Structural Similarity Index

Topics to be Covered

Why Image quality measure

What is Image quality measure

Types of quality assessment

MSE – Mean square error

SSIM- Structural similarity index method

VIF – Virtual information fidelity

Simulation results

Conclusion

References

2

Why Image quality?

Digital images are subject to wide variety of distortions

during transmission, acquisition, processing,

compression, storage and reproduction any of which

may result in degradation of visual quality of an image.

E.g. lossy compression technique – used to reduce

bandwidth, it may degrage the quality during

quantization process.

So the ultimate aim of data compression is to remove

the redundancy from the source signal. Therefore its

reduces the no of binary bits required to represent the

information contained within the source.

3

What is Image Quality Assessment?

Image quality is a characteristic of an image that

measures the perceived image degradation

It plays an important role in various image processing

application.

Goal of image quality assessment is to supply quality

metrics that can predict perceived image quality

automatically.

Two Types of image quality assessment

– Subjective quality assessment

– Objective quality assessment

4

Subjective Quality Measure

The best way to find quality of an image is to look at it

because human eyes are the ultimate viewer.

Subjective image quality is concerned with how image is

perceived by a viewer and give his or her opinion on a

particular image.

The mean opinion score (MOS) has been used for

subjective quality assessment from many years.

In standard subjective test where no of listeners rate the

heard audio quality of test sentences reas by both male

and female speaker over the communication medium being

tested.

Too Inconvenient, time consuming and expensive 5

Example of MOS score

The MOS is generated by avaragin the result of a set of standard, subjective tests.

MOS is an indicator of the perceived image quality.

MOS score [24]

MOS score of 1 is worst image quality and 5 is best.

Mean Opinion Score (MOS)

MOS Quality Impairment

5 Excellent Imperceptible

4 Good Perceptible but not annoying

3 Fair Slightly annoying

2 Poor Annoying

1 Bad Very annoying

6

Objective Quality Measure

Mathematical models that approximate results of

subjective quality assessment

Goal of objective evalution is to devlope quantative

measure that can predict perceived image quality

It plays variety of roles

– To monitor and control image quality for quality control

systems

– To benchmark image processing systems;

– To optimize algorithms and parameters;

– To help home users better manage their digital photos and

evaluate their expertise in photographing.

7

Objective evaluation

Three types of objective evaluation

It is classified according to the availability of an

original image with which distorted image is to

be compared

– Full reference (FR)

– No reference –Blind (NR)

– Reduced reference (RR)

8

Full reference quality metrics

MSE and PSNR: the most widely used video quality

metrics during last 20 years.

SSIM: new metric (was suggested in 2004) shows

better results, than PSNR with reasonable

computational complexity increasing.

some other metrics were also suggested by VQEG,

private companies and universities, but not so popular.

A great effort has been made to develop new objective

quality measures for image/video that incorporate

perceptual quality measures by considering the human

visual system (HVS) characteristics

9

http://en.wikipedia.org/wiki/2004

HVS – Human visual system

Quality assessment (QA) algorithms predict visual

quality by comparing a distorted signal against a

reference, typically by modeling the human visual

system.

The objective image quality assessment is based on

well defined mathematically models that can predict

perceived image quality between a distorted image and

a reference image.

These measurement methods consider human visual

system (HVS) characteristics in an attempt to

incorporate perceptual quality measures.

10

MSE – Mean square error

MSE and PSNR are defined as

(1)

(2)

Where x is the original image and y is the

distorted image. M and N are the width

and height of an image. L is the dynamic

range of the pixel values.

11

Property of MSE

If the MSE decrease to zero, the pixel-by-pixel

matching of the images becomes perfect.

If MSE is small enough, this correspond to a

high quality decompressed image.

Also in general MSE value increases as the

compression ratio increases.

12

Original “Einstein” image with different distortions, MSE value [6]

(a) Original Image MSE=0

(b) MSE=306 (c) MSE=309 (d) MSE=309

(e) MSE=313 (f) MSE=309 (g) MSE=308 13

SSIM – Structural similarity index

Recent proposed approach for image quality

assessment.

