A no-reference image quality measure based on CPBD and noise

In order to meet the needs of the objective evaluation of no-reference image, a no-reference image quality measure is presented. The measure is based on edge analysis and is suitable for images with noise. Taking properties of the Human Visual System(HVS) into account, we compute the probability of blur after getting the edge width and the local contrasts. And at last the image quality probability can be got considering cumulative probability of blur detection and the noise pollution degree. Experimental results show that the metric has a wide application, good anti-noise ability, simple calculation, as well as in high consistence with the subjective evaluation results.


Introduction
A With the widespread use of image information technology, the image which contains a lot of valuable information is more and more valued as the source of visual information.In the process of image acquisition and transmission, it is inevitable that images will be affected by noise, blur, the loss of data and so on, resulting in a decrease of image quality.The image quality directly affects the amount of information acquired and the subjective feelings of people.So, an automated and objective no-reference image quality evaluation assessment is needed considering the errors caused by subjective judgment and the time-consuming staff.
So far research on reference and semi-reference image quality evaluation has achieved good results.And the non-reference evaluation which is most practical has gradually become the researches emphasis in recent years.Some scholars have studied the image quality evaluation method, and have achieved some results.
These algorithms are either based on edge analysis to evaluate the image blurriness as in literature [1][2][3][4], or simply estimate the variance of the noise as in [7][8][9][10].For images which contain blur and noise in the same time, these motheds lack the ability to evaluate them.Considering the situation, this paper proposes an evaluation method combining the edge analysis and image noise level, which can be used more broadly.

The Improved Quality Probability Model
To evaluate the quality of an image with blur and noise, an improved probability model based on the edge analysis is proposed by taking noise into account.In this section, the implementation procedures of the algorithm are given in detail.

Cumul
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tter the imag width largely nt, and the along the t is the edge measurement [11]  e n order to increase the consistency between Eq. 2 and the experimental results of the fuzzy psychological perception.The experimental results in literature [6] find that the probability is more accurate and credible when β has a median value of 3.6, and the JNB edge widths for different contrasts can be modeled as: An improved method of JNB is proposed in literature [9].The method divides an image into 64x64 blocks.Then the blocks whose edge pixels are accounted for more than 0.2% of the total number of pixels in the block are defined as edge blocks to be processed further.It is pointed out that, at the JNB with, w e w e corresponds to a probability of blur detection p P 63%.Thus, for a given edge pixel e , when p P , it is considered that there is no blur to be detected at that edge.With the increase of the blur degree of an image, the edges' width increases, which leads to a bigger value of w e and to a higher probability of blur detection.Considering all of the edge blocks and the edges, the entire image's sharpness probability is modeled as a cumulative probability of blur detection (CPBD) as follows: Where is defined as the value of the probability distribution function at a given .

Measurement of Noise Effects.
Noise, as one of the most common image degradation factors, is often produced in the process of generation and propagation of an image.When evaluating the image quality with noise, people pay more attention to the intensity of noise rather than it's random characteristic， for it is the intensity of noise that effects people's subjective perception.The differences between the images before and after denoising reflect the noise pollution degree.Thus, after using the total variation (TV) [10] minimization method which can save the image details as much as possible to denoise, the signal noise ratio(SNR) which denotes the ratio of the original image signal intensity and the intensity of noise can be got : Where and are the size of the image., denotes the denoised image and , denotes the original image with noise.Considering the randomness of the noise, the probability that the detected edges are effected by the noise to reduce the quality of the image is 1/ 1 , that is, the ratio of the image quality to be reduced.

The Comprehensive Evaluation Probability Model.
There is a CPBD model which denotes the probability of edge sharpness of the whole image and the ratio noise affects the quality of an image.Since the noise is random and independent of the image signal or the edge sharpness, a new probability model can be obtained by putting them together: Where P is the final quality evaluation value, that is, the probability of a high quality image.And the higher the image quality is, the higher the value will be.

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Fig. 3
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