# Image quality assessment with lasso regression and pairwise score differences

## Abstract

The reception of multimedia applications often depends on the quality of processed and displayed visual content. This is the main reason for the development of automatic image quality assessment (IQA) techniques which try to mimic properties of human visual system and produce objective scores for evaluated images. Most of them require a training step in which subjective scores, obtained in tests with human subjects, are used for parameters tuning. In this paper, it is shown that pairwise score differences (PSD) can be successfully used for training a full-reference hybrid IQA measure based on the least absolute shrinkage and selection operator (lasso) regression. The results of extensive experimental evaluation on four largest IQA benchmarks show that the proposed IQA technique is statistically better than its version trained using raw scores, and both approaches are statistically better than state-of-the-art full-reference IQA measures. They are also better than other hybrid approaches. In the paper, the evaluation protocol is extended with tests using PSD.

## Keywords

Image quality assessment Full-reference Lasso regression Evaluation## 1 Introduction

With the rapid development of different image processing methods, a considerable attention is given to techniques which are trying to mimic human visual perception [9, 46]. Image quality assessment (IQA) techniques measure the quality of presented images, and often support, or compare, image enhancement, restoration, or denoising techniques [3, 5, 29, 32]. According to the availability of a reference image, IQA measures are divided into full-reference, reduced-reference, and no-reference techniques [4, 19, 31, 47].

This paper focuses on the full-reference IQA measures. Over the last decade, many different full-reference IQA approaches have been introduced [4], starting from simple peak signal-to-noise ratio (PSNR), or noise quality measure (NQM) [8] in which a linear frequency distortion and an additive noise injection are used for the distorted image modelling. The popular structural similarity (SSIM) measure [41], being the extension of universal image quality index (UQI) [39], uses loss of correlation, luminance distortion, and contrast distortion. SSIM was further extended using a multi-scale approach (MSSIM) [41] or statistical models of natural scenes, as it can be seen in information content weighted SSIM (IWSSIM) [40]. Such statistical models are also utilised in information fidelity criterion (IFC) [33] and visual information fidelity (VIF) [34]. Feature similarity index (FSIM) [51] and its version for colour images (FSIMc), employ phase congruency and image gradient magnitude. In Riesz-transform based feature similarity measure (RFSIM) [50] Riesz-transform features at key locations between the distorted image and its reference image are used. SURF-SIM [38], in turn, uses Speed Up Robust Features (SURF) in order to detect multiscale differences between features. Spectral residual based similarity measure (SRSIM) [49] and visual saliency-induced index (VSI) [52] use visual saliency maps. Contrast changes and luminance distortions are used in gradient similarity (GSM) measure [16], and inter-patch and intra-patch similarities were modelled in [54] using modified normalised correlation coefficient and image curvature. The edge based image quality assessments (EBIQA) measure is based on different edge features extracted from distorted image and its pristine equivalent [1]. In [11], a novel pooling strategy based on the harmonic mean was proposed.

In the literature, there are also approaches in which several IQA techniques are combined into a hybrid, or a fusion, measure. For example, in the most apparent distortion algorithm (MAD) [14] local luminance and contrast masking evaluate high-quality images, while changes in the local statistics of spatial-frequency components are used for images with a low quality. Information obtained using saliency maps, gradient and contrast information was fused in [30]. In [21, 22], scores of MSSIM, VIF and R-SVD were non-linearly combined. A preliminary work with non-linear combination of several IQA measures selected by a genetic algorithm was shown in [23]. In [17], SNR, SSIM, VIF, and VSNR were combined using canonical correlation analysis, and a regularised regression was used to combine up to seven IQA models in [13]. In [25], a support vector machines classifier used for predicting of the distortion followed by a fusion of SSIM, VSNR, and VIF using k-nearest-neighbour regression, was proposed. An adaptive combination of IQA approaches with an edge-quality based on preservation of edge direction was introduced in [26]. In [18], a fusion measure using a support vector regression approach was proposed. Lukin et al. [20] introduced a fusion measure which combines six IQA measures using a neural network. In [48], kernel ridge regression was used to combine found perceptually meaningful structures and local distortion measurements. In other approaches, adaptive weighting [2] or internal generative mechanism [43] were considered in order to obtain hybrid measures.