Method for measuring the similarity between

two images.Full reference metrics

Value lies between [0,1]

The SSIM is designed to improve on traditional

metrics like PSNR and MSE, which have

proved to be inconsistant with human eye

perception. Based on human visual system.

14

SSIM measurement system

Fig. 2. Structural Similarity (SSIM) Measurement System [6]

15

Example images at different quality levels and their SSIM index maps[6]

16

Equation for SSIM

If two non negative images placed together

Mean intensity (3)

Standard deviation (4)

- Estimate of signal contrast

Contrast comparison c(x,y) - difference of σx

and σy (5)

Luminance comparison (6)

C1, C2 are constant.

17

Equation for SSIM

Structure comparison is conducted s(x,y) on

these normalized signals (x- µx )/σx and(y- µy )/ σy

(7)

(8)

(9)

(10)

α, β and γ are parameters used to adjust the

relative importance of the three components.

18

Property of SSIM

Symmetry: S(x,y) = S(y,x)

Bounded ness: S(x,y)

MSE vs. MSSIM

20

MSE vs. SSIM simulation result

Type of Noise MSE MSSIM VIF

Salt & Pepper Noise 228.34 0.7237 0.3840

Spackle Noise 225.91 0.4992 0.4117

Gaussian Noise 226.80 0.4489 0.3595

Blurred 225.80 0.7136 0.2071

JPEG compressed 213.55 0.3732 0.1261

Contrast Stretch 406.87 0.9100 1.2128

21

MSE vs. MSSIM

MSE=226.80 MSSIM =0.4489 MSE = 225.91 MSSIM =0.4992

22

MSE vs. MSSIM

MSE = 213.55 MSSIM = 0.3732 MSE = 225.80 MSSIM =0.7136

23

MSE vs. MSSIM

MSE = 226.80 MSSIM = 0.4489 MSE = 406.87 MSSIM =0.910

24

Why MSE is poor?

MSE and PSNR are widely used because they are simple and easy to calculate and mathimatically easy to deal with for optimization purpose

There are a number of reasons why MSE or PSNR may not correlate well with the human perception of quality.

– Digital pixel values, on which the MSE is typically computed, may not exactly represent the light stimulus entering the eye.

– Simple error summation, like the one implemented in the MSE formulation, may be markedly different from the way the HVS and the brain arrives at an assessment of the perceived distortion.

– Two distorted image signals with the same amount of error energy may have very different structure of errors, and hence different perceptual quality. 25

Virtual Image Fidelity (VIF)

Relies on modeling of the statistical image

source, the image distortion channel and the

human visual distortion channel.

At LIVE [10], VIF was developed for image and

video quality measurement based on natural

scene statistics (NSS).

Images come from a common class: the class

of natural scene.

26

VIF – Virtual Image Fidelity

Mutual information between C and E quantifies the information that the brain could ideally extract from the reference

image, whereas the mutual information between C and F quantifies the corresponding information that could be

extracted from the test image [11].

Image quality assessment is done based on information

fidelty where the channel imposes fundamental limits on

how mauch information could flow from the source (the

referenceimage), through the channel (the image

distortion process) to the receiver (the human observer).

VIF = Distorted Image Information / Reference Image

Information

27

VIF quality

The VIF has a distinction over traditional quality

assessment methods, a linear contrast enhancement

of the reference image that does not add noise to it will

result in a VIF value larger than unity, thereby

signifying that the enhanced image has a superior

visual quality than the reference image

No other quality assessment algorithm has the ability

to predict if the visual image quality has been

enhanced by a contrast enhancement operation.

