EXPLORE: Web colours

Optimising your colour table in Photoshop

You can change the palette - or set of colors - in the color table by selecting a color reduction option in the Optimize panel/palette. There are three categories of options:

• Dynamic options use a color reduction algorithm to build a palette based on the colors in the image and the number of colors specified in the optimization setting. The colors in the palette are regenerated every time you change or reoptimize the image. Perceptual, Selective, and Adaptive are dynamic options.
  
• Fixed options use a set palette of colors. In other words, the set of available colors is constant, but the actual colors in the palette will vary depending on the colors in the image. Web, Mac OS, Windows, Black & White, and Grayscale tables are fixed options.
  
• The Custom option 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.

To select a color reduction algorithm in Photoshop:
Choose a colour table option from the Color Reduction Algorithm pop-up menu (below the file format menu in the Optimize panel/palette):

Properties of colour tables

Adaptive Creates a palette by sampling the colours from the spectrum appearing most commonly in the image. For example, an RGB image with only the colours green and blue produces a palette made primarily of greens and blues. Most images concentrate colours in particular areas of the spectrum.

Perceptual Creates a custom colour table by giving priority to colours for which the human eye has greater sensitivity.

Selective Creates a colour table similar to the Perceptual colour table, but favouring broad areas of colour and the preservation of Web colours. This colour table usually produces images with the greatest colour integrity. Selective is the default option in Photoshop.

Exact Creates a palette using the exact colours appearing in the RGB imagean option available only if the image uses 256 or fewer colours. Because the image's palette contains all colours in the image, there is no dithering.

If a picture contains fewer than 256 colours, the exact palette for that picture will contain only those colours actually in the picture. However, they may not be the 216 Web safe colours.

Web Uses the palette most often used by Web browsers to display 8-bit images. This palette is a subset of the Windows and Macintosh systems' palettes.

System allows you to save the 256 colours specific to either the Windows or Macintosh platform.

Weighted optimisation

Weighted optimization lets you smoothly vary optimization settings across an image using an alpha channel. This technique produces higher-quality results in critical image areas without sacrificing file size. With weighted optimization, you can produce gradual variations in GIF dithering, lossy GIF settings, and JPEG compression. Weighted optimization also lets you favor colors in selected image areas when you generate a color table.

About alpha channels and weighted optimization

Alpha channels lets you store selections as grayscale images called masks. When you use an alpha channel to apply optimization settings, the white areas of the mask describe the highest level of image quality, while the black areas for the mask describe the lowest level of image quality.

Weighted optimization is available for specific settings in the Optimize panel/palette, as indicated by the channel button (). This button becomes available when your image has an alpha channel.

To create a channel for use during optimization
Use a selection tool to select an area of the image. Save the selection (Select: Save selection), or create a new alpha channel using the 'Channels' window, and use the painting and editing tools to modify it. This channel will then be available for weighting the optimisation of your web image.