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Devices (displays, printers, scanners, cameras) don’t treat color the same way; each has its own range of colors that the device can produce faithfully. A color produced on one device might not be able to be produced on another device.

To work with color effectively and to understand the Quartz 2D functions for using color spaces and color, you should be familiar with the terminology discussed in Color Management Overview. That document discusses color perception, color values, device-independent and device color spaces, the color-matching problem, rendering intent, color management modules, and ColorSync.

In this chapter, you’ll learn how Quartz represents color and color spaces, and what the alpha component is. This chapter also discusses how to:

Create color spaces
Create and set colors
Set rendering intent

About Color and Color Spaces

A color in Quartz is represented by a set of values. The values are meaningless without a color space that dictates how to interpret color information. For example, the values in Table 4-1 all represent the color blue at full intensity. But without knowing the color space or the allowable range of values for each color space, you have no way of knowing which color each set of values represents.

**Table 4-1** Color values in different color spaces
Values	Color space	Components
240 degrees, 100%, 100%	HSB	Hue, saturation, brightness
0, 0, 1	RGB	Red, green, blue
1, 1, 0, 0	CMYK	Cyan, magenta, yellow, black
1, 0, 0	BGR	Blue, green, red

If you provide the wrong color space, you can get quite dramatic differences, as shown in Figure 4-1. Although the green color is interpreted the same in BGR and RGB color spaces, the red and blue values are flipped.

Color spaces can have different numbers of components. Three of the color spaces in the table have three components, while the CMYK color space has four. Value ranges are relative to that color space. For most color spaces, color values in Quartz range from 0.0 to 1.0, with 1.0 meaning full intensity. For example, the color blue at full intensity, specified in the RGB color space in Quartz, has the values (0, 0, 1.0). In Quartz, color also has an alpha value that specifies the transparency of a color. The color values in Table 4-1 don’t show an alpha value.

The Alpha Value

The alpha value is the graphics state parameter that Quartz uses to determine how to composite newly painted objects to the existing page. At full intensity, newly painted objects are opaque. At zero intensity, newly painted objects are invisible. Figure 4-2 shows five large rectangles, drawn using alpha values of 1.0, 0.75, 0.5, 0.1, and 0.0. As the large rectangle becomes transparent, it exposes a smaller, opaque red rectangle drawn underneath.

You can make both the objects on the page and the page itself transparent by setting the alpha value globally in the graphics context before painting. Figure 4-3 compares a global alpha setting of 0.5 with the default value of 1.0.

In the normal blend mode (which is the default for the graphics state) Quartz performs alpha blending by combining the components of the source color with the components of the destination color using the formula:

destination = (alpha * source) + (1 - alpha) * destination

where source is one component of the new paint color and destination is one component of the background color. This formula is executed for each newly painted shape or image.

For object transparency, set the alpha value to 1.0 to specify that objects you draw should be fully opaque; set it to 0.0 to specify that newly drawn objects are fully transparent. An alpha value between 0.0 and 1.0 specifies a partially transparent object. You can supply an alpha value as the last color component to all routines that accept colors. You can also set the global alpha value using the CGContextSetAlpha function. Keep in mind that if you set both, Quartz multiplies the alpha color component by the global alpha value.

To allow the page itself to be fully transparent, you can explicitly clear the alpha channel of the graphics context using the CGContextClearRect function, as long as the graphics context is a window or bitmap graphics context. You might want to do this when creating a transparency mask for an icon, for example, or to make the background of a window transparent.

Creating Color Spaces

Quartz supports the standard color spaces used by color management systems for device-independent color spaces and also supports generic, indexed, and pattern color spaces. Device-independent color spaces represent color in a way that is portable between devices. They are used for the interchanges of color data from the native color space of one device to the native color space of another device. Colors in a device-independent color space appear the same when displayed on different devices, to the extent that the capabilities of the device allow. For that reason, device-independent color spaces are your best choice for representing color.

Applications that have precise color requirements should always use a device-independent color space. A common device independent color space is the generic color space. Generic color spaces let the operating system provide the best color space for your application. Drawing to the display looks as good as printing the same content to a printer.

Important: iOS does not support device-independent or generic color spaces. iOS applications must use device color spaces instead.

Creating Device-Independent Color Spaces

To create a device-independent color space, you provide Quartz with the reference white point, reference black point, and gamma values for a particular device. Quartz uses this information to convert colors from your source color space into the color space of the output device.

The device-independent color spaces supported by Quartz, and the functions that create them are:

L*a*b* is a nonlinear transformation of the Munsell color notation system (a system that specifies colors by hue, value, and saturation—or chroma —values). This color space matches perceived color difference with quantitative distance in color space. The L* component represents the lightness value, the a* component represents values from green to red, and the b* component represents values from blue to yellow. This color space is designed to mimic how the human brain decodes color. Use the function CGColorSpaceCreateLab.
ICC is a color space from an ICC color profile, as defined by the International Color Consortium. ICC profiles define the gamut of colors supported by a device along with other device characteristics so that this information can be used to accurately transform the color space of one device to the color space of another. The manufacturer of the device typically provides an ICC profile. Some color monitors and printers contain embedded ICC profile information, as do some bitmap formats such a

About Color and Color Spaces

The Alpha Value

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Creating Color Spaces

Creating Device-Independent Color Spaces