The ZSoft Paintbrush (*.pcx) is one of the original graphics formats.
The dialog allows you to set the bits per pixel i.e. how many colors are to be used to constitute the image: 1-bit for 2 colors (typically black and white), 4-bits for 16 colors, 8-bits for 256 colors, etc.
If you click on 8-bits, and make sure Color and User palette are selected, the Define button is activated. Click on this to enter the Color Table box. You can choose from a variety of color ranges from the drop-down list in the Table line at the top of the box: Custom, Black Body, Grayscale, Macintosh System, Spectrum, Windows System, Web. Each of these presents a different color palette, the image itself using only the colors that are available to it from the chosen palette.
The Dither Type i.e. either Color or Gray, refers to how the image format tries to simulate the colors when moving from a more highly defined image format (i.e. greater bits per pixel) to a less highly defined one (i.e. fewer bits per pixel). For instance, a patch of turquoise may be rendered by a combination of blue and green pixels as an approximation. If you select Gray the dithering will use gray-scale values instead.
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