Optimization options for Images and Animated GIFs

Web graphics formats can be either bitmap (raster) or vector. The bitmap formats—GIF, JPEG, and PNG—are resolution-dependent, meaning that a bitmap image’s dimensions, and possibly image quality, will change at different monitor resolutions. The vector formats—SVG and SWF—are resolution-independent and can be scaled up or down without losing any image quality. The vector formats can also include raster data. 

JPEG optimization options

JPEG is the standard format for compressing continuous-tone images such as photographs. Optimizing an image as a JPEG format relies on lossy compression, which selectively discards data.

Determines the level of compression. The higher the Quality setting, the more detail the compression algorithm preserves. However, using a high Quality setting results in a larger file size than using a low Quality setting. View the optimized image at several quality settings to determine the best balance of quality and file size.

Creates an enhanced JPEG with a slightly smaller file size. The Optimized JPEG format is recommended for maximum file compression; however, some older browsers do not support this feature.

Displays the image progressively in a web browser. The image appears as a series of overlays, enabling viewers to see a low-resolution version of the image before it downloads completely. The Progressive option requires use of the Optimized JPEG format.

 Progressive JPEGs require more RAM for viewing and are not supported by some browsers.

Specifies the amount of blur to apply to the image. This option applies an effect identical to that of the Gaussian Blurfilter and allows the file to be compressed more, resulting in a smaller file size. A setting of 0.1 to 0.5 is recommended.

Preserves color profiles in the optimized file. Some browsers use color profiles for color correction.

Specifies a fill color for pixels that were transparent in the original image. Click the Matte color swatch to select a color in the color picker, or select an option from the Matte menu: Eyedropper Color (to use the color in the eyedropper sample box), Foreground Color,Background Color, White, Black, or Other (to use the color picker).

Pixels that were fully transparent in the original image are filled with the selected color; pixels that were partially transparent in the original image are blended with the selected color.

GIF and PNG‑8 optimization options

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, a color lookup table is built to store and index 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.

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%.

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

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.

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.

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.

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 Color (to use the color in the eyedropper sample box), Foreground Color, Background Color, White, Black, or Other (to use the color picker).

 The Foreground Color and Background Color options are only available in Photoshop.

A. Original image B. Transparency selected with a matte color C. Transparency selected with no matting D. Transparency deselected with a matte color

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.

• DiffusionTransparency 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.

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.

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.

Optimize transparency in GIF and PNG images

Transparency makes it possible to create nonrectangular images for the web. Background transparency preserves transparent pixels in the image.

This allows the background of the web page to show through the transparent areas of your image. Background matting simulates transparency by filling or blending transparent pixels with a matte color that can match the web page background. Background matting works best if the web page background is a solid color and if you know what that color is.

Use the Transparency and Matte options in the Save For Web & Devices dialog box to specify how transparent pixels in GIF and PNG images are optimized.

• (GIF and PNG‑8) 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.
• (GIF and PNG‑8) To make all pixels with greater than 50% transparency fully transparent and all pixels with 50% or less transparency fully opaque, select Transparency and select None from the Matte menu.
• (PNG‑24) To save an image with multilevel transparency (up to 256 levels), select Transparency. The Matte option is disabled since multilevel transparency allows an image to blend with any background color.

 In browsers that do not support PNG‑24 transparency, transparent pixels may be displayed against a default background color, such as gray.

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 Color (to use the color in the eyedropper sample box), Foreground Color, Background Color, White, Black, or Other (to use the color picker).

 The Foreground Color and Background Color options are only available in Photoshop.

View the color table for an optimized slice

The color table for a slice appears in the Color Table panel in the Save For Web & Devices dialog box.

Select a slice that is optimized in GIF or PNG‑8 format. The color table for the selected slice appears in the SaveFor Web & Devices color table.

If an image has multiple slices, the colors in the color table may vary between slices (you can link the slices first to prevent this from happening). If you select multiple slices that use different color tables, the color table is empty and its status bar displays the message “Mixed.”

Customize the color table for GIF and PNG‑8 images

You use the color table in the Save For Web & Devices dialog box to customize the colors in optimized GIF and PNG‑8 images. Reducing the number of colors often preserves image quality while reducing the file size of the image.

You can add and delete colors in the color table, shift selected colors to web‑safe colors, and lock selected colors to prevent them from being dropped from the palette.

Sort a color table

Choose a sorting order from the Color Tablepalette menu:

• Unsorted restores the original sorting order.
• Sort By Hue sorts by the location of the color on the standard color wheel (expressed as a degree from 0 to 360). Neutral colors are assigned a hue of 0 and located with the reds.
• Sort By Luminance sorts by the lightness or brightness of a color.
• Sort By Popularity sorts by the colors’ frequency of occurrence in the image.

Add a new color to the color table

You can add colors that were left out in building the color table. Adding a color to a dynamic table shifts the color in the palette closest to the new color. Adding a color to a fixed or Custom table adds an additional color to the palette.

Select colors in the color table

A white border appears around selected colors in the Color Table.

• To select a color, click the color in the Color Table.
• To select multiple colors in the color table, press Shift and click another color. All colors in the rows between the first and second selected colors are selected. To select a non-adjacent group of colors, press Ctrl (Windows) or Command (Mac OS) and click each color that you want to select. The Color Table palette menu also provides commands for selecting colors.
• To select a color in the preview image, click in the preview with the Save For Web & Devices Eyedropper tool. Shift‑click to select additional colors.
• To deselect all colors, choose Deselect All Colors from the Color Tablepalette menu.

Shift a color

You can change a selected color in the color table to any other RGB color value. When you regenerate the optimized image, the selected color changes to the new color wherever it appears in the image.

Shift colors to the closest web palette equivalent

To protect colors from dithering in a browser, you can shift the colors to their closest equivalents in the web palette. This ensures that the colors won’t dither when displayed in browsers on either Windows or Macintosh operating systems capable of displaying only 256 colors.

Map colors to transparency

You can add transparency to an optimized image by mapping existing colors to transparency.

Lock or unlock a color

You can lock selected colors in the color table to prevent them from being dropped when the number of colors is reduced and to prevent them from dithering in the application.

 Locking colors does not prevent them from dithering in a browser.

Delete selected colors

You can delete selected colors from the color table to decrease the image file size. When you delete a color, areas of the optimized image that previously included that color are rerendered using the closest color remaining in the palette.

When you delete a color, the color table automatically changes to a Custom palette. This is because the Adaptive, Perceptual, and Selective palettes automatically add the deleted color back into the palette when you reoptimize the image—the Custom palette does not change when you reoptimize the image.

Save a color table

You can save color tables from optimized images to use with other images and to load color tables created in other applications. Once you load a new color table into an image, the colors in the optimized image are changed to reflect the colors in the new color table.

Load a color table

PNG‑24 optimization options

PNG‑24 is suitable for compressing continuous-tone images; however, it produces much larger files than JPEG format. The advantage of using PNG‑24 is that it can preserve up to 256 levels of transparency in an image.

Determine how transparent pixels in the image are optimized.

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.