For evaluation of IQA approaches, specific IQA benchmark databases have been introduced [14, 27, 28, 42]. They contain pristine images, their distorted equivalents and subjective human evaluations in the form of mean opinion scores (MOS) or differential MOS (DMOS). Some images from these benchmarks with subjective scores are often used for tuning parameters of many developed methods, e.g., [13, 20, 44, 52, 54]. Here, the number of used images should be small in order to obtain the benchmark-independent solution. In this paper, a novel full-reference hybrid IQA measure is proposed which employs regularized least-squares regression using the least absolute shrinkage and selection operator (lasso) [36, 37]. This technique combines objective scores produced by up to 16 full-reference IQA measures. The lasso regression was applied since it performs selection of the most important predictors, what makes the usage of such combined measure more practical. Finally, only several IQA measures take part in the fusion. In the proposed approach, the regression coefficients are determined using part of images and their scores from benchmark databases. It is shown that the proposed hybrid measure is significantly better if pairwise score differences (PSD) are used instead of raw scores. These differences can be also used for performance evaluation of IQA measures. The application of PSD can be motivated by the organisation of some IQA tests with human subjects [28], where the observer compares distorted images with each other, taking into account the pristine image. It can be assumed that PSD can also be used in the development of IQA measures that require supervised learning. The hybrid measures developed using raw scores or PSD in the lasso regression are compared with the state-of-the-art techniques on four largest IQA benchmark image datasets using well-established evaluation protocol, as well as statistical significance tests.

The rest of this paper is organised as follows. Section 2 presents the proposed hybrid IQA measure. In Section 3, the approach is compared with state-of-the-art measures using four IQA benchmarks, and, finally concluding remarks are presented in Section 4.

## 2 Proposed approach

*Q*

_{1}, …,

*Q*

_{ M }be the objective scores of

*M*IQA measures seen as predictor variables in multiple linear regression model [36]. In the model,

**S**^{ o }is the estimated response, or objective score, of the resulted hybrid IQA measure. It can be written as follows:

*contains fitted coefficients estimated by minimising the mean squared difference between the outcome, i.e., the vector of subjective scores*

**B**

**S**^{ s }, and predicted outcome,

**S**^{ o };

*𝜖*represents a relationship between

*and*

**Q**

**S**^{ s }which is rejected from the equation.

*λ*, the lasso determines

*solving the following optimisation problem:*

**B***N*is the number of objective scores, and

*λ*is a regularization parameter. In other words, the lasso minimises the residual sum of squares with the constant

*α*:

In the approach, *λ* value which minimises the mean squared error was used to determine coefficients.

In experiments, the following *M*=16 publicly available full-reference IQA measures were used: VSI [52], FSIM [51], FSIMc [51], GSM [16], IFC [33], IW-SSIM [40], MAD [14], MSSIM [41], NQM [8], PSNR [35], RFSIM [50], SR-SIM [49], SSIM [42], VIF [34], IFS [7], and SFF [6]. They were used for assessment of processed images and then PSD were obtained. Most of these approaches present state-of-the-art performance, and their inclusion was mainly influenced by the need of achieving a broad sample of various approaches mimicking human visual system. It is assumed that the lasso regression would be able to select several IQA measures and develop the well-performing hybrid measure.

The proposed approach uses first 20 % images and their subjective scores from a given benchmark dataset, in order to obtain the regression coefficients. There are four largest IQA benchmark image datasets; therefore, four hybrid measures are introduced. In the literature, different numbers of images with scores were used for this purpose, ranging from 20 % [38], through 30 % [52], 50 % [48], and 100 % [13, 26, 44], to several datasets jointly [54].

