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Bitmap Image Information

Quartz supports a wide variety of image formats and has built-in knowledge of several popular formats. In Mac OS X v10.1, Quartz provides a convenience function for working with bitmap images that use JPEG-encoded data. A similar function is available for working with PNG-encoded data starting with Mac OS X v10.2. Mac OS X v10.4 supports many formats, including JPEG, PNG, TIFF, GIF, and JPEG2000. Other bitmap image formats or proprietary formats require that you specify details about the image format to Quartz in order to ensure that images are interpreted correctly.

This section describes the information associated with a bitmap image. When you create and work with Quartz images (which use the CGImageRef data type), you’ll see that some Quartz image-creation functions require you to specify all this information, while other functions require a subset of this information. What you provide depends on the encoding used for the bitmap data, and whether the bitmap represents an image or an image mask.

Quartz uses the following information when it creates a bitmap image (CGImageRef):

• A bitmap data source, which can be a Quartz data provider or a Quartz image source. “Data Management” describes both and discusses the functions that provide a source of bitmap data.

• An optional “Decode Array.”

• An interpolation setting, which is a Boolean value that specifies whether Quartz should apply an interpolation algorithm when resizing the image.

• A rendering intent that specifies how to map colors that are located within the destination color space of a graphics context. This information is not needed for image masks. See “Setting Rendering Intent” for more information.

• The image dimensions.

• “Pixel Format,” which includes bits per component, bits per pixel, and bytes per row.

• For images, “Color Spaces and Bitmap Layout” information to describe the location of alpha and whether the bitmap uses floating-point values. Image masks don’t require this information.

Decode Array

A decode array maps the image color values to other color values, which is useful for such tasks as desaturating an image or inverting the colors. The array contains a pair of numbers for each color component. When Quartz renders the image, it applies a linear transform to map the original component value to a relative number within the designated range appropriate for the destination color space. For example, the decode array for an image in the RGB color space contains six entries, one pair for each red, green, and blue color component.

Pixel Format

The pixel format consists of the following information:

• Bits per component, which is the number of bits in each individual color component in a pixel. For an image mask, this value is the number of significant masking bits in a source pixel. For example, if the source image is an 8-bit mask, specify 8 bits per component.

• Bits per pixel, which is the total number of bits in a source pixel. This value must be at least the number of bits per component times the number of components per pixel.

• Bytes per row. The number of bytes per horizontal row in the image.

Color Spaces and Bitmap Layout

To ensure that Quartz correctly interprets the bits of each pixel, you must specify:

• Whether a bitmap contains an alpha channel. Quartz supports RGB, CMYK, and Gray color spaces. It also supports alpha, or transparency, although alpha information is not available in all bitmap image formats. When it is available, the alpha component can be located in either the most significant bits of a pixel or the least significant bits.

• For bitmaps that have an alpha component, whether the color components are already multiplied by the alpha value. Premultiplied alpha describes a source color whose components are already multiplied by an alpha value. Premultiplying speeds up the rendering of an image by eliminating an extra multiplication operation per color component. For example, in an RGB color space, rendering an image with premultiplied alpha eliminates three multiplication operations (red times alpha, green times alpha, and blue times alpha) for each pixel in the image.

• The data format of the samples—integer or floating-point values. Quartz supports floating-point formats starting in Mac OS X v10.4.

When you create an image using the function CGImageCreate, you supply a bitmapInfo parameter, of type CGImageBitmapInfo, to specify bitmap layout information. The following constants specify the location of the alpha component and whether the color components are premultiplied:

• kCGImageAlphaLast—the alpha component is stored in the least significant bits of each pixel, for example, RGBA.

• kCGImageAlphaFirst—the alpha component is stored in the most significant bits of each pixel, for example, ARGB.

• kCGImageAlphaPremultipliedLast—the alpha component is stored in the least significant bits of each pixel, and the color components have already been multiplied by this alpha value.

• kCGImageAlphaPremultipliedFirst—the alpha component is stored in the most significant bits of each pixel, and the color components have already been multiplied by this alpha value.

• kCGImageAlphaNoneSkipLast—there is no alpha channel. If the total size of the pixel is greater than the space required for the number of color components in the color space, the least significant bits are ignored.

• kCGImageAlphaNoneSkipFirst—there is no alpha channel. If the total size of the pixel is greater than the space required for the number of color components in the color space, the most significant bits are ignored.

• kCGImageAlphaNone—equivalent to kCGImageAlphaNoneSkipLast.

You use the constant kCGBitmapFloatComponents to indicate a bitmap format that uses floating-point values. For floating-point formats, you logically OR this constant with the appropriate constant from the previous list. For example, for a 128 bits per pixel floating-point format that uses premultiplied alpha, with the alpha located in the least significant bits of each pixel, you supply the following information to Quartz:

kCGImageAlphaPremultipliedLast|kCGBitmapFloatComponents

Figure 11-2 visually depicts how pixels are represented in CMYK and RGB color spaces that use 16- or 32-bit integer formats. The 32-bit integer pixel formats use 8 bits per component. The 16-bit integer format uses 5 bits per component. As of Mac OS X v10.4, Quartz 2D also supports 128-bit floating-point pixel formats that use 32 bits per component. The 128-bit formats are not shown in the figure.

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