How do varying N (resolution of image) and k (number of gray level in image) affect images? An early study by Huang [1965] attempted to quantify experimentally the effects on image quality produced by varying N and k simultaneously. The experiment uses images that is similar to those shown in below figure. The woman’s face has relatively little detail; the picture of the cameraman contains an intermediate amount of detail; and the crowd picture contains, by comparison, a large amount of detail. 


(a) Image with a low detail. (b) Image with a intermediate amount of detail. (c) Image with a large amount of detail.


Sets of these three types of images were generated by varying N and k, and participants were then asked to rank them according to their subjective quality. Results were plotted in the form of so-called iso-preference curves in the Nk-palne (see below graph). 


Representative iso-preference curves for the three types of images in the above figure.


Each point in the Nk-plane represents an image having values of N and k equal to the coordinates of that point. Points lying on an iso-preference curve correspond to images of equal subjective quality. It was found in the course of the experiments that the iso-preference curves tended to shift right and upward, but their shapes in each of the three image categories were similar to those shown in above graph. This is not unexpected, since a shift up and right in the curves simply means larger values for N and k, which implies better picture quality.  The key point of interest in the context of the present discussion is that iso-preference curves tend to become more vertical as the detail in the image increases. This result suggests that for images with a large amount of detail only a few gray levels may be needed. For example, the iso-preference curve of crowd image is nearly vertical. This indicates that, for a fixed value of N, the perceived quality for this type of image is nearly independent of the number of gray levels used. It is also of interest to note that perceived quality in the other two image categories remained the same in some intervals in which the spatial resolution was increased, but the number of gray levels actually decreased. The most likely reason for this result is that a decrease in k tends to increase the apparent contrast of an image, a visual effect that humans often perceive as improved quality in an image.


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