*M*=16 IQA measures, as well as trained with pairwise differences of these scores. The obtained fitted coefficients,

*, indicate the number and contribution of the most informative IQA measures. Only these measures were used in the quality assessment of the test images. For a selected reference image, all score differences between its distorted equivalents are calculated. For example, for 5 images with 24 distortions and 5 distortion levels, 600 images and scores are available in the typical learning scenario, or, as it is introduced in this paper, \({\sum }_{k=1}^{5} \displaystyle {120 \choose 2} = 7140\) pairwise score differences for these images can be used. It is assumed that only scores of distorted images that have the same reference image are compared. The usage of PSD can be also motivated by the tristimulus methodology for performing tests with human observers [28], in which two distorted images are presented with their pristine equivalent at the same time. Then, the observer selects which distorted image has the better quality, what requires evaluation of each distorted image separately, looking at the pristine image, and jointly, while making the decision on their relative quality. Such pairwise image comparison is used to determine subjective opinions for assessed images [28].*

**B***lasso regression SImilarity Measures*(lrSIMs), obtained for the first 20 % of benchmark images are written as follows:

In the (4)–(11), the number in the subscript denotes the benchmark whose part of images were used for the development of the measure, 1 for TID2013, 2, 3, 4 for TID2008, CSIQ, and LIVE, respectively; the number in superscript, in turn, indicates that the measure was developed using scores ”1” or PSD ”2”; the letter in the subscript denotes which subset of training images was used (five letters: *a*-*e*). In the evaluation (see Section 3), results for the *a* subset or for all subsets together in the form of the mean value are reported. Taking into account all obtained hybrid measures, it can be said that one hybrid measure uses 5.725 single IQA measures, on average. Among the mostly used IQA measures, VIF was selected 40 times, MAD 39, IFS 29, FSIM 25, RFSIM 22, PSNR 20, SFF 19, and VSI 17. The remaining IQA measures were used less than 15 times each. Interestingly, FSIMc was not used at all, and NQM, SSIM, or MSSIM were used less than five times, what can indicate that their features were replaced by the remaining IQA measures. Some measures contributed more than others, what is reflected by the weights. For example, in hybrid measures (4)–(5) VSI was the most contributing technique, in (6)–(7) MAD with SFF contributed more than other techniques, and in (8)–(11) MAD with VIF. The sign the weight mostly depends on the sign of correlation between the objective scores produced by the measure and the subjective scores in the benchmark. Experiments were performed using Matlab 7.14 with Statistics Toolbox.

## 3 Experimental evaluation

**S**^{ o }, and subjective scores, MOS or DMOS, denoted here by

**S**^{ s }, using the following mapping function for the non-linear regression [35]:

*=[*

**β***β*

_{1},

*β*

_{2}, …,

*β*

_{5}] are parameters of the non-linear regression model [35], and \(\boldsymbol {{{S_{m}^{o}}}}\) is the non-linearly mapped

**S**^{ o }. PCC and RMSE use

**S**^{ o }:

*d*

_{ i }is the difference between

*i*

^{ t h }image in \(\boldsymbol {\hat {S}}\) and

*,*

**S***m*is the total number of images. In KRCC, the number of concordant pairs in the dataset,

*m*

_{ c }, is used, as well as the number of discordant pairs,

*m*

_{ d }:

The values of RMSE closer to 0 are considered better, in contrary to SRCC, KRCC, and PCC whose values should be close to 1.

Comparison of mean values of RMSE for developed hybrid measures on four IQA benchmark datasets using raw scores or PSD

lrSIM\(_{1}^{1}\) | lrSIM\(_{1}^{2}\) | lrSIM\(_{2}^{1}\) | lrSIM\(_{2}^{2}\) | lrSIM\(_{3}^{1}\) | lrSIM\(_{3}^{2}\) | lrSIM\(_{4}^{1}\) | lrSIM\(_{4}^{2}\) |
---|---|---|---|---|---|---|---|

