Modulation transfer function measurements, image quality metrics, and subjective image quality for soft-copy color images

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Virginia Tech


The effect of spatial frequency manipulation of color images and its impact on subjective image quality were examined by measuring subjective image quality and the changes in several prominent image quality metrics. The Modulation Transfer Function Area (M1F A), the Square Root Integral (SQRI), the Integrated Contrast Sensitivity (ICS), the Subjective Quality Factor (SQF), and several other acutance-derived image quality metrics were evaluated for their ability to predict image quality. Five distinct spatial frequency filters were applied to each of four pictorial color scenes in the horizontal dimension, the vertical dimension, and in a combined two dimensional format. The same spatial frequency filters also were applied in a circular format. Thirty-two subjects, 24 college students and 8 Eastman Kodak employees, participated in a paired comparison method in which 21 stimuli for each of the four scenes were evaluated for their perceived image quality.

Horizontal and vertical modulation transfer functions were acquired by photometric scans of a cross-hair one-pixel wide white line target as well as by luminance profile manipulation of square-waves of differing frequencies, both utilized as input signals. From the modulation transfer functions, values for the various image quality metrics were calculated and related to the subjective image quality data.

In the evaluation of perceived image quality, several experimental procedures are available, such as magnitude estimation, rank ordering, rating-scale, categorical judgments, and the method of paired comparison. The method of paired comparison is frequently avoided for its time consuming nature. However, results indicate that with the use of computer automatization it is a powerful and reliable experimental procedure for testing subjective preferences between digital stimuli containing small perceptual differences.

The highest correlation between perceived image quality and image quality metrics was obtained for the pictorial scene which contained a uniform and dense set of frequencies in the horizontal and vertical directions. The lowest R 2 values were reported from the pictorial scene that contained more scattered frequencies in both directions. Therefore, it is advised when performing perceived image quality evaluations of frequency-manipulated pictorial stimuli to use those stimuli that contain a broad range and uniform set of spatial frequencies. The impact of frequency manipulation is then more apparent and, in addition, it may provide for a more reliable transfer of results across experiments.

Small increases in modulation produced perceived quality increases in the pictorial color images. Furthermore, improved image quality was obtained with low frequency (less than 9 c/deg) modulation enhancement. In addition, vertical filtering produced greater subjective image quality improvement than did horizontal filtering. For all scenes, the two-dimensional filtered images were perceived as possessing equal or better quality than the circular filtered images. Low-frequency enhancement, close to zero spatial frequency, and no high-frequency enhancement, only minutely (if at all), increased perceived image quality; however, continuing the enhancement process from the low frequencies to the higher frequencies significantly improved perceived image quality.

The SQRI metric is not recommended for use in the evaluation of image quality when changes in the MTFs occur at spatial frequencies of 3 cycles per degree and higher. The ICS and MTFA behaved in an acceptable manner with changes in subjective image quality and should be considered for their computational accuracy and practicality. The SQF and the acutance metrics were highly recommended for predicting subjective image quality. In addition, the development of a standardized display measurement technique for color CRTs and a standardized verification process of display image quality are recommended.