Colors, Palettes and Dot Patterns

Color Models.

ATOMS is designed to give the best quality display and output for the most common types of screen display, printers and plotters. However, because these devices vary widely in their capabilities for displaying color, it may be necessary to understand several different color models. The models are best discussed in terms of the number of bits used to represent each pixel in a raster or bit-mapped display or output.

1-bit or black-and-white. This is used for two-color or black-and-white printing and files destined to be printed. Each pixel is either off (white) or on (black). For filled areas dot-patterns with varying percentages of black dots simulate shades of gray. Every input atom, bond or polyhedron has a black-and-white pattern number for rim and fill. For raster files, ATOMS supplies the patterns in two series, coarse and fine (see below). For printer output in Windows, you can select either the ATOMS patterns or the screen-driver patterns; for Macintosh the ATOMS patterns are always used. Direct PostScript output uses the patterns supplied by the output device.

The dot pattern is used for lines only if wide lines (Line Widths dialog in the Input2 Menu) are selected; if lines are one-dot width, they are black. If atoms, bonds or polyhedra are not shaded, that is given variable coloring depending on presumed illumination, then the pattern number can be used without modification as a means of distinguishing different atoms, etc. Generally, if shading is used, it is best to treat all atoms, bonds and polyhedra alike, that is show a white color (no dithering) at maximum illumination. However, it is possible to assign an initial shade to each type of atom.

(Shading dialog in the Input2 menu).

8-bit or 256 color. This may be used by older types of screen displays, and it is often advantageous for non-3D drawings in raster files. The 256 colors in these displays (in Windows minus 20 colors set aside for system use) may each be set to any admixture of red, green and blue. ATOMS sets up a 256-color palette designed for displaying shaded

atoms, bonds and polyhedra. It is based on 16 principal colors, which can be defined in the Palette dialog in the Settings menu. See below (Palettes) for further information on palettes.

15-, 16, 24- or 32-bit or RGB color. This is becoming standard for screen displays, and it is used for Postscript color output and for 16- and 24-bit raster files. One-third of the available bits are used to represent each of the color components red, green and blue.

For 24- or 32-bit screen displays, there are virtually no restrictions on colors, and any colors and shadings thereof can be rendered faithfully.

In non-3D display modes, for the most part RGB color does not greatly improve the quality of ATOMS drawings over 8-bit color. However, it does remove restrictions in two areas: (1) Any number of separate colors are valid for shaded atoms, bonds and polyhedra, rather than only 15 colors for 16 or fewer shading zones, or 7 colors for 32 zones (see under palettes below); (2) when front-back fading (Shading dialog in the Input2 menu) is in effect, the atoms etc. in the rear of the drawing are not restricted to a few of the darker zones in a shading series, as in 8-bit color.

When generating raster files of color images in non-3D modes, it may advantageous to use use 8-bit (256-color) rather than RGB (15-bit through 32-bit) color because the files tend to be much smaller. This is because of the smaller number of bits in the pixels, but also because the run-length encoding used in some file types does not compress RGB files well.

The 3D mode does not use the ATOMS 256-color palette on 8-bit displays, in either Windows or Macintosh. Windows uses a special palette for this mode and the Macintosh must use the system color table. This often results in dithering, so RGB color should generally be used for 3D modes.

Printing. The choice between 4-bit (standard VGA), 8-bit (256-color) and 15-32-bit display may may a great difference in the appearance on screen, but hard-copy devices such as printers actually use only a very limited number of colors (typically cyan,

magenta, yellow and black, although some inkjet printers may use more by mixing inks), simulating more complex colors with dithering. Some degradation of color quality from screen to printed image is to be expected. If color match of the printed output is very poor, check the settings for the printer in the Print dialog.

Palettes.

Although Windows and Macintosh programs can specify colors made up of any admixture of red, green and blue components, it is generally a good idea to use simple colors whenever possible, to increase the chances of faithful reproduction on a variety of display and output devices.