28

SSIM vs. VIF

29

VIF and SSIM

Type of Noise MSE MSSIM VIF

Salt & Pepper Noise 101.78 0.8973 0.6045

Spackle Noise 119.11 0.7054 0.5944

Gaussian Noise 65.01 0.7673 0.6004

Blurred 73.80 0.8695 0.6043

JPEG compressed 49.03 0.8558 0.5999

Contrast Stretch 334.96 0.9276 1.1192

30

VIF and SSIM

VIF = 0.6045 MSSIM = 0.8973 VIF = 0.5944 MSSIM = 0.7054

31

VIF and SSIM

VIF = 0.60 MSSIM = 0.7673 VIF = 0.6043 MSSIM = 0.8695

32

VIF and SSIM

VIF = 0.5999 MSSIM = 0.8558 VIF = 1.11 MSSIM = 0.9272 33

Simulation Result

MSE vs. SSIM – Lena.bmp

– Goldhill.bmp

– Couple.bmp

– Barbara.bmp

SSIM vs. VIF – Goldhill.bmp

– Lake.bmp

JPEG compressed image – Lena.bmp

– Tiffny.bmp

34

JPEG compressed Image- Tiffny.bmp

Quality Factor Compression Ratio MSSIM

100 0 1

1 52.79 0.3697

4 44.50 0.4285

7 33.18 0.5041

10 26.81 0.7190

15 20.65 0.7916

20 17.11 0.8158

25 14.72 0.8332

45 9.36 0.8732

60 7.68 0.8944

80 4.85 0.9295

90 3.15 0.9578

99 1.34 0.9984

35

Comparison of QF, CR and MSSIM

CR= 0 MSSIM = 1 Q.F = 1 CR= 52.79 MSSIM =0.3697

36

Comparison of QF, CR and MSSIM

Q.F = 4 CR= 44.50 MSSIM = 0.4285 Q.F = 7 CR= 33.18 MSSIM = 0.5041

37

Comparison of QF, CR and MSSIM

Q.F = 10 CR= 26.81MSSIM = 0.7190 Q.F = 15 CR= 20.65 MSSIM = 0.7916

38

Comparison of QF, CR and MSSIM

Q.F = 20 CR= 17.11 MSSIM = 0.8158 Q.F = 25 CR= 14.72 MSSIM = 0.8332

39

Comparison of QF, CR and MSSIM

40 Q.F = 45 CR= 9.36 MSSIM = 0.8732 Q.F = 80 CR= 4.85 MSSIM = 0.9295

41 Q.F = 45 CR= 3.15 MSSIM = 0.9578 Q.F = 99 CR= 1.34 MSSIM = 0.9984

Comparison of QF, CR and MSSIM

Conclusion

The main objective of this project was to

analyze SSIM Index in terms of compressed

image quality.

I explained why MSE is a poor metric for the

image quality assessment systems [1] [6].

In this project I have also tried to compare the

compressed image quality of SSIM with VIF.

By simulating MSE, SSIM and VIF I tried to

obtain results, which I showed in the previous

slides.

42

Conclusion

As shown in the simulation figure: 1, where the original “Einstein” image is altered

with different distortions, each adjusted to yield nearly identical MSE relative to the

original image. Despite this, the images can be seen to have drastically different

perceptual quality.

Only VIF has the ability to predict the visual image quality that has been enhanced

by a contrast enhancement operation.

For the JPEG compression, quality factor, compression ratio and MSSIM are

related with each other. So as quality factor increases compression ratio

decreases and so MSSIM increases.

The distortions caused by movement of the image acquisition devices, rather than

changes in the structures of objects in the visual scene. To overcome this problem

to some extent the SSIM index is extended into the complex wavelet transform

domain.

The quality prediction performance of recently developed quality measure, such as

the SSIM and VIF indices, is quite competitive relative to the traditional quality

measure.

43

References

[1] Z. Wang and A. C. Bovik, “Image quality assessment: from error visibility to structural similarity,” IEEE Trans. Image Processing, vol. 13, pp. 600 – 612, Apr. 2004.

www.ece.uwaterloo.ca/~z70wang/publications/ssim.html

[2] Z. Wang and A. C. Bovik, “Modern image quality assessment”, Morgan & Claypool Publishers,

Jan. 2006.

[3] M. Sendashonga and F Labeau, “Low complexity image quality assessment using frequency

domain transforms,” IEEE International Conference on Image Processing, pp. 385 – 388, Oct.

2006.

[4] S. S. Channappayya, A. C. Bovik, and R. W. Heath Jr, “A linear estimator optimized for the

structural similarity index and its application to image denoising,” IEEE International

Conference on Image Processing, pp. 2637 – 2640, Oct. 2006.

[5] Z. Wang and A.C. Bovik, “A universal image quality index,” IEEE signal processing letters, vol.

9, pp. 81-84, Mar. 2002.

