Photoshop
Saving Files
Colour Tables
A colour table is another term for colour palette. When you first open the Save for Web & Devices dialog and select either the GIF or PNG-8 format, a colour table is generated that is displayed in the Colour Table panel. There are numerous options for sorting, adding, deleting and editing the colour table but we're just going to look at the colour reduction algorithms. These produce different colour tables and it's a good idea to understand the differences so you can choose the one that best suits the image you're saving.
Use the Color Reduction Algorithm pop-up menu to select an option for generating a colour table. Some options generate a table based on the colours in the image and others use a predetermined palette. Each option is described below.
- Perceptual
- When building a colour table, this option gives greater weight to colours in the image for which the human eye has a greater sensitivity.
- Selective
- This is the default option. It works similar to Perceptual but additional weight is given to Web colours and colours that appear in extended areas in the image.
- Adaptive
- A colour table is created by sampling colours from the spectrum that appear in the image. Other options tend to give more weight to specific areas of the spectrum.
- Web
- This colour table consists of the 216 Web-safe colours. It includes the Web-safe colours that are common to both Windows and Macintosh operating systems. Use of these colours prevents browser dithering which can happen if colours outside the Web palette are used. The file sizes can be larger, however.
- Custom
- Retains the current palette so that any subsequent changes to the image will not change the palette.
- Black & White
- This colour table consists only of black and white.
- Greyscale
- Uses the 256-colour greyscale palette.
- Mac OS
- Uses the Macintosh operating system's default 256 colour palette.
- Windows
- Uses the Windows operating system's default 256 colour palette.
We've put together examples of images and the colour tables used to generate them as GIFs so you can compare the differences.
Colour Tables Summary
- A colour table is another name for a colour palette. A colour table is generated for images being optimized in GIF and PNG-8 format.
- A colour table can contain up to 256 colours, including one colour for transparency.
- The Perceptual, Selective and Adaptive colour tables are based on colours found in the image.
- The Custom option lets you lock in a colour table so that changes to the image won't modify the table.
- The Web, Black & White, Greyscale, Mac OS and Windows colour tables are based on a fixed set of colours.
Photoshop - TOC - Introduction - Books -
Saving Files - Links - Questions -
1 - 2 - 3 - 4 - 5 - 6 - [ 7 ] -
