Setting the Color Depth for an Image

You can alter the color depth of an image.

Each pixel in a raster image stores color information. The amount of information within each pixel is called depth and is measured in bits. Pixels with more depth can store more information and therefore display more colors. For instance, an 8-bit pixel can display one of 256 indexed colors (or, in the case of grayscale images, shades of gray), while a 24-bit pixel can display one of 16.7 million colors.

The following table shows the color depths supported by AutoCAD Raster Design toolset and the number of colors each depth can display.

Type of Image	Color Depth	Available Color(s)
Bitonal	1-Bit	1 color
Grayscale	8-Bit	256 shades of gray
Indexed Color	8-Bit	256 colors
True Color	24-Bit	16.7 million colors

Changing the pixel depth is useful because it allows you to manipulate file size or add colors to the palette. Decreasing pixel depth decreases the number of colors that can be displayed, and decreases the file size of the image. Likewise, increasing pixel depth increases the number of colors that can be displayed and increases the file size.

The following table shows the depth conversions supported by the idepth command:

Type of Image	Bitonal	Color	Grayscale	True Color
Bitonal	N/A	Yes	Yes	Yes
Indexed Color	Yes	N/A	Yes	Yes
Grayscale	Yes	N/A	N/A	Yes
True Color	Yes	N/A	True Color	Yes

In most cases, when you convert an image for which you have already specified a transparent color, the transparent color is maintained. However, there are two exceptions:

• When converting to a bitonal image, transparent pixels are OFF if background transparency is enabled. If background transparency is disabled, transparent pixels are treated the same as other pixels.
• When converting to grayscale, a transparent color is mapped to its corresponding gray value, whether background transparency is ON or OFF.