[6] X. Shang, “Structural similarity based image quality assessment: pooling strategies and

applications to image compression and digit recognition” M.S. Thesis, EE Department, The

University of Texas at Arlington, Aug. 2006.

44

http://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.htmlhttp://www.ece.uwaterloo.ca/~z70wang/publications/ssim.html

References

[7] H. R. Sheikh and A. C. Bovik, “A visual information fidelity approach to video quality assessment,” The First International Workshop on Video Processing and Quality Metrics for

Consumer Electronics, Scottsdale, AZ, Jan. 23-25, 2005

http://live.ece.utexas.edu/publications/2005/hrs_vidqual_vpqm2005.pdf

[8] H. R. Sheikh and A. C. Bovik, “Image information and visual quality,” IEEE Trans. Image

Processing, vol. 15, pp. 430 – 444, Feb. 2006.

[9] A. Stoica, C. Vertan, and C. Fernandez-Maloigne, “Objective and subjective color image quality

evaluation for JPEG 2000- compressed images,” International Symposium on Signals, Circuits

and Systems, 2003, vol. 1, pp. 137 – 140, July 2003.

[10] H. R. Sheikh, et al, “Image and video quality assessment research at LIVE,”

http://live.ece.utexas.edu/research/quality/.

[11] A. C. Bovik and H. R. Sheikh, “Image information and visual quality- a visual information

fidelity measure for image quality assessment,”

http://live.ece.utexas.edu/research/Quality/VIF.htm.

[12] H. R. Wu and K. R. Rao, “Digital video image quality and perceptual coding,” Boca Raton,

FL: Taylor and Francis 2006.

[13] A. M. Eskicioglu and P. S. Fisher, “Image quality measure and their performance,” IEEE signal

processing letters, vol. 43, pp. 2959-2965, Dec. 1995.

45

http://live.ece.utexas.edu/publications/2005/hrs_vidqual_vpqm2005.pdfhttp://live.ece.utexas.edu/research/quality/http://live.ece.utexas.edu/research/Quality/VIF.htm

References

[14] Z. Wang, H. R. Sheikh and A. C. Bovik, “Objective video quality assessment”, Chapter 41 in The handbook of video databases: design and applications, B. Furht and O. Marqure, ed., CRC Press, pp. 1041-1078, September 2003. http://www.cns.nyu.edu/~zwang/files/papers/QA_hvd_bookchapter.pdf

[15] Z. Wang, A. C. Bovik and Ligang Lu , “Why is image quality assessment so difficult", IEEE International Conference on Acoustics, Speech, and Signal Processing, Proceedings. (ICASSP '02), vol. 4, pp. IV-3313 - IV-3316, May 2002.

[16] T. S. Branda and M. P. Queluza, “No-reference image quality assessment based on DCT domain statistics” Signal Processing, vol. 88, pp. 822-833, April 2008.

[17] B. Shrestha, C. G. O’Hara and N. H. Younan, “JPEG2000: Image quality metrics”

[18] http://media.wiley.com/product_data/excerpt/99/04705184/0470518499.pdf

[19] http://en.wikipedia.org/wiki/Subjective_video_quality

[20] H. R. Sheikh, A. C. Bovik, and G. de Veciana, "An Information Fidelity Criterion for Image Quality Assessment Using Natural Scene Statistics," IEEE Transactions on Image Processing, in Publication, May 2005.

[21] http://www.cns.nyu.edu/~zwang/files/research/quality_index/demo_lena.html

[22] http://live.ece.utexas.edu/research/Quality/vif.htm

[23] http://www.ece.uwaterloo.ca/~z70wang/research/ssim/

[24] http://en.wikipedia.org/wiki/Mean_Opinion_Score

[25] www-ee.uta.edu/dip

46

http://www.cns.nyu.edu/~zwang/files/papers/QA_hvd_bookchapter.pdfhttp://en.wikipedia.org/wiki/Subjective_video_qualityhttp://www.cns.nyu.edu/~zwang/files/research/quality_index/demo_lena.htmlhttp://live.ece.utexas.edu/research/Quality/vif.htmhttp://www.ece.uwaterloo.ca/~z70wang/research/ssim/http://en.wikipedia.org/wiki/Mean_Opinion_Score

Thank You

47