Gray Balance
General
For some images, special lighting conditions (leading to undesirable color casts) mean that it is not possible to achieve satisfactory results using either the white balance or the color curves. This is where the gray balance can be useful. By setting four reference points the various color casts of the image are calculated according to preset gray values. The result is a curve which can be adjusted afterwards. However, it is usually sufficient just to set the reference points.
Usage
When the mouse pointer is moved over the image, one of the four reference areas in the bottom right corner of the dialog is given a red border. The area (1) is associated with dark areas, while area (4) is for very light areas. In other words, the areas represent the parts of the histogram.
To set a reference point, double-click a part of the image with the left mouse key. Now the corresponding brightness value appears in the upper half of the area which had the red border. Below, the gray value for the area which will be used as the basis for the correction of the selected area is displayed. At the same time, control points are placed in the color curves ((1)=A, (2)=B, (3)=C, (4)=D) which cause the selected brightness range to be converted in accordance with the respective gray value. The control points can be adjusted at will if further correction is necessary.
Sometimes adjacent reference points have a similar degree of brightness because they are too close together. This can be minimized by displaying the brightness of the respective area. If the mouse point hovers over a given area of brightness, the corresponding reference area has a vertical line running through it (see (3)). This shows the brightness of the given area. If the line tends to the left, then the associated image area is rather dark; if it is more in the right hand part, then the image area is lighter.
You may need some practice in setting reference values, as not every brightness value proves to be suitable. You should start with reference points in the lighter areas. For areas with a lot of noise it is better to use the polygon selection method (5). This produces a brightness mix from all the pixels contained within the selection polygon.
Areas of use
- For removing color casts in images with gray areas of differing brightness.
- For retouching slight color casts on reproductions (of paintings) where a color/grayscale was photographed simultaneously. For paintings it is especially important to achieve complete neutrality throughout the whole brightness range.
