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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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