Adobe Photoshop CS4 - Manual (Page 122)

115 USING PHOTOSHOP CS4
Color fundamentals
Last updated 1/10/2010
Pattern Dither conversion method, and Diffusion Dither conversion method
Convert a Bitmap mode image to Grayscale mode
You can convert a Bitmap mode image to Grayscale mode in order to edit it. Keep in mind that a Bitmap mode image
edited in Grayscale mode may not look the same when you convert it back to Bitmap mode. For example, suppose a
pixel that is black in Bitmap mode is edited to a shade of gray in Grayscale mode. When the image is converted back
to Bitmap mode, that pixel is rendered as white if its gray value is above the middle gray value of 128.
1 Choose Image > Mode > Grayscale.
2 Enter a value between 1 and 16 for the size ratio.
The size ratio is the factor for scaling down the image. For example, to reduce a grayscale image by 50%, enter 2 for the
size ratio. If you enter a number greater than 1, the program averages multiple pixels in the Bitmap mode image to
produce a single pixel in the grayscale image. This process lets you generate multiple shades of gray from an image
scanned on a 1-bit scanner.
Convert a grayscale or RGB image to indexed color
Converting to indexed color reduces the number of colors in the image to at most 256—the standard number of colors
supported by the GIF and PNG-8 formats and many multimedia applications. This conversion reduces file size by
deleting color information from the image.
To convert to indexed color, you must start with an image that is 8 bits per channel and in either Grayscale or RGB mode.
1 Choose Image > Mode > Indexed Color.
Note: All visible layers will be flattened; any hidden layers will be discarded.
For grayscale images, the conversion happens automatically. For RGB images, the Indexed Color dialog box appears.
2 Select Preview in the Indexed Color dialog box to display a preview of the changes.
3 Specify conversion options.
Conversion options for indexed-color images
When converting an RGB image to indexed color, you can specify a number of conversion options in the Indexed
Color dialog box.
Palette Type A number of palette types are available for converting an image to indexed color. For the Perceptual,
Selective, and Adaptive options, you can choose using a local palette based on the current image’s colors. These are the
available palette types:
• Exact Creates a palette using the exact colors appearing in the RGB image—an option available only if the image
uses 256 or fewer colors. Because the image’s palette contains all colors in the image, there is no dithering.
• System (Mac OS) Uses the Mac OS default 8-bit palette, which is based on a uniform sampling of RGB colors.