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Using Indexed mode effectively

Software: Canvas 5
Platform: Mac OS and Windows
 
This article discusses choosing Indexed image modes to reduce an image's memory requirements. The information presented here is important if you want to create the smallest possible ".gif" files.


Image modes and memory requirements

In Canvas, every image has an image mode. The image mode determines the color model, the number of colors, and the filters and effects you can apply to an image. Image modes also affect how much memory an image needs. You can often reduce an image's memory requirements by changing the image mode. For example, you can reduce the memory requirements for an RGB Color image by converting it to Grayscale, and reduce the memory requirements for a Grayscale image by converting it to Duotone.

The number of colors in an image is directly proportional to the amount of memory it needs. An image in Indexed mode uses up to 256 colors. Because Indexed mode uses a maximum of 256 colors, it usually requires less memory than RGB Color mode, and CMYK Color mode, both of which can use millions.

Indexed mode is often used when you save an image in GIF format, the most common for Web graphics. In Canvas, you can render an image into Indexed or Grayscale mode when you save in GIF format.

To convert an existing image to Indexed mode

1. Click the image with a Selection tool to select it.

2. Choose Mode > Indexed in the Image menu to open the Indexed Color dialog box.

3. Choose one of the following color table computational methods in the Palette area:

Exact: Canvas creates a color table from the colors in the image, if the image contains 256 colors or less.
 
System: Canvas creates a color table based on the operating system's palette of 256 colors.
 
Uniform: You choose the number of colors in the color table in the colors pop-up menu in the Colors area. You can choose 8, 27, 125, 64, 125, 216, or 256 colors. This method works best when the image contains less than 256 colors, otherwise the Adaptive method usually works best.
 
Adaptive: You enter a number of colors in the colors text box in the Colors area. Canvas creates a color table based on that number of the most frequently occurring colors in the image. When using the Adaptive method, vary the number of colors until you get the results you want.
 
Custom: Lets you load a color palette from a disk file.
 
Previous: Uses the last color table created by the Indexed Color dialog box during the current Canvas session.

4. Choose a color-distribution option in the Dither area:

None: Changes colors to their closest equivalent in the selected color table without dithering.
 
Pattern: Estimates colors not in the palette by arranging palette color in geometric patterns. This option is available if System is chosen in the Palette area.
 
Diffusion: Estimates non-palette colors by randomly dithering available colors. This option creates the most natural effect. Using fewer colors makes the diffusion effect more obvious.

5. After entering the settings you want, Click OK.

Examples

In these examples, we converted a 72 dpi image to Indexed mode seven different ways.

In example 1, we chose Adaptive using 256 colors and Diffusion dithering.

 

 (1)

Colors: 256

Palette: Adaptive

Dither: Diffusion

Size: 7,741 bytes

Format: GIF

In example 2, we chose Adaptive using 256 colors, but no dithering. Notice example 1 and 2 both display 256 colors, but the dithered example has a posterized appearence and a smaller file size.  (2)

Colors: 256

Palette: Adaptive

Dither: None

Size: 5,843 bytes

Format: GIF

In example 3, we chose Adaptive using 216 colors and no dithering. Notice there is no significant change in the appearance of the image, but the file size is slightly smaller.  (3)

Colors: 216

Palette: Adaptive

Dither: None

Size: 5,723 bytes

Format: GIF

In example 4, we chose Adaptive using only 27 colors and no dithering. Notice the posterization effects and the large reduction in file size compared with the previous examples.  (4)

Colors: 27

Palette: Adaptive

Dither: None

Size: 2,979 bytes

Format: GIF

In example 5, we chose Adaptive using 27 colors and Diffusion dithering. This improves the image quality, but increases the file size.  (5)

Colors: 27

Palette: Adaptive

Dither: Diffusion

Size: 4,057 bytes

Format: GIF

In example 6, we chose Uniform using 125 colors and Pattern dithering.  (6)

Colors: 125

Palette: Uniform

Dither: Pattern

Size: 4,052 bytes

Format: GIF 

 In example 7, we chose Uniform using 125 colors and Diffusion dithering. In examples 6 and 7, you can see the difference in the background pattern created using Diffusion and Pattern dithering.  (7)

Colors: 125

Palette: Uniform

Dither: Diffusion

Size: 6,022 bytes

Format: GIF

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