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Inside Macintosh: Advanced Color Imaging on the Mac OS /
Chapter 4 - Developing ColorSync-Supportive Applications / Developing Your ColorSync-Supportive Application

Checking Colors Against a Destination Device's Gamut

Different imaging devices (scanners, displays, printers) work in different color spaces, and each can have a different gamut or range of colors that they can produce. The process of matching colors between devices entails adjusting the colors of an image from the color gamut of one device to the color gamut of another device so that the resulting image looks as similar as possible to the original image. Not all colors can be rendered on all devices. The rendering intent used in the color transformation process dictates how the colors are matched, strongly influencing the outcome. Your application can give your user some control over the outcome by allowing the user to select the rendering intent. However, some users might want to know in advance which colors are out of gamut for the destination device so that they can choose other appropriate colors within the gamut.

Using the ColorSync Manager low-level color-checking functions, your application can check the colors of a pixel map (using the CWCheckPixMap function), the colors of a bitmap (using the CWCheckBitmap function), or a list of colors (using the CWCheckColors function) against the color gamut of the destination device and warn your user when a color she or he chooses is out of gamut for that device.

There are a number of ways in which your application can provide gamut-checking services. For example, you can use gamut checking to see if a given color is reproducible on a particular printer. If the color is not directly reproducible--that is, if it is out of gamut--you could alert the user to that fact.

You can allow your user to specify a list of colors that fall within the gamut of a source device to see if they fit within the gamut of a destination device before the user color matches an image. Your application could display the results in a window, indicating which colors are in the gamut and which are out. This feature, too, gives the user the opportunity to test colors and select different ones for portions of an image whose colors fall out of gamut. To handle this feature, your application can call the CWCheckColors function.

In addition to providing features that allow your user to anticipate which colors are out of gamut for a particular device, your application can also show results. Your application can provide a print preview dialog box, showing which colors in a printed image, for example, are out of gamut for the image as it appears on the screen.

For an image that your application prepares, for example, your application can present a print preview dialog box that signifies those colors within the image that the printer cannot accurately reproduce. Your application can also allow users to choose whether and how to match colors in the image with those available on the printer.

You can provide a gamut-checking feature that marks the areas of a displayed image, showing the colors that do not fall within the destination device's gamut. For example, your application can color check an image against a destination device and create a black-and-white version of the image drawn to the display using black to indicate the portions of the source image that are out of gamut. The CSDemo sample application takes this approach. For information on how to obtain the CSDemo application, see "Extracting Profiles Embedded in Pictures" (page 4-37).

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© Apple Computer, Inc.
13 NOV 1996