Optimizing Images for the Web > Optimizing images > Setting optimization options for GIF and PNG-8 formats
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Setting optimization options for GIF and PNG-8 formats


GIF is the standard format for compressing images with flat color and crisp detail, such as line art, logos, or illustrations with type. (See About GIF format.) Like the GIF format, PNG-8 efficiently compresses solid areas of color while preserving sharp detail; however, not all Web browsers can display PNG-8 files. (See About PNG-8 format.)
Optimization options for GIF format: A. Optimization format B. Color reduction algorithm C. Dither algorithm

GIF format and PNG-8 format can use up to 256 colors to describe an image. The process of determining which colors to use is called indexing (which is why images in GIF and PNG-8 formats are sometimes called indexed color images). To convert an image to indexed color, Photoshop Elements 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 program chooses the closest one or simulates the color using available colors.

To optimize an image in GIF or PNG-8 format:

1 Choose GIF or PNG-8 for the optimization format.

2 Choose a color reduction algorithm for generating the color lookup table:

  • Perceptual to create a custom color table by giving priority to colors for which the human eye has greater sensitivity.
  • Selective to create 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 to create a custom color table by sampling colors from the spectrum appearing most commonly 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.
  • Web to use 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.) If your image has fewer colors than the total number specified in the color palette, unused colors are removed. Using the Web palette can create larger files, and is recommended only when avoiding browser dither is a high priority. (See Previewing and controlling dithering.)
  • Custom to preserve the current color table as a fixed palette that does not update with changes to the image.

    • 3 To specify the maximum number of colors in the color palette, select a number from the Colors pop-up menu, enter a value in the text box, or use the arrows to change the number of colors. If the image contains fewer colors than the number specified, the color table will contain only the number of colors in the image.

    You can choose the Auto option when working with a fixed color palette. This option determines the number of colors in the color table based on the frequency of colors in the image. Choose Auto if you want Photoshop Elements to determine the optimal number of colors in the color table.

    4 Choose a dithering algorithm option, and specify a percentage for Dither. (See Previewing and controlling dithering.)

    5 If the image contains transparency, select Transparency to preserve transparent pixels as transparent; deselect Transparency to fill fully and partially transparent pixels with the Matte color. (See Making transparent and matted images.)

    6 Select Interlaced to create an image that displays as low-resolution versions in a browser while the full image file is downloading. Interlacing can make downloading time seem shorter and assures viewers that downloading is in progress.

    Optimizing Images for the Web > Optimizing images > Setting optimization options for GIF and PNG-8 formats