM 0.9389 0.9466 0.9635 0.9046 0.9393 0.9129
MS-SSIM 0.9627 0.9785 0.9773 0.9542 0.9386 0.9535
CBIQ 0.8935 0.9418 0.9582 0.9324 0.8727 0.8954
LBIQ 0.9040 0.9291 0.9702 0.8983 0.8222 0.9063
BLIINDS-II 0.9323 0.9331 0.9463 0.8912 0.8519 0.9124
DIIVINE 0.9123 0.9208 0.9818 0.9373 0.8694 0.9250
BRISQUE 0.9139 0.9647 0.9786 0.9511 0.8768 0.9395
TMIQ 0.8412 0.8734 0.8445 0.8712 0.7656 0.8010
NIQE 0.9172 0.9382 0.9662 0.9341 0.8594 0.9135
E. NIQE Index
The new OU-DU IQA index, called NIQE, is applied by
computing the 36 identical NSS features from patches of the
same size P ×P from the image to be quality analyzed, fitting
them with the MVG model (9), then comparing its MVG fit
to the natural MVG model. The sharpness criterion (4) is not
applied to these patches because loss of sharpness in distorted
images is indicative of distortion and neglecting them would
lead to incorrect evaluation of the distortion severity. The patch
size was set to 96 × 96 in our implementation. However, we
observed stable performance across patch sizes ranging from
32 × 32 to 160 × 160.
Finally, the quality of the distorted image is expressed as
the distance between the quality aware NSS feature model and
the MVG fit to the features extracted from the distorted image:
D(ν
1
, ν
2
, Σ
1
, Σ
2
) =
v
u
u
t
(ν
1
− ν
2
)
T
Σ
1
+ Σ
2
2
−1
(ν
1
− ν
2
)
!
(10)
where ν
1
, ν
2
and Σ
1
, Σ
2
are the mean vectors and covariance
matrices of the natural MVG model and the distorted image’s
MVG model.
III. PERFORMANCE EVALUAT ION
A. Correlation with Human Judgments of Visual Quality
To test the performance of the NIQE index, we used the
LIVE IQA database [2] of 29 reference images and 779
distorted images spanning five different distortion categories
– JPEG and JPEG2000 (JP2K) compression, additive white
Gaussian noise (WN), Gaussian blur (blur) and a Rayleigh
fast fading channel distortion (FF). A difference mean opinion
score (DMOS) associated with each image represents its
subjective quality.
Since all of the OA IQA approaches that we compare
NIQE to require a training procedure to calibrate the regressor
module, we divided the LIVE database randomly into chosen
subsets for training and testing. Although our blind approach
and the FR approaches do not require this procedure, to
ensure a fair comparison across methods, the correlations of
predicted scores with human judgments of visual quality are
only reported on the test set. The dataset was divided into 80%
training and 20% testing – taking care that no overlap occurs
between train and test content. This train-test procedure was
4
Fig. 2. Variation of performance with the number of natural images K . Error
schmears around each point indicate the standard deviation in performance
across 100 iterations for 5 < K < 125.
TABLE II
MEDIAN LINEAR CORRELATION COEFFICIENT ACROSS 1000 TRAIN-TEST
COMBINATIONS ON THE LIVE IQA DATAB ASE. Italics INDICATE
(OA/OU)-DA NO-REFERENCE ALGORITHMS AND bold face INDICATES
THE NEW OU-DU MODEL ALGORITHM.
JP2K JPEG WN Blur FF All
PSNR 0.8762 0.9029 0.9173 0.7801 0.8795 0.8592
SSIM 0.9405 0.9462 0.9824 0.9004 0.9514 0.9066
MS-SSIM 0.9746 0.9793 0.9883 0.9645 0.9488 0.9511
CBIQ 0.8898 0.9454 0.9533 0.9338 0.8951 0.8955
LBIQ 0.9103 0.9345 0.9761 0.9104 0.8382 0.9087
BLIINDS-II 0.9386 0.9426 0.9635 0.8994 0.8790 0.9164
DIIVINE 0.9233 0.9347 0.9867 0.9370 0.8916 0.9270
BRISQUE 0.9229 0.9734 0.9851 0.9506 0.9030 0.9424
TMIQ 0.8730 0.8941 0.8816 0.8530 0.8234 0.7856
NIQE 0.9370 0.9564 0.9773 0.9525 0.9128 0.9147
repeated 1000 times to ensure that there was no bias due to
the spatial content used for training. We report the median
performance across all iterations.
We use Spearman’s rank ordered correlation coefficient
(SROCC), and Pearson’s (linear) correlation coefficient (LCC)
to test the model. The NIQE scores are passed through a
logistic non-linearity [2] before computing LCC for mapping
to DMOS space. We compared NIQE with three FR indices:
PSNR, SSIM [9] and multiscale SSIM (MS-SSIM) [18], five
general purpose OA-DA algorithms - CBIQ [6], LBIQ [7],
BLIINDS-II [5], DIIVINE [4], BRISQUE [3] and the DA-OU
approach TMIQ [8].
As can be seen from Tables I and II, NIQE performs better
than the FR PSNR and SSIM and competes well with all of
the top performing OA-DA NR IQA algorithms. This is a
fairly remarkable demonstration of the relationship between
quantified image naturalness and perceptual image quality.
B. Number of Natural Images
We addressed the question: ‘How many natural images are
needed to obtain a stable model that can correctly predict
image quality?’ Such an analysis provides an idea of the
quality prediction power of the NSS features and how well
they generalize with respect to image content.
To undertake this evaluation, we varied the number of
natural images K from which patches are selected and used
for model fitting. Figure 2 shows the performance against the
number of images. An error band is drawn around each point
to indicate the standard deviation in performance across 100
iterations of different sample sets of K images. It may be
observed that a stable natural model can be obtained using a
small set of images.
IV. CONCLUSION
We have created a first of a kind blind IQA model that as-
sesses image quality without knowledge of anticipated distor-
tions or human opinions of them. The quality of the distorted
image is expressed as a simple distance metric between the
model statistics and those of the distorted image. The new
model outperforms FR IQA models and competes with top
performing NR IQA trained on human judgments of known
distorted images. Such a model has great potential to be
applied in unconstained environments.
ACKNOWLEDGMENT
This research was supported by Intel and Cisco corpora-
tion under the VAWN program and by the National Science
Foundation under grants CCF-0728748 and IIS-1116656.
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