In the low-resolution pre-scanning process, some platforms and roller scanners use automatic density control prior to scanning to perform a special exposure setting for the variation density of the original. The brightest density (Dmin) The darkest density (Dmax) is automatically positioned to indicate the existing density range. The automatic setting is very easy to use for bright, medium, and dark originals, but some originals require additional settings to achieve good results when selecting highlights and dark spots. Therefore, manually set bright spots and dark spots to intentionally retain a limited range of highlight or dark original tonality. These modifications are very similar to the effects of the linear tone correction in the previous section.
For dark-sounding images, the automatic white point density is too deep, causing the dark tone to be too bright and out of touch with reality. For highly-original originals, automatic black points will make the density of the selected points too bright, resulting in an image that is too dark. When the image contains some key tonal areas, it is necessary to insert a gray scale when scanning the image so that the white and black points can be correctly defined. Some scanners' interface programs include timoing scales used for this purpose.
If the image contains some direct light sources, or high light spots such as metal or glass surfaces, the effect should be kept pure white. An automatically defined white point will be incorrectly selected at these very high light points, causing the other highlights and midtone levels to become too dark. At this time, it is required to set the white point in the light and visible detail neutral light region. If the white point is set in the light gray area, the density area that is lighter than this gray density will be cut into white, thereby making the remaining gradation lighter. The black point setting is not as critical as the white point and should be defined in the darkest detailed neutral gray tone zone. Any darker tone areas will be cut to pure black, thus removing all details of these areas