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VT340 Color Planes

The VT340 has four 1-bit color planes. Each pixel makes up a four-bit number that is looked up in a colormap table to find the actual color. However, the bit planes can be written to individually, as well, by using the ReGIS W(Fn) "Field" bitmask which is ANDed against any graphical write.

Example of Using Bitplanes

The bitplane() function in the registest.sh file exercises the ReGIS's W(F_n_) bitmask.

Click here to see the bitplane() function 
bitplane() {
    # Test of writing to each of the four bitmap planes of the VT340.
    echo "W(I15)"		# Color "15", normally would turn all 4 bits on
    echo "W(F1)"		# F1 bitmask: Only plane 0
    echo "P[400,0]W(S1)C[+250]W(S0)"
    echo "W(F2)"		# F2 bitmask: Only plane 1
    echo "P[200,380]W(S1)C[+250]W(S0)"
    echo "W(F4)"		# F4 bitmask: Only plane 2
    echo "P[600,380]W(S1)C[+250]W(S0)"
    echo "W(F8)"		# F8 bitmask: Only plane 3
    echo "P[400,240]W(S1)C[+100]W(S0)"
    echo "P[400,240]W(S1N1)C[+50]W(S0N0)" # Negate inner circle

    echo "W(F15)"		# Re-enable writing to all planes.
    echo "S("		# Adjust colormap so intersections look right
    echo "M4(H240 L49 S59)	M3(H60 L49 S59)" # Swap green & magenta.
    echo "M12(H240 L46 S28)	M11(H60 L46 S28)" # Same for low sat G&M.
    echo "M15(H0 L46 S0)	M7(H0 L100 S0)"	  # Swap medium & bright gray.
    echo ")"

    echo -n ${ST}
    tput cup 1000
    read -s -n1 -p "Magenta & green swapped. Hit any key to reset colormap to default..."
    echo -n $'\r'
    tput el
    echo -n ${DCS}p

    echo "S("		# Undo changes to colormap
    echo "M3(H240 L49 S59)	M4(H60 L49 S59)"
    echo "M11(H240 L46 S28)	M12(H60 L46 S28)"
    echo "M7(H0 L46 S0)		M15(H0 L100 S0)"
    echo ")"
}

Here is what the output from a VT340 looks like, at the point when the script swaps magenta and green:

Default VT340 colors, seen as bit planes

Index F8 F4 F2 F1 Color name H L S
0 0 0 0 0 Black 0 0 0
1 0 0 0 1 Blue 0 49 59
2 0 0 1 0 Red 120 46 71
3 0 0 1 1 Green 240 49 59
4 0 1 0 0 Magenta 60 49 59
5 0 1 0 1 Cyan 300 49 59
6 0 1 1 0 Yellow 180 49 59
7 0 1 1 1 Medium Gray (50%) 0 46 0
8 1 0 0 0 Dark Gray (25%) 0 26 0
9 1 0 0 1 Desat Blue 0 46 28
10 1 0 1 0 Desat Red 120 42 38
11 1 0 1 1 Desat Green 240 46 28
12 1 1 0 0 Desat Magenta 60 46 28
13 1 1 0 1 Desat Cyan 300 46 28
14 1 1 1 0 Desat Yellow 180 46 28
15 1 1 1 1 White (Gray 75%) 0 79 0

Color mixing peculiarities

Click here for discussion of color mixing

The apparent intersections where colors should mix via bitplanes does not make a lot of sense using the default color map on the VT340. In particular, BLUE (0001) and RED (0010) make GREEN (0011)! Of course, the colors could be remapped easily enough. It turns out only three pairs need to be swapped to be make intersections look correct.

  1. Swap magenta and green (3 and 4)
  2. Swap low saturation magenta and green (11 and 12)
  3. Swap medium gray with light gray (7 and 15)

<details

Click for a swapped color table

Index F8 F4 F2 F1 Color name H L S
0 0 0 0 0 Black 0 0 0
1 0 0 0 1 Blue 0 49 59
2 0 0 1 0 Red 120 46 71
3 0 0 1 1 Magenta 60 49 59
4 0 1 0 0 Green 240 49 59
5 0 1 0 1 Cyan 300 49 59
6 0 1 1 0 Yellow 180 49 59
7 0 1 1 1 White (Gray 75%) 0 79 0
8 1 0 0 0 Dark Gray (25%) 0 26 0
9 1 0 0 1 Desat Blue 0 46 28
10 1 0 1 0 Desat Red 120 42 38
11 1 0 1 1 Desat Magenta 60 46 28
12 1 1 0 0 Desat Green 240 46 28
13 1 1 0 1 Desat Cyan 300 46 28
14 1 1 1 0 Desat Yellow 180 46 28
15 1 1 1 1 Medium Gray (50%) 0 46 0

Colormap affects text rendering

As mentioned in the colormap notes, some of the locations in the color map affect the screen text. Briefly: 0 Screen Background, 7 Foreground Text, 8 Bold+Blink FG, 15 Bold Foreground.

If you need to reset the colormap to the VT340 defaults, you can cat the file resetpalette.regis.

Click to see data 
Esc P0p
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)
)
Esc \