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Convert an image to Bitmap mode
Converting an image
to Bitmap mode reduces the image to two colors, greatly simplifying
the color information in the image and reducing its file size.
When
converting a color image to Bitmap mode, first convert it to Grayscale mode.
This removes the hue and saturation information from the pixels
and leaves just the brightness values. However, because only a few
editing options are available for Bitmap mode images, it’s usually
best to edit the image in Grayscale mode and then convert it to
Bitmap mode.
Note: Images in Bitmap mode are 1
bit per channel. You must convert a 16‑ or 32‑bits-per-channel image
to 8‑bit Grayscale mode before converting it to Bitmap mode.
- Do one of the following:
If the image is in color, choose Image >
Mode > Grayscale. Then choose Image > Mode >
Bitmap.
If the image is grayscale, choose Image >
Mode > Bitmap.
- For Output, enter a value for the output resolution of
the Bitmap mode image, and choose a unit of measurement. By default,
the current image resolution appears as both the input and the output
resolutions.
- Choose one of the following bitmap conversion methods
from the Use pop-up menu:
- 50% Threshold
- Converts pixels with gray values above the middle gray
level (128) to white and pixels with gray values below that level
to black. The result is a very high-contrast, black-and-white representation
of the image.
- Pattern Dither
- Converts an image by organizing the gray levels into
geometric configurations of black and white dots.
- 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 it, 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.
- Halftone Screen
- Simulates the appearance of halftone dots in the converted image.
Enter values in the Halftone Screen dialog box:
For Frequency, enter a value for the screen frequency, and
choose a unit of measurement. Values can range from 1.000 to 999.999
for lines per inch and from 0.400 to 400.00 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.
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.
For Shape, choose the dot shape you want.
Important: The halftone screen becomes part of the image.
If you print the image on a halftone printer, the printer will use
its own halftone screen as well as the halftone screen that is part
of the image. On some printers, the result is a moiré pattern.
- Custom Pattern
- Simulates
the appearance of a custom halftone screen in the converted image.
Choose a pattern that lends itself to thickness variations, typically
one with a variety of gray shades.
To use this option, you
first define a pattern and then screen the grayscale image to apply
the texture. To cover the entire image, the pattern must be as large
as the image. Otherwise, the pattern is tiled. Photoshop comes with
several self-tiling patterns that can be used as halftone screen
patterns.
 To prepare a black-and-white pattern
for conversion, first convert the image to grayscale and then apply
the Blur More filter several times. This blurring technique creates
thick lines tapering from dark gray to white.
 Original grayscale image, and 50% Threshold conversion method  Pattern Dither conversion method, and Diffusion Dither conversion
method
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