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colorreset.md

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Resetting the VT340 color map

The VT340's palette of colors has strange entwinings between text, Sixel graphics, and ReGIS graphics as discussed in the colormap page. It is easy for the VT340 to end up with unreadable text and no obvious way to fix it.

Simplest solution, using the keyboard

It is possible to reset colors from the VT340's builtin setup, but tricky because the current color map is used for the setup screen's text so you can't see what you're doing. To reset colors from the keyboard (without needing to look) press this sequence of keys:

    Set-Up Prev Screen Do Set-Up

That pulls up the palette editor and recalls the saved color map.

Programmatic solutions

Of course, it would be better if there was a way to reset the VT340 colormap using an escape sequence. That is still being investigated, but here is what we know so far.

Current best solution: set colors in a script

While way too verbose to type blindly, one can create a script that programs all 16 colors or only the ones which are used for text. There are at least three ways to do that:

  • ReGIS: one can create a cattable file or a shell function:

    # Function to fix a common problem: unreadable text colors
resetpalette() {
    printf '\ePp
    S(
  	  M 0     (H  0 L  0 S  0)
  	  M 1     (H  0 L 49 S 59)
  	  M 2     (H120 L 46 S 71)
  	  M 3     (H240 L 49 S 59)
  	  M 4     (H 60 L 49 S 59)
  	  M 5     (H300 L 49 S 59)
  	  M 6     (H180 L 49 S 59)
  	  M 7     (H  0 L 46 S  0)
  	  M 8     (H  0 L 26 S  0)
  	  M 9     (H  0 L 46 S 28)
  	  M 10    (H120 L 42 S 38)
  	  M 11    (H240 L 46 S 28)
  	  M 12    (H 60 L 46 S 28)
  	  M 13    (H300 L 46 S 28)
  	  M 14    (H180 L 46 S 28)
  	  M 15    (H  0 L 79 S  0)
    )\e\\'
}

  • resetpalette.sh: Uses DECRSTS (\eP2$p...) to send the actual Color Table Report returned by hackerb9's VT340+ in response to DECRQTSR (Request Terminal State Report).

    Give the -t option to only reset the foreground and background text colors. Unlike ReGIS's Hue-Lightness-Saturation, colors are specified using the familiar RGB colorspace, with values ranging from 0 to 100%. Note that DECRSTS is explicitly documented as not portable to other terminals.

  • Sixel graphics can technically change the color palette in a repeatable way, but it is needlessly grotesque. Unlike the situation in ReGIS, Sixel color index numbers do not correspond directly to the VT340's actual palette. Instead, they are mapped based on the order in which the colors are assigned.

    • The SIXTH color you assign will change the text foreground color,
    • The SEVENTH changes the bold + blinking foreground color,
    • The FIFTEENTH changes the bold text foreground color and
    • the SIXTEENTH changes the text background.

    This confusing indirection has the benefit that four-color sixel images (as were common on the VT240) will not affect the background or text colors. The VT340 designers probably felt free to implement it this way because ReGIS provides direct access to the true colormap if necessary.

What does NOT work

According to the DEC Printer Programming Language 2 Reference — although focused on printers, it is the best documentation yet found for Sixel Level 2 — there are four ways in which the color palette is reset.

Color numbers remain assigned even after leaving and reentering Sixel Graphics mode. At power-up, Soft Terminal Reset (DECSTR), Select Conformance Level (DECSCL), and Reset to Initial State (RIS), the device assigns all color numbers to black.

Upon entering Sixel Graphics mode, the device selects color 0.

(For printers the initial palette is all black because the paper is implicitly white, but of course that default doesn't make sense for a video terminal which must define both foreground and background colors.)

DECSTR

Soft Terminal Reset does not reset the color palette on the VT340, despite what the PPL2 says.

    CSI ! p
    printf '\e[!p'

Hackerb9 believes this is a misfeature in the VT340 and that emulators should reset the palette upon receiving DECSTR.

"At power up"

Power cycling the VT340 technically works, but booting up requires a lengthy self-test so it is not a practical solution.

RIS

Hard Terminal Reset (aka Reset to Initial State) does reset the palette, but it takes a long time to execute as it goes through the whole power-on self-test again.

printf '\ec'

DECSCL

"Select Conformance Level" does not reset the palette on hackerb9's VT340. The VT340 manual says DECSCL calls DECSTR but does not otherwise mention resetting anything.

The DEC PPL2 documentation does explicitly state that DECSCL will reset the color palette, the same as DECSTR.

CSI Ps " p
printf '\e["p'			# Default operating mode
printf '\e[63;1"p'		# VT300 mode, 7-bit controls
printf '\e[72"p'		# PPL Level 2

Tektronix mode

Switching to Tektronix mode and back does not restore the palette.

echo $'\e[?38h\e[?38l'		# Colors do not change

DECSTGLT

Technically using Set Text/Graphics Look-Up Table works, but it causes graphical glitches on the screen where old text reappears in a rectangle on the screen. Clearing the screen can erase the glitches, but is less than ideal.

#!/bin/bash
# DECSTGLT - Set Text/Graphics Look-Up Table
# Changing to colormap #0 then colormap #1 resets the color table to default.
echo -n $'\e[0){\e[1){'

# However, there are strange text glitches, so clear the screen.
echo -n $'\e[2J'