Transition points are points in the brightness scale where we move from a normal image zone into an inverted zone – and of course where we move out of an inverted image area into a normal one. With complex manipulations you may have inversions adjacent to each other, without a region of normal gradient.
Often the most interesting effects come from setting a transition point in the middle of the tonal range of an object, especially when the object is a reflective surface like chrome or polished paint.
The following transform shows the tonal range of the oil tank between the outer red lines, with the middle line showing a point on the curve of the tank that I selected as a starting point for experimentation. This formed the experimentation range for my first example and as you experiment you will see that moving the transition point between the red lines produces some interesting effects on the oil tank as well as the image as a whole.
Compare this image with the predecessor. There is already quite a change in the effect, although there has only been a relatively small change in the transition point.
Some other points to note