The mean value mapping technique corresponds to the simplest photographers' approach, where the average light is measured and the aperture is automatically set such that the average is mapped to the medium gray. More advanced photographers know that they have to adjust the aperture manually if the scene is not a typical one. By setting the aperture manually the final photo can be darker or lighter. Photographers also know how to enhance the overall contrast of the image. Sometimes it is necessary to use a flashlight even on a sunny day if portraits are shot. Experienced photographers will choose an appropriate film speed as well.
Just as in real life, we did not want to use just the mean value mapping method, so we have proposed to use interactive calibration. Together with chapters 6, 7, and 8 this is the main contribution of this thesis.
Up to now we have been describing the mapping functions that are applied to the whole raw image. We introduce another approach here, namely that a clipping interval is selected first, and the mapping function is applied to this interval only. Values outside the interval are simply clipped to the interval borders.
Our intention is to find a clipping interval [s,e]. The start point of the interval will be mapped to the minimum displayable value and the end point to the maximum displayable value. Just as in photography the center of the interval is not always the average value, actually in professional photo this is almost never true. The interval is found by varying aperture and contrast, as explained in the next chapters.