In Photoshop you can easily create an image in one color mode and convert it to another, perhaps to get it ready for a specific print job.
You can change an image from its original mode (source mode) to a different mode (target mode). When you choose a different color mode for an image, you permanently change the color values in the image. For example, when you convert an RGB image to CMYK mode, RGB color values outside the CMYK gamut (defined by the CMYK working space setting in the Color Settings dialog box) are adjusted to fall within gamut. As a result, some image data may be lost and can’t be recovered if you convert the image from CMYK back to RGB.
Before converting images, it’s best to do the following:
Do as much editing as possible in the original image mode (usually RGB for images from most scanners or digital cameras, or CMYK for images from traditional drum scanners or imported from a Scitex system).
Save a backup copy before converting. Be sure to save a copy of your image that includes all layers so that you can edit the original version of the image after the conversion.
Flatten the file before converting it. The interaction of colors between layer blending modes changes when the mode changes.
In most cases, you’ll want to flatten a file before converting it. However, it isn't required and, in some cases, it isn’t desirable (for example, when the file has vector text layers).
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.
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.
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.
Converts an image by organizing the gray levels into geometric configurations of black and white dots.
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.
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.
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.
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.
The technique above minimizes file size but discards color information and can convert adjacent colors to the exact same shade of gray. Using a Black & White adjustment layer increases file size but retains color information, letting you map colors to shades of gray.
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.
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.
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.
When converting an RGB image to indexed color, you can specify a number of conversion options in the Indexed Color dialog box.
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:
Creates a palette by uniformly sampling colors from the RGB color cube. For example, if Photoshop takes six evenly-spaced color levels each of red, green, and blue, the combination produces a uniform palette of 216 colors (6 cubed = 6 x 6 x 6 = 216). The total number of colors displayed in an image corresponds to the nearest perfect cube (8, 27, 64, 125, or 216) that is less than the value in the Colors text box.
Creates a palette by sampling the colors from the spectrum appearing most commonly in the image. For example, an RGB image with only the colors green and blue produces a palette made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum. To control a palette more precisely, first select a part of the image containing the colors you want to emphasize. Photoshop weights the conversion toward these colors.
Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of web colors. This option usually produces images with the greatest color integrity. Applies when you have multiple documents open; takes all open documents into account.
Creates a palette by sampling the colors from the spectrum appearing most commonly in the image. For example, an RGB image with only the colors green and blue produces a palette made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum. To control a palette more precisely, first select a part of the image containing the colors you want to emphasize. Photoshop weights the conversion toward these colors. Applies when you have multiple documents open; takes all open documents into account.
Creates a custom palette using the Color Table dialog box. Either edit the color table and save it for later use or click Load to load a previously created color table. This option also displays the current Adaptive palette, which is useful for previewing the colors most often used in the image.
Number Of Colors
For the Uniform, Perceptual, Selective, or Adaptive palette, you can specify the exact number of colors to be displayed (up to 256) by entering a value for Colors. The Colors text box controls only how the indexed color table is created. Adobe Photoshop still treats the image as an 8‑bit, 256‑color image.
Color Inclusion And Transparency
To specify colors to be included in the indexed color table or to specify transparency in the image, choose from the following options:
Provides options to force the inclusion of certain colors in the color table. Black And White adds a pure black and a pure white to the color table; Primaries adds red, green, blue, cyan, magenta, yellow, black, and white; Web adds the 216 web‑safe colors; and Custom lets you define custom colors to add.
Specifies the background color used to fill anti-aliased edges that lie adjacent to transparent areas of the image. When Transparency is selected, the matte is applied to edge areas to help blend the edges with a web background of the same color. When Transparency is deselected, the matte is applied to transparent areas. Choosing None for the matte creates hard-edged transparency if Transparency is selected; otherwise, all transparent areas are filled with 100% white. The image must have transparency for the Matte options to be available.
Unless you’re using the Exact color table option, the color table may not contain all the colors used in the image. To simulate colors not in the color table, you can dither the colors. Dithering mixes the pixels of the available colors to simulate the missing colors. Choose a dither option from the menu, and enter a percentage value for the dither amount. A higher amount dithers more colors but may increase file size. You can choose from the following dither options: