Adobe 29180155 User Guide - Page 152

ADOBE PHOTOSHOP ELEMENTS 4.0 147 User Guide Diffusion Dither Converts an image by using an error-diffusion process, starting at the pixel in the upper left corner of the image. If the pixel's value is above middle gray (128), the pixel is changed to white-if it's below middle gray, it's changed to black. Because the original pixel is rarely pure white or pure black, error is inevitably introduced. This error is transferred to surrounding pixels and diffused throughout the image, resulting in a grainy, film-like texture. This option is useful for viewing images on a black-and-white screen. Pattern Dither conversion method, and Diffusion Dither conversion method To convert a bitmap-mode image to grayscale Before converting, keep in mind that a bitmap-mode image edited in grayscale mode may not look the same when converted back to bitmap mode. For example, consider a pixel that is black in bitmap mode and then edited to a shade of gray in grayscale mode. If the gray value of the pixel is light enough, it will become white when converted back to bitmap mode. 1 In the Editor, choose Image > Mode > Grayscale. 2 Enter a value from 1 to 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. To convert an image to indexed color Converting to indexed color reduces the number of colors in the image to a maximum of 256-the standard number of colors supported by the GIF and PNG-8 formats and many web browsers. This conversion reduces file size by deleting color information from the image. Note: To convert to indexed color, you must start with either a grayscale or an RGB image. 1 In the Editor, choose Image > Mode > Indexed Color. 2 Click OK to flatten layers. Note: This will flatten all visible layers and discard hidden layers. For grayscale images, the conversion happens automatically. For RGB images, the Indexed Color dialog box appears. 3 Select Preview in the Indexed Color dialog box to display a preview of the changes. 4 Specify any of the following conversion options and click OK. Palette Specifies the color palette to apply to the indexed-color image. There are 10 color palettes available: • Exact Creates a palette using the exact colors that appear in the RGB image-an option available only if the image uses 256 or fewer colors. Because the image's palette contains all of the 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.