Adobe 13101332 - Photoshop - Mac User Manual page 96

Adobe Photoshop Help
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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 below, 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, filmlike texture. This
option is useful for viewing images on a black-and-white screen.
Original grayscale image, and 50% Threshold conversion method
Pattern Dither conversion method, and Diffusion Dither conversion method
Specifying halftone screens for Bitmap-mode images
The Halftone Screen option in the Bitmap dialog box lets you convert a grayscale image to
simulated halftone dots.
Important: The halftone screen becomes part of the image. If you print the image on a
halftone printer, it will print with the halftone screen you choose here plus that of the
printer. On some printers, the result probably will be a moiré pattern in the image.
To specify the halftone screen for a Bitmap-mode image:
1 Choose Image > Mode > Bitmap.
2 Click Halftone Screen; then click OK.
3 For Frequency, enter a value for the screen frequency, and choose a unit of
measurement. Values can range from 1 to 999 for lines per inch and from 0.400 to 400 for
lines per centimeter. You can enter decimal values.
The screen frequency specifies the ruling of the halftone screen in lines per inch (lpi).
The frequency depends on the paper stock and type of press used for printing.
Newspapers commonly use an 85-line screen. Magazines use higher resolution screens,
such as 133 lpi and 150 lpi. Check with your print shop for correct screen frequencies.
4 Enter a value for the screen angle in degrees from –180 to +180.
The screen angle refers to the orientation of the screen. Continuous-tone and black-and-
white halftone screens commonly use a 45° angle.
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