Registered: Jan 28, 2005
Total Posts: 6632
Country: Canada
I think that the curve illustrated in this window from the tutorial is a bad example. One can't argue with the final result, where Mahesh has produced an excellent image after all the processing, but this curve is probably not a good example to teach with:
This shows partial tone reversal in the highlights, i.e. the highest tones will actually become darker than the next highest tones. The slope goes flat and then negative on the right of the curve. The curves are not multipliers, they are transformers (transfer functions). If you create a flat line across the middle it does not leave the tones unaltered, it flattens them all out to middle gray (modulo some small variation due to the local adaptation algorithm).
Partial tone reversal is a specialty effect sometimes called solarization or partial solarization or the Sabattier effect (sometimes spelled Sabatier). Solarization was a total tone reversal resulting from a massive overexposure of a film negative. The Sabattier effect is most commonly known through the prints of Man Ray and is achieved by briefly exposing the B&W silver gelatin print to white light during development, often accomplished by turning on the white darkroom light for a second.
In Curves windows like this, the curve should be "monotonically increasing", meaning that the curve should never dip down but should get higher for each step along the bottom to the right. The slope would never go flat or negative and would always be positive.
One can accomplish things like "darkening the highlights" by moving the curve below a 45 degree slope in that area, without making the slope ever go flat or negative.
Another thing about the curve as shown is that there are no highlight values above medium light luminance because the curve peaks in that range. The later curve applied a little boost, but still not much.
It is an artist's choice to introduce partial reversal of tones or to reduce the luminance of highlights so they never reach full value, but those choices should be considered very carefully and I would advise not using them except in exceptional cases, and not as the default way of proceeding.