16-Color Palettes. ATOMS uses a Main Palette composed of 16 colors for two reasons:

to aid in the consistent selection of simple colors; and to form a basis for the 256-color palette used for 8-bit displays or other devices. The first eight colors in the default Main palette are black, white, red, green and blue, and then the binary combinations of red, green and blue which are cyan, magenta (violet) and yellow. The second eight colors are two shades of gray, then the binary combinations of red, green, blue, cyan, magenta and yellow (these six have no universally-recognized names). The colors in this palette are the same as those in the Alternate palette which is used in the DOS versions of ATOMS, but in a different order.

The RGB values in both the 16-color palettes can be changed with dialogs in the Settings menu. You can also save and re-read palette files.

256-Color Palettes. The palettes which are used in 256-color screen-display modes are primarily designed for shading. There are two distinct palettes; the first has 15 principal colors, each with 16 gradations from full intensity to almost black, and is used whenever the number of atom shading zones (Shading dialog in the Input2 menu) is 16 or fewer;

the second has only 7 principal colors, each with 32 gradations from full intensity to almost black, and is used when the number of atom shading zones is 32.

The first or "16-zone" palette is made up as follows. The first 15 colors (0-14) start with full-intensity color number 1 (white), and grade with decreasing intensity to near black;

the second 15 colors (15-29) start with full intensity color number 2, grading to near black, and so on. When shading is in effect, ATOMS finds the nearest color in the 16-color palette to the 16-color selected in the input for the atoms, bond or polyhedron. Any shading zone of an atom, bond or polyhedron receiving full illumination is given this full-intensity color and shaded parts are given colors in the series of 15 colors derived from that principal color. The palette defined in this way actually contains only 225 colors.

In the second or "32-zone" palette, the first 31 colors (0-30) grade from full-intensity color 1 to near black, the next 31 from full-intensity color 2 to near black and so on, through color number 7 and its derivatives. When shading is in effect, ATOMS again

"forces" the color to one of those in the 16-color palette, but the choices this time are only colors 0-7. As the colors are arranged in the default palette, this results in changing colors 8-15 to 0-7 respectively. Actually, colors 0-7 can be regarded as "simplified" or more basic versions of colors 8-15. The above shaded colors add up to only 217, so colors 8-15 are also added to the palette to be usable for lines and unshaded fills.

Again, if shading is in effect and you are using more than 16 shading zones (Shading dialog in the Input2 menu), any fill colors for atoms, stick bonds or polyhedra in the range 8-15 will be automatically "downgraded" to colors 0-7.

All these complexities and restrictions pertain only to 8-bit or 256-color display or raster files. For standard VGA (4-bit) display in Windows, the RGB values are sent to the

display driver, which determines in its own way what combination of pixels to mix to represent a given color. This is likewise true for printer and PostScript output. The RGB values are also sent directly to 16-, 24- and 32- bit screen displays, which can presumably render any color without dithering.

Dot Patterns for Black-and-White Display and Output.

The dot-patterns which are the "colors'' for atom, bond and polyhedron fills and wide lines in black-and-white printer output (if ATOMS patterns are selected in preference to those supplied by the printer driver in Windows), and which are also used for shading in black-and-white raster files, are shown in the diagram below. The patterns comprise two series of dot-patterns of increasing density with shade numbers 0-15; the choice between them is made in the Print or Raster File dialogs (File menu).

The fine series is recommended for 9-pin or other low resolution dot-matrix plotting, for very small drawings done at high resolution, or for raster files which will be shown on a low-resolution screen. The coarse series contains patterns more like the half-tone screening used in printing. It is appropriate for gray tones and shading in high-resolution dot-matrix plotting. Actually, for shading with 32 zones, there are 32 patterns in the coarse series, only half of which are shown in Figure IV-1. The fine series has only 16 unique patterns even if the number of shading zones is 32, although this could be changed in the SCRPTR file (Direct Modification...).

ATOMS has the ability to use macrosopic hachure patterns on both polyhedra and atoms.

In many cases it may be more appropriate to use these patterns to differentiate atoms and polyhedra than to use grey shades.