Selective Color Correction

This transformation lets you apply pinpoint color corrections to an image that modify only certain colors and leave the rest unchanged. The ability to localize adjustments to certain colors often makes this transformation more useful than Color Balance or Filter which change all the colors in an image. For example, you can use the Color Correction transformation to adjust skin tones in a portrait without altering other colors in the clothing or background.

The Selective Correction transformation is based on the hue and saturation components of the HSV color space, as represented by the color wheel. You specify a correction by placing control points on the color wheel; each control point has two parts: an input color and an output color. When the correction is applied, all pixels in the input image whose hue and saturation match one of the input colors is changed to the corresponding output color with the same value (brightness). Colors in between the control points are interpolated to make smooth transitions.

In addition to modifying hue and saturation, you can also assign a brightness change to each of the control points. This lets you apply brightness corrections at the same time you make color corrections. For example, using this feature you can make a clear blue sky lighter or darker without altering other colors in the image. Both color corrections and brightness changes are interpolated between control points, so the effects have smooth transitions based on the colors in the input image.

For making very fine adjustments to control points, you can zoom the color wheel in or out and scroll or re-center the display. You can also enlarge the color wheel by resizing the dialog box.

Amount

The amount control lets you control how much of the color correction 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.

Radius

Adjusting this slider controls how closely colors must match those of a control point to be affected by the transformation. Larger radius values affect more of the neighboring colors; smaller radius values narrow the effect to only those colors that match more closely.

Show Modified Color Wheel

Normally, Picture Window displays a color wheel at the bottom of the dialog box (see below). When the Show Modified Color Wheel checkbox is checked the Color Correction transformation applied to the color wheel itself and the results of this are displayed in its place. This gives you a visual representation of which colors are being changed and by how much. To return to the normal color wheel display, uncheck the check box.

Show Affected Region

This control is hidden until a control point is selected. When checked, the preview display blacks out colors that are not affected by the current control point. Only the preview is affected – if you click OK, the final image will show the full color corrected version.

Save As Mask

This control is hidden until a control point is selected. When checked, instead of computing a color corrected version of the input image, the result will be an 8-bit black and white mask that highlights the affected colors associated with the current control point.

Color Wheel

The color wheel and the control points displayed over it are central to this transformation. A control point has two parts – an input color and an output color. If both are the same, the control point in displayed as a circle. If the output color differs from the input color, a smaller circle is displayed at the input color, connected by an arrow pointing to the output color. All colors in the input image whose hue and saturation match the input color are changed in the output image to the output color. Colors in between control points are interpolated to make a smooth transition.

The control point you most recently added or clicked on is called the current control point. The current control point, if any, is displayed as a filled square instead of a hollow circle. To make a control point the current control point, simply click on it.

Initially, there are 7 control points set on white, red, yellow, green, cyan, blue, and magenta. Each of these control points has both its input and output colors set the same. These control points ensure that white and the fully saturated primary and secondary colors in the input image are left unchanged in the output image, which is often a good starting point if you just want to make a minor change. Applying the transformation with this initial configuration, however, will have no effect on the input image as none of the output colors differs from its input color.

To add a new control point, shift-click (hold down either Shift key while clicking with the left mouse button) on the color wheel at the desired input color location. Or, you can add a control point by using the Probe. To do this, make sure button is depressed and then click once on a color in the input image window you want to change. You can create up to 32 control points; Picture Window beeps if you try to create more than 32.

To remove a control point, control-click (hold down the either Ctrl key while clicking with the left mouse button) on it.

To change the output color of a control point, click and drag its circle on the color wheel. This will extend an arrow from the control point to the desired output color. Once an arrow has been created, you can make additional changes to the output color by dragging the arrowhead.

To change the input color of a control point, first make the output color different from the input color, if necessary. Then you can change the input color by dragging the small circle at the base of the arrow.

To select a control point, simply click on it. The selected control point is displayed on the color wheel as a filled square instead of a hollow circle.

As you move the mouse cursor over the color wheel, a readout just below the color wheel displays, for reference purposes, the hue and saturation values corresponding to the cursor location.

Palette

A set of palette colors is displayed just below the color wheel. These colors represent the current custom palette as defined using the Palette Tool or from within the Color Picker. Clicking on a palette color inserts a new control point in the color wheel at the corresponding color. This feature is useful if you want to match a color from an image other than the current input image. To do this, first click on the image you want to select from, activate the palette tool and select the color, and then edit the image you want to perform selective color correction on and select the color from the palette.

Brightness Change

This control is hidden until a control point is selected. Each control point has an associated color change as indicated by the size and direction of the arrow displayed on the color wheel. In addition, each control point also has an associated brightness change; positive settings make colors that match the input color of a control point brighter while negative values make matching colors darker (using the HSV or HSL color space).

The Brightness Change slider at the bottom of the Color Correction dialog box lets you assign brightness adjustments to the current control point. To make a control point the current control point, simply click on it. This value displayed in the slider is the value of the brightness change setting for the current control point -- any changes you make to the slider alter the brightness change setting for the current control point.

Settings Menu

Preserve Black and White/Preserve Black

These options relate to brightness corrections. When Preserve Black and White is selected (the default setting), brightness corrections are made in such a way as to leave pure black and pure white unchanged – the primary effect is on the mid-tones. When Preserve Black is selected, an equal percentage brightness increase is applied to all brightness levels. This provides a more uniform brightening effect but can lead to a loss of highlight detail due to clipping when you increase brightness. The types of brightness curves used are illustrated below.

HSV/HSL

These options specify the color space in which brightness corrections are made. Lightening in HSL makes colors closer to white while lightening in HSV preserves the ratios between the RGB channels. In either color space, darkening makes colors closer to black.

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 an 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.

The  tool bar lets you zoom the color wheel in or out.

Plus zooms the color wheel in, displaying a magnified region around the current control point or, if no control point is selected, about the center of the window. With the color wheel zoomed in, it is easier to make fine corrections to control points. When the color wheel is scrolled to a magnification greater than 1:1, you can use the scroll bars to bring hidden parts of the color wheel into view if necessary. You can zoom in up to four levels.

Minus zooms the color wheel out, reducing its size by one level.

Right Button Menu

Clicking near one of the control points in the color wheel using the right mouse button selects the nearest control point and pops up a menu of options you can use to alter that control point. The menu contains the following items:

Reset Color – This clears any color change the control point specifies; the display for the control point reverts to a small circle with no arrowhead. Any brightness change the control point may have is retained.

Reset Brightness – This clears any brightness change the control point specifies; the brightness change slider is returned to its center. Any color change the control point may have is retained.

Reset Brightness and Color – This clears any color and brightness change the control point specifies.

Remove Control Point – This deletes the control point.

Right-clicking away from any of the control points in the color wheel pops up a single-item menu:

Add New Control Point – This creates a new control point at the location you clicked on the color wheel. This is the same as Shift-Clicking on the color wheel.

Tips

To make a correction of one color in an image, click on a part of the input image having the color you want to change. Next go to the color wheel and click and drag on the new control point you just created. To make the color more or less saturated, drag it toward the edge or toward the center of the color wheel. To adjust its hue, drag the control point either clockwise or counterclockwise about the center. Finally check the results and click OK if it looks good. To make further adjustments, click again on the input image to create additional control points.

If the transformation is changing colors close to one you selected and you want these to remain unchanged, create additional control points whose input and output colors are the same to prevent the nearby colors from being modified.

If there are several elements in the image that have the same color, and you only want to adjust some of them, create a mask to isolate the parts you want to change.