By your question, I’m assuming that you’re attempting to identify objects in an image, correct?
Most of the automatic thresholding algorithms employed by IdentifyPrimAutomatic employ some model of the image histogram. The histogram is basically a function describing the frequency of each pixel intensity, which is what you’re looking for.
Each method does this differently, and does it on a per-image basis (unless you specify ‘All’). If your background is very large, you might want to try Background or RobustBackground.
Have you given these methods a try and found that they’re not working for you? If I’ve misunderstood what you’re looking for, please correct me.