TID2013 | |||||||

0.5266 | 0.5225 | 0.6092 | 0.6056 | 0.6203 | 0.6333 | 0.6709 | 0.6412 |

TID2013, PSD | |||||||

0.7150 | 0.6910 | 0.8487 | 0.8252 | 0.9412 | 0.8899 | 0.9845 | 0.9090 |

TID2008 | |||||||

0.5794 | 0.5794 | 0.5532 | 0.5518 | 0.5994 | 0.5917 | 0.6677 | 0.6190 |

TID2008, PSD | |||||||

0.8015 | 0.7813 | 0.7467 | 0.7385 | 0.8158 | 0.7856 | 0.9116 | 0.8187 |

CSIQ | |||||||

0.0735 | 0.0690 | 0.0659 | 0.0638 | 0.0679 | 0.0605 | 0.0766 | 0.0639 |

CSIQ, PSD | |||||||

0.1133 | 0.1041 | 0.0912 | 0.0892 | 0.0887 | 0.0767 | 0.1003 | 0.0807 |

LIVE | |||||||

7.1739 | 6.9237 | 6.7223 | 6.5120 | 6.6943 | 6.2567 | 6.9266 | 6.1925 |

LIVE, PSD | |||||||

13.835 | 13.142 | 11.040 | 10.951 | 9.1425 | 8.9324 | 9.8535 | 8.1755 |

Comparison of hybrid measures with the six best state-of-the-art IQA measures on four benchmark datasets

VSI | FSIM | MAD | SRSIM | IFS | SFF | lrSIM\(_{1}^{2a}\) | lrSIM\(_{2}^{2a}\) | lrSIM\(_{3}^{2a}\) | lrSIM\(_{4}^{2a}\) | |
---|---|---|---|---|---|---|---|---|---|---|

