Color Balance

This transformation lets you remove color casts from an image by adjusting its white balance. Before starting, Picture Window analyzes the input image to try to find a color cast to remove automatically. You can accept this auto white balance, or you can select the white balance manually. In addition, you can add a color cast by specifying a second color cast to add to the image.

Removing a color cast means making colors that match a color become neutral; adding a color cast is the reverse; it makes neutrals take on a color. Removing a color cast is like passing an image through a filter whose color is the complement of the color cast; adding a color cast is like passing an image through a filter whose color is the same color as the cast. Removing and adding a color cast at the same time makes colors which match the remove cast become the color of the add cast.

For example, to remove a greenish cast from an image, select a greenish color to remove. The more saturated you make the cast to remove, the more green you take out of the image. To add a warming cast to an image to simulate early morning or late afternoon light, add a reddish to yellowish cast. Again, the more saturated the color, the stronger the effect.

The color balance transformation works by applying three curves to the input image -- one for the red channel, one for green channel, and one for the blue channel. These curves are displayed on the right side of the dialog box. Each curve is a straight line starting at the lower left corner. You can either set the curves by choosing colors to add and remove, or you can drag points on the curve to select the colors.

Two color patches are displayed against a white surround on the left side of the dialog box -- a color to remove and a color to add. Clicking on either of these patches pops up a color picker you can use to change its color. You can also set the color to remove by clicking on the input image. A convenient way to set the color to remove is to find a part of the input image that should be neutral (such as the whites of the eyes) and click on it. You can add a color cast by setting the color to add. For example, to warm up an image, add a pale orange color.

For a more powerful version of this transformation that allows you to remove different color casts from lighter and darker parts of the image by creating more complex correction curves, see Multi Color Balance.

Amount

The amount control lets you control how much of the color balance transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

White

This control lets you select how the white balance is specified.

None -- sets the cast to remove and the cast to add both to pure white -- this produces curves that have no effect on the input image.

Auto -- sets the color to remove to be the auto white that Picture Window detected automatically on startup.

Auto - Full Range -- sets the color to remove to be the auto white that Picture Window detected automatically on startup, and makes the image as bright as possible without clipping.

Custom -- lets you choose the highlight cast to remove with the color picker or by clicking on the input image.

Settings Menu

White Threshold -- The White Threshold options (Low/Medium/High) apply only when White is set to Auto or Auto - Full Range. They adjust the internal threshold used when computing the automatic white balance. Normally a low threshold works best, but it the input image is overexposed or some of the highlights are clipped, a larger threshold may work better, although it can result in the clipped highlights being tinted.

Probe Size -- These options let you control the sample size used by the probe, should you choose to enable it (see below). Larger probe sizes are useful for obtaining an average reading of any area of the image that does not have a smooth texture; smaller probe sizes let you get precise readings of very small areas. The currently selected probe size is shown with a check mark.

Tips

Color matching to a reference image

If you want, for example, to match a skin tone from one image to a skin tone in another image, all you have to do is select the skin tone in the input image as the highlight cast to remove and the skin tone you want to change to as the highlight cast to add. The procedure for doing this is as follows:

Click on the input image to select the skin tone as the color to be removed.

Click on the color square for highlight cast to add; this pops up a color picker.

Click on the skin tone you want to match; this sets the color to add.