I have a variable "x" that takes real values between 1 to 6. I have 6 data points in this interval which each of them corresponds to an image. I read the histogram of the images for finding out the changes in each data point. I want to find a relationship between the histograms and the variable x. So that later on I can estimate (and interpolate) the value of the variable "x" by analyzing a new image. Any suggestions on reading material and/or algorithm?