6.2. Mode

The Mode submenu of the Image menu

Overview

This command can be accessed via the image menu: Image/ Mode....

It allows you to change the color mode of your image. Three modes are available:

Mode Menuitems

The RGB mode

You are normally working in RGB mode, well adapted to screen. See Glossary. You can change RGB to Grayscale or Indexed, but be careful: once the image saved you can no longer retrieve the RGB colors. Work on a copy.

The Grayscale mode

Grayscale: Converts your image to a 256 gray level image. See the glossary for mor information about the grayscale mode.

The Indexed mode

This option opens the Convert Image to Indexed Colors dialog:

Convert Image to Indexed Colors

See Glossary for explanations about the Indexed Color Mode.

General Palette Options
  • Generate Optimate Palette: This option generates the best possible palette with a default maximum number of 256 colors (classical GIF format). You can reduce this Maximum Number of Colors. This may create unwanted effects on smooth transitions. You can reduce them by using a dithering option.

  • Use Black and White (1-bit) Palette: This option generates line-art images.

  • Use Custom Palette: The button allows you to select a custom palette in a list. The number of colors is indicated for each palette. The Web palette, 216 colors, is the Web-safe palette. It was originally created by Netscape to provide colors that would look the same on both Macs and PCs and Internet Explorer 3 could manage it. Since version 4, MSIE handles a 212 colors palette... This problem of color similarity between all platforms is not solved yet and probably will never be. You have to keep to two principles in your Web page: light text on dark background or dark text on light background and never entrust an information to a color.

    Some colors of the palette may not be used if your image is poorly colored. They will be removed from the palette if the option Remove Unused Colors from the Final Palette is checked.

Dithering Options

As an indexed image contains 256 colors or less, some colors in the original image may be absent in the palette. This may result in some blotchy or solid colors in subtle color areas. These options allows you to correct these unwanted effects induced by the Palette Options.

The filter will attempt to approximate the missing color by spreading pixels of the nearest palette colors going clustering. These clustered pixels seen from a distance simulate a new color. See the glossary for more information on dithering.

Three filters are available. It's not possible to foresee what the result of a specific filter will be and you will have to try all to get the best result. The Positioned Color Dithering is well adapted to animations.

Here is the image of a smooth transition as an example.

After transformation to GIF 4 colors without dithering.

GIF 4 colors with Floyd-Sternberg (normal).

GIF 4 colors with Floyd-Sternberg-2.

In a GIF image transparency is coded on 1 bit: transparent or not transparent. To give an illusion of partial transparency use the option Enable Dithering of Transparency, but the semi-flatten plugin will give you a better result.