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VT340 Default Color Map

Index H L S R G B Text attribute
0 0 0 0 0 0 0 Screen Background
1 0 49 59 20 20 79
2 120 46 71 79 13 13
3 240 49 59 20 79 20
4 60 49 59 79 20 79
5 300 49 59 20 79 79
6 180 49 59 79 79 20
7 0 46 0 46 46 46 Foreground Text
8 0 26 0 26 26 26 Bold+Blink FG
9 0 46 28 33 33 59
10 120 42 38 59 26 26
11 240 46 28 33 59 33
12 60 46 28 59 33 59
13 300 46 28 33 59 59
14 180 46 28 59 59 33
15 0 79 0 79 79 79 Bold Foreground

Unlike modern systems, on the VT340, R, G, B range from 0 to 100 percent in decimal. Another difference is that a Hue angle of 0 degrees is blue on the VT340, not red.

Documentation differs

Note that the above table contains the actual values retrieved from a VT340+ after doing a factory reset of the settings. Almost all values have slightly lower lightness or saturation than what is printed in the manual. Most significantly, the foreground color (7) in the manual is listed as 53% (#878787) in the manual, but in reality it is 46% (#757575). Perhaps in an earlier firmware revision it did match the manual. At some point, it seems it was decided, for every color except the foreground, to subtract 1 from the percent saturation and subtract 0 or 1 from the lightness.

Here is the HLS side of table 2-3 VT340 Default Color Map from the VT340 Graphics Programming manual (2nd ed.), with the delta of how it changed in the actual hardware.

Index	 H   	  L	    S
0      	 0   	  0         0
1      	 0   	 50 -1     60 -1
2      	120  	 46        70 -1
3      	240  	 50 -1     60 -1
4      	 60  	 50 -1     60 -1
5      	300  	 50 -1     60 -1
6      	180  	 50 -1     60 -1
7      	 0   	 53 -6      0
8      	 0   	 26         0
9      	 0   	 46        29 -1
10     	120  	 43 -1     39 -1
11     	240  	 46        29 -1
12     	 60  	 46        29 -1
13     	300  	 46        29 -1
14     	180  	 46        29 -1
15     	 0   	 80 -1      0

Connection between text attributes and colormap

Although the VT340 supports text colors, it is not "ANSI color". Instead, it represents some text attributes by using four specific colors in the colormap: 0, 7, 8, and 15.

Style Color Index Used as color for VT340 Default
Background 0 Background screen Black
Foreground 7 Normal text Medium gray
Bright 15 Bold text Light gray
Blink 8 Blink+Bold text Dark gray

For example, sending the ANSI escape sequence for bold text, ␛[1m, uses color number 15 as the foreground instead of 7. Conversely, to change the bold text color on a VT340, set color index 15 using ReGIS: ␛P0pS(M15(AH60L80s60))␛\

Note that this color map is shared with sixel and ReGIS graphics, which means displaying images can mess up text legibility. Although DEC has mitigated that somewhat by making sixel's access to the colormap indirect and by starting at index 1: the first six colors used do not affect the text. The seventh color in the image changes the foreground (index 7) and only an image which needs all 16 colors will wrap all the way back to index 0 and modify the background.

Resetting the colormap

Since displaying sixel images with 7 or more colors can make text unreadable, it would be good if one could reset the colormap without being able to see what i on the screen. The easiest way is to use the builtin Color Set-up by pressing these keys on a VT340 keyboard:

    Set-Up Prev Screen Do Set-Up

Character attributes in combination

For the most part, the use of colors behave as one would expect when attributes are used in combination. The one perhaps surprising design choice is how the VT340 renders Bold+Blinking text.

For normal text, The Blink attribute is displayed as normal text (7 on 0) alternating with reversed normal text (0 on 7) once a second. However, for Bold text, Blink does not look the same as Bold text (15 on 0) alternating with Reverse Bold text (0 on 15). Instead, it shows Bold text (15 on 0) alternating with Blink+Bold text (8 on 7). This is the only time that hackerb9 has found the VT340 using color number 8 for text attributes.

Attributes Foreground Background Escape Sequence
Normal 7 0 ␛[0m
Bold 15 0 ␛[1m
Reverse 0 7 ␛[7m
Reverse Bold 0 15 ␛[1;7m
Blink (off) 7 0 ␛[5m
" (on) 0 7 "
Bold Blink (off) 15 0 ␛[1;5m
" (on) 8 7 "
Reverse Bold Blink (off) 0 15 ␛[1;5;7m
" (on) 7 8 "

Note that the Underline character attribute (␛[5m) is not mentioned in this table because the VT340 renders it as an actual underline, not through color.

Inverse colors

The VT340 has a concept of "inverse" colors in at least one sense: when the cursor is on top of sixel data, it flashes between normal and some other color picked from the palette. To find the inverse index number, xor the normal index number with 7. That flips the low three bits but keeps the high bit of the nybble.

INVERSE COLOR PAIRS: XOR 7
	0--7		 8--15
	1--6		 9--14
	2--5		10--13
	3--4		11--12

Note that this mapping works somewhat like the way that text attributes are already inverted during Blink and Reverse (as above). Both Blink and Reverse swap foreground text color 7 with background color 0. When blinking, bold text color 15 is switched with its inverse, color 8. However, this analogy is not perfect: reversed, bold text color 15 is swapped with background color 0, not 8 as would be expected.

Todo: Does the ReGIS graphics cursor invert the colors in the same way? What about ReGIS logical NOT operator?

Inverse when using the default VT340 colormap

One benefit of keeping the high bit is that inverting a color will not change the saturation when using the default colormap. Colors 8 to 15 are lower saturation versions of colors 0 to 7.

The default colormap also makes the relationship between a color and its inverse simple to describe in terms of Hue, Saturation, and Lightness:

  • If Saturation is 0 (gray), then the Lightness Percent is offset by around 50 percentile points.

  • If Saturation is >0 (color), then the Hue Angle is rotated by 180 degrees.

Interactively choosing screen colors for text

Built-in color chooser

The VT340 includes in its firmware a Color Set-up screen which allows one to recall the saved color map, save the color map, or adjust the RGB values of any of the 16 colors in the palette. Set color #0 for background and #7 for text foreground.

rgb.sh script for text colors

An easier way to pick text colors is to use the rgb.sh script to interactively choose colors from a list for Background, Foreground, Bold, and Blink. Once favorite colors are known, it can be used non-interactively via arguments: rgb.sh 12 14 55 41.

The script was originally written for the VT241 and is a bit clunky. Unlike the VT340's built-in Color Set-up, rgb.sh does not show a preview of the color choices, only names such as "Cornflower Blue", "Goldenrod", or "Turquoise". Additionally, only certain color names (RGBCMYKW) are recognized, all others must be chosen by number.

VT340 Emulation Question

Which, if any, VT340 emulators can run rgb.sh? Emulators typically do not use the colormap to represent text attributes. Additionally, they would need to support ReGIS graphics for setting individual colors in the palette.