There are Exceptions
Here is an example where setting the White Point aggressively or automatically is not appropriate. There is no color in this image that should be as bright as white (grayscale white). Most images are not so single-color as this. Most images (like scenic landscapes and portraits of people) will have a more complete tonal range.
Scanners have automatic modes too, and the option works by having the
software automatically examine the histogram tones in this same way, and
then both ends are clipped automatically. Auto mode will always clip both
ends somewhat, it is simply how it works. Often that's good, but
it does not work well in every case. It would produce incorrect
results if we set the brightest tones in THIS image to be white.
Sometimes a human eye has to see it and judge it and control it. The
preview image shows what is happening, and we say "Oops!" and put it
back. So how do you know what to do in every case? Well,
you try something, and see what happens. In this case, the White Point
was hardly moved, to brighten it only slightly.
So get out your scanner and try this procedure once, and it will become clear and easy.
Note: Histograms have Output Level controls too.
The histogram controls will often have both an Input and an Output level fields. The Input Levels are as described above, setting the Black and White points, and these newly marked end values become 0 and 255, and all other values are stretched to fill that range. The final range is always 0 to 255 regardless of the tone values where we place those points.
The Output Levels are very different. These are designed to limit the total contrast for output devices using halftones (laser printers and printing presses). Halftones do not have maximum tonal range, the blackest blacks and the whitest whites have to be limited slightly.
Setting Output levels to say 20 and 235 will compress all values to fit in the range of 20 to 235. 0 becomes 20, and 255 becomes 235. No tone values will be outside that range, those end points will be the darkest and brightest levels. The Input 0 to 255 range is modified on output to the specified values. Output level is more for B&W halftones that can't make pure white. If images are to be reproduced, the printing process will make them more contrasty than the originals were. You can limit both ends by about 10% with the Output Control to compensate for this. Obviously different processes have different requirements, discuss it with your printer.
Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.
