GIF is the standard format for compressing images with flat color and crisp detail, such as line art, logos, or illustrations with type. Like the GIF format, the PNG‑8 format efficiently compresses solid areas of color while preserving sharp detail.

PNG‑8 and GIF files support 8‑bit color, so they can display up to 256 colors. The process of determining which colors to use is called indexing, so images in GIF and PNG‑8 formats are sometimes called indexed color images. To convert an image to indexed color, Photoshop builds a color lookup table, which stores and indexes the colors in the image. If a color in the original image does not appear in the color lookup table, the application either chooses the closest color in the table or simulates the color using a combination of available colors.

In addition to the following options, you can also adjust the number of colors in the image’s color table. See Customize the color table for GIF and PNG‑8 images.

Lossy (GIF only)

Reduces file size by selectively discarding data. A higher Lossy setting results in more data being discarded. You can often apply a Lossy value of 5–10, and sometimes up to 50, without degrading the image. The Lossy option can reduce file size by 5% to 40%.

Note: You cannot use the Lossy option with the Interlaced option or with Noise or Pattern Dither algorithms.

Color Reduction Method and Colors

Specifies a method for generating the color lookup table and the number of colors you want in the color lookup table. You can select one of the following color reduction methods:

Perceptual

Creates a custom color table by giving priority to colors for which the human eye has greater sensitivity.

Selective

Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of web colors. This color table usually produces images with the greatest color integrity. Selective is the default option.

Adaptive

Creates a custom color table by sampling colors from the predominant spectrum in the image. For example, an image with only the colors green and blue produces a color table made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum.

(Restrictive) Web

Uses the standard 216‑color color table common to the Windows and Mac OS 8‑bit (256‑color) palettes. This option ensures that no browser dither is applied to colors when the image is displayed using 8‑bit color. (This palette is also called the web‑safe palette.) Using the web palette can create larger files, and is recommended only when avoiding browser dither is a high priority.

Custom

Uses a color palette that is created or modified by the user. If you open an existing GIF or PNG‑8 file, it will have a custom color palette.

Use the Color Table palette in the Save For Web & Devices dialog box to customize the color lookup table.

Black and White, Grayscale, Mac OS, Windows

Use a set palette of colors.

Dithering Method and Dither

Determines the method and amount of application dithering. Dithering refers to the method of simulating colors not available in the color display system of your computer. A higher dithering percentage creates the appearance of more colors and more detail in an image, but can also increase the file size. For optimal compression, use the lowest percentage of dither that provides the color detail you require. Images with primarily solid colors may work well with no dither. Images with continuous-tone color (especially color gradients) may require dithering to prevent color banding.

GIF image with 0% dither (left), and with 100% dither (right)

You can select one of the following dithering methods:

Diffusion

Applies a random pattern that is usually less noticeable than Pattern dither. The dither effects are diffused across adjacent pixels.

Pattern

Applies a halftone-like square pattern to simulate any colors not in the color table.

Noise

Applies a random pattern similar to the Diffusion dither method, but without diffusing the pattern across adjacent pixels. No seams appear with the Noise dither method.

Transparency and Matte

Determines how transparent pixels in the image are optimized.

• To make fully transparent pixels transparent and blend partially transparent pixels with a color, select Transparency and select a matte color.

• To fill fully transparent pixels with a color and blend partially transparent pixels with the same color, select a matte color and deselect Transparency.

• To select a matte color, click the Matte color swatch and select a color in the color picker. Alternatively, select an option from the Matte menu: Eyedropper (to use the color in the eyedropper sample box), Foreground Color, Background Color, White, Black, or Other (to use the color picker).

Examples of transparency and matting

A.

Original image

B.

Transparency selected with a matte color

C.

Transparency selected with no matting

D.

Transparency deselected with a matte color

Transparency Dithering

When the Transparency option is selected, you can choose a method for dithering partially transparent pixels:

• No Transparency Dither applies no dither to partially transparent pixels in the image.

• Diffusion Transparency Dither applies a random pattern that is usually less noticeable than Pattern dither. The dither effects are diffused across adjacent pixels. If you select this algorithm, specify a Dither percentage to control the amount of dithering that is applied to the image.

• Pattern Transparency Dither applies a halftone-like square pattern to partially transparent pixels.

• Noise Transparency Dither applies a random pattern similar to the Diffusion algorithm, but without diffusing the pattern across adjacent pixels. No seams appear with the Noise algorithm.

Example of Pattern Transparency dithering (left) and applied to a web page background (right)

Interlace

Displays a low-resolution version of the image in a browser while the full image file is downloading. Interlacing can make downloading time seem shorter and can assure viewers that downloading is in progress. However, interlacing also increases file size.

Web Snap

Specifies a tolerance level for shifting colors to the closest web palette equivalents (and prevent the colors from dithering in a browser). A higher value shifts more colors.