The effects of image quality produced by varying N and m simultaneously. There are different types of images taken into consideration as shown a group of people. The first image has a lot of details example a face. The second image was one which had intermediate amount of information for example a small group standing together and the third image was one which had a lot of information i.e. crowd. In these images, N and m were varied. Now we observe the rank
of the image according to the subjective quality. These results were summarized in the form of Isoprefrence curves in the N-m plane. Point lying on an isopreference curve corresponds to image of equal subjective quality. From the isporefrence curve, Haung concluded that images with large amount of details require a few grey levels.

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Since the isoprefrence curve of the crowd is near vertical, it means that for affixed value of N, the image is nearly independent of m.