QuickTime 4 Reference

Color Limits

Transfer mode operations allow you to specify limits on the acceptable input values for the source or destination color, and on the acceptable output values for the result color. For example, in converting CMYK color to RGB, you may wish to limit the intensities to values that can be displayed without oversaturating the phosphors on a monitor's screen. Or, to create a special effect, you may want to draw only the extreme light and dark portions of an image, leaving out its midrange entirely.

Each color component in the component field of the transfer mode structure can have a maximum and a minimum permitted value. The permissible ranges can be interpreted as shown in Figure 48 . In the figure, the large cube represents all of RGB space; the small cube represents one possible example of the limits that could be imposed on allowable values for all three components.

Figure 48 Maximum and minimum color-component values in RGB space

In the case of source and destination colors, color values outside the range of acceptable values (that is, outside the small cube in Figure 48 ) are ignored; if any single component value is outside of its acceptable range, no drawing occurs at all for that color. In the case of the calculated colors that result from a given transfer mode operation, color values outside of the acceptable range are pinned to, or moved so that they don't exceed, the nearest acceptable value (the closest edge of the small cube). See Figure 49 .

Figure 49 How minimum and maximum color limits affect drawing

For a given component, the maximum value for a color limit can be either greater or smaller than the minimum. If the maximum is less than the minimum, only the extreme color values (that is, values outside of the small cube area in Figure 48 ) are allowed. See Figure 50 .

Figure 50 How reversed minimum and maximum color limits affect drawing

Each of the components in a color space can have its limits set entirely independently of the others. Figure 51 shows the effects of reversing, in turn, the maximum and minimum values for each of the three axes in RGB space.

Figure 51 The effects of reversing maximum and minimum in a color space

Where the words Min and Max are bold in Figure 51 , the minimum is greater than the maximum. Refer to Figure 48 for the positions of the color axes on the RGB cube in this figure:

In drawing (A), all minimum limits are less than their respective maximums; the allowable color ranges form a small cube, just as in Figure 48 .
In drawing (B), the maximum on the red axis is less than the minimum; only red color values outside of the range of the small cube are permitted, whereas blue and green must still be within the limits of the small cube. The acceptable color values form two rectangular solids within RGB space.
In drawing (C), the maximum and minimum on the green axis are also reversed; the acceptable color values form a more complicated set of solids.
In drawing (D), all maximum and minimum color limits are reversed. In that case, only color values at the outer corners of the color space (all components outside of the range of the small cube) are acceptable.