TID2013 | ||||||||||

SRCC | | 0.8015 | 0.7807 | 0.7999 | | | | 0.8211 | 0.8070 | 0.8082 |

KRCC | | 0.6289 | 0.6035 | 0.6314 | | | | 0.6463 | 0.6298 | 0.6307 |

PCC | | 0.8589 | 0.8267 | 0.8590 | | | | 0.8676 | 0.8604 | 0.8604 |

RMSE | | 0.6349 | 0.6975 | 0.6347 | | | | 0.6164 | 0.6318 | 0.6317 |

TID2008 | ||||||||||

SRCC | | 0.8805 | 0.8340 | 0.8913 | 0.8903 | 0.8767 | | | 0.8916 | |

KRCC | | 0.6946 | 0.6445 | | 0.7009 | 0.6882 | | | 0.7083 | 0.7089 |

PCC | 0.8762 | 0.8738 | 0.8306 | 0.8866 | 0.8810 | 0.8817 | | | | |

RMSE | 0.6466 | 0.6525 | 0.7473 | 0.6206 | 0.6349 | 0.6333 | | | | |

CSIQ | ||||||||||

SRCC | 0.9423 | 0.9242 | 0.9466 | 0.9319 | 0.9582 | 0.9627 | | | | |

KRCC | 0.7857 | 0.7567 | 0.7970 | 0.7725 | 0.8165 | 0.8288 | | | | |

PCC | 0.9279 | 0.9120 | 0.9500 | 0.9250 | 0.9576 | 0.9643 | | | | |

RMSE | 0.0979 | 0.1077 | 0.0820 | 0.0997 | 0.0757 | 0.0695 | | | | |

LIVE | ||||||||||

SRCC | 0.9524 | 0.9634 | 0.9669 | 0.9618 | 0.9599 | 0.9649 | | | | |

KRCC | 0.8058 | 0.8337 | 0.8421 | 0.8299 | 0.8254 | 0.8365 | | | | |

PCC | 0.9482 | 0.9597 | 0.9675 | 0.9553 | 0.9586 | 0.9632 | | | | |

RMSE | 8.6816 | 7.6781 | 6.9073 | 8.0813 | 7.7765 | 7.3461 | | | | |

Overall direct | ||||||||||

SRCC | | 0.8924 | 0.8821 | 0.8962 | | 0.9139 | | | 0.9097 | 0.9105 |

KRCC | 0.7555 | 0.7285 | 0.7218 | 0.7372 | 0.7553 | 0.7529 | | | | |

PCC | 0.9131 | 0.9011 | 0.8937 | 0.9065 | 0.9191 | 0.9200 | | | | |

RMSE | | 0.4650 | 0.5089 | 0.4517 | 0.4338 | 0.4376 | | | | 0.4333 |

Overall weighted | ||||||||||

SRCC | | 0.8598 | 0.8412 | 0.8628 | | | | 0.8836 | 0.8725 | 0.8735 |

KRCC | | 0.6898 | 0.6711 | 0.6981 | | 0.7121 | | | 0.7084 | 0.7096 |

PCC | | 0.8828 | 0.8625 | 0.8876 | | 0.8981 | | | 0.8991 | 0.8986 |

RMSE | | 0.5566 | 0.6150 | 0.5456 | | 0.5313 | | | 0.5319 | 0.5337 |

Comparison of hybrid measures with the six best state-of-the-art IQA measures on four benchmark datasets

VSI | FSIM | MAD | SRSIM | IFS | SFF | lrSIM\(_{1}^{2a}\) | lrSIM\(_{2}^{2a}\) | lrSIM\(_{3}^{2a}\) | lrSIM\(_{4}^{2a}\) | |
---|---|---|---|---|---|---|---|---|---|---|

TID2013 | ||||||||||

SRCC | | 0.8489 | 0.8203 | 0.8447 | | | | 0.8604 | 0.8420 | 0.8423 |

KRCC | | 0.6660 | 0.6459 | 0.6616 | | 0.6786 | | | 0.6694 | 0.6689 |

PCC | | 0.8585 | 0.8314 | 0.8530 | 0.8658 | | | | 0.8557 | 0.8557 |

RMSE | | 0.9017 | 0.9771 | 0.9177 | 0.8799 | | | | 0.9099 | 0.9101 |

TID2008 | ||||||||||

SRCC | 0.8655 | 0.8695 | 0.8535 | 0.8786 | 0.8684 | 0.8756 | | | | |

KRCC | 0.6780 | 0.6829 | 0.6718 | 0.6948 | 0.6812 | 0.6896 | | | | |

PCC | 0.8492 | 0.8629 | 0.8550 | 0.8714 | 0.8632 | 0.8761 | | | | |

RMSE | 1.0086 | 0.9650 | 0.9904 | 0.9369 | 0.9641 | 0.9206 | | | | |

CSIQ | ||||||||||

SRCC | 0.8853 | 0.8544 | 0.9543 | 0.8478 | 0.9348 | 0.9408 | | | | |

KRCC | 0.7188 | 0.6837 | 0.8167 | 0.6750 | 0.7811 | 0.7897 | | | | |

PCC | 0.8874 | 0.8652 | 0.9573 | 0.8664 | 0.9316 | 0.9346 | | | | |

RMSE | 0.1674 | 0.1821 | 0.1050 | 0.1814 | 0.1320 | 0.1292 | | | | |

LIVE | ||||||||||

SRCC | 0.9031 | 0.9273 | | 0.9182 | 0.9361 | 0.9378 | 0.9603 | | | |

KRCC | 0.7266 | 0.7605 | | 0.7467 | 0.7744 | 0.7793 | 0.8249 | | | |

PCC | 0.8902 | 0.9159 | | 0.9030 | 0.9254 | 0.9244 | 0.9522 | | | |

RMSE | 17.748 | 15.637 | | 16.735 | 14.764 | 14.858 | 11.904 | | | |

Overall direct | ||||||||||

SRCC | 0.8856 | 0.8750 | 0.8989 | 0.8723 | 0.9019 | 0.9048 | | | | |

KRCC | 0.7084 | 0.6983 | 0.7442 | 0.6945 | 0.7300 | 0.7343 | | | | |

PCC | 0.8767 | 0.8756 | 0.9021 | 0.8734 | 0.8965 | 0.9006 | | | | |

RMSE | 0.6703 | 0.6830 | 0.6909 | 0.6787 | 0.6587 | 0.6417 | | | | |

Overall weighted | ||||||||||

SRCC | 0.8838 | 0.8649 | 0.8662 | 0.8633 | 0.8861 | 0.8875 | | | | |

KRCC | 0.7048 | 0.6848 | 0.7011 | 0.6830 | 0.7078 | 0.7096 | | | | |

PCC | 0.8741 | 0.8678 | 0.8718 | 0.8660 | 0.8817 | 0.8861 | | | | |

RMSE | 0.7819 | 0.8068 | 0.8429 | 0.8068 | 0.7869 | 0.7708 | | | | |

Comparison of the approach with other IQA approaches and hybrid measures based on SRCC values reported in the literature

IQA measure | TID2008 | CSIQ | LIVE | Overall direct | Overall weighted |
---|---|---|---|---|---|

Other IQA measures | |||||

ESSIM [53] | | 0.9326 | 0.9622 | 0.9264 | 0.9151 |

GMSD [45] | 0.8910 | 0.9570 | 0.9600 | 0.9360 | 0.9244 |

SURF-SIM [38] | | 0.9460 | 0.9440 | 0.9270 | 0.9178 |

[44] | | | | – | – |

[54] | | | | – | – |

SM-HM-FSIM [11] | | 0.9530 | 0.9640 | 0.9377 | 0.9241 |

Other hybrid measures | |||||

ADM [15] | 0.8617 | 0.9333 | 0.9460 | 0.9137 | 0.9001 |

[2] | 0.8100 | | | 0.9100 | 0.8843 |

BME [25] | | | | – | – |

BMMF [12] | | – | – | – | – |

CNNM [20] | – | – | – | – | – |

CQM [21] | | – | – | – | – |

DOG-SSIM [24] | | 0.9204 | 0.9423 | 0.9295 | 0.9282 |

EHIS [22] | | | | – | – |

ESIM [23] | | 0.9620 | 0.9420 | 0.9420 | |

GLD-PFT [30] | 0.8849 | 0.9549 | 0.9631 | 0.9343 | 0.9186 |

IGM [43] | 0.8902 | 0.9401 | 0.9580 | 0.9294 | 0.9190 |

[13] | – | – | | – | – |

MAD [14] | 0.8340 | 0.9466 | 0.9669 | 0.9158 | 0.8944 |

MMF [18] | | | | – | – |

RMSSIM [26] | | | | – | – |

[48] | | | | – | – |

lrSIM\(_{1}^{2a}\) | | | | | |

lrSIM\(_{2}^{2a}\) | | | | | |

lrSIM\(_{3}^{2a}\) | 0.8916 | | | 0.9439 | 0.9275 |

lrSIM\(_{4}^{2a}\) | | | | | 0.9283 |

The results of comparison based on SRCC are presented in Table 5. They reveal that lrSIMs outperformed other measures, being in most cases among the three best IQA techniques. For TID2008, SM-HM-FSIM [11] was the second best technique. However, SM-HM-FSIM is non-hybrid approach, what makes all lrSIMs better than other compared hybrid approaches. The presented lrSIM family, together with the approach introduced by Barri et al. [2] outperformed other techniques on CSIQ. Overall results, as well as tests on LIVE, showed superior performance of lrSIMs over the other measures. More specifically, lrSIM\(_{1}^{2a}\) and lrSIM\(_{2}^{2a}\) were clearly better than other measures, what was indicated in the previous experiments.

## 4 Conclusions

In this paper, a hybrid full-reference IQA was introduced. The measure was obtained using the lasso regression and pairwise score differences of up to 16 IQA measures seen as predictors. The lasso was able to select the most important several IQA measures. This resulted in the family of hybrid measures, lrSIMs, which was extensively evaluated on four largest IQA image benchmarks employing SRCC, KRCC, PCC, and RMSE. The evaluation was also based on PSD. The introduced approach outperformed widely used full reference IQA measures, as well as other hybrid techniques. It can be assumed that the usage of PSD will support the development of other IQA measures based on supervised learning.

The Matlab code of the approach that allows adding other IQA measures, scripts performing pairwise score differences on used benchmarks, as well as the evaluation of compared approaches, are available to download at: http://marosz.kia.prz.edu.pl/lrSIMpsd.html.

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