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Media Copy details & investigations

Media Copy is how one can take a screenshot from a VT340. It can be of the entire screen or a region. Depending upon complexity of the graphics, it can take a long time (~5m) to transmit.

A script for making screenshots can be found in mediacopy.sh.

The format of the output is discussed in Sixel Printer Port.

Sequence for sending

  1. Set VT340 to Level 2 graphics in Printing Set-Up menu.
  2. Set printing options (color, transparent background).
  3. Send REGIS "HardCopy" command.
  4. Receive sixel data from VT340.

Details

To send the screen to the host as sixel data, you set the MediaCopy output to "2", optionally change settings, and then send the ReGIS "Hard Copy" command.

CSI=$'\e['			# Control Sequence Introducer
DCS=$'\eP'			# Device Control String
ST=$'\e\\'			# String Terminator

# MC: Media Copy, "2" means send graphics to host, not printer
echo -n ${CSI}'?2i'		

# DECGEPM: Graphics Expanded Print Mode (only for Level 1 Graphics)
#echo -n ${CSI}'?43l'		# Compressed:  6" x 3" printout, 800x240
echo -n ${CSI}'?43h'		# Expanded:   12" x 8" printout, 1600x480

# DECGPCM: Print Graphics Color Mode
#echo -n ${CSI}'?44l'		# Print in Black and White
echo -n ${CSI}'?46h'		# Print in Color

# DECGPCS: Print Graphics Color Syntax
#echo -n ${CSI}'?45l'		# Print using HLS colors
echo -n ${CSI}'?45h'		# Print using RGB colors (ImageMagick reqs)

# DECGPBM: Print Graphics Background Mode
echo -n ${CSI}'?46l'		# Do not send background when printing
#echo -n ${CSI}'?46h'		# Include background when printing

echo -n ${DCS}'p'		# Enter REGIS mode
echo -n $'S(H)'			# Screen hard copy
echo -n ${ST}			# Exit REGIS mode

Media Copy response from VT340

I will describe the output informally. For the precisely specified output, see sixel-printer-port which is an extract from DEC STD 070.

Here is what a typical Level 2 Graphics response from a VT340 might look like:

[Esc] \ [Newline]
[Esc] [ 2 [Space] I
[Esc] P 0 ; 1 ; 6 q
" 1 ; 1 ; 800 ; 480
# 1 ; 2 ; 20 ; 20 
# 1 !255? !133? !8_ !9o !8_ $-
# 1 !255? !108? __oooowwww{{{{}}}} !39~ }}}}{{{{wwwwoooo__ $-
...
[ Esc ] \

Level 2 is required

The response varies depending upon what options you've chosen and whether your VT340 is set up for Level 2 graphics. There appears to be a bug in the Level 1 code where the final line of sixels, including the ST (string terminator) are not sent to the host. Fortunately, you don't want Level 1 anyway since it uses wacky rectangular pixels.

read -p -d idiom

The typical read idiom in bash for getting a response from a terminal does not work. Sending the inquiry in the prompt is fine, but one cannot stop reading at the first \ character because the VT340 sends ST (Esc \) at the beginning and the end of the data. Also note that, while having ST at the beginning is valid, it confuses ImageMagick and libsixel (as of October 2021).

Recommended reading method

Since the String Terminator can be at the start of the data, it is recommended to look for the sixel header, Esc P...q — where "..." is a string of digits and semicolons — before looking for the String Terminator, Esc \.

While sixel images can theoretically be made up of multiple sixel sequences — for example, setting palette colors in one and using them in another — the VT340 never sends data like that during a Media Copy command. After Esc P...q, the first backslash found will be the end of the data.

A timeout is not reliable for guessing when the data is done. Due to the 8-bit glitch mentioned below, the VT340 pauses for a long time in the middle of transmission.

Scattered notes as I figure things out

  • There are multiple options that affect the sixel data:

    1. [Level 1]/Level 2 graphics
      Only in Set-Up
    2. [No Terminator]/Send formfeed
      Only in Set-Up
    3. [Compressed]/Expanded
      DECGEPM, 43
    4. [Mono]/Color
      DECGPCM, 44
    5. [HLS]/RGB
      DECGPCS, 45
    6. [No Background]/Print Background
      DECGPBM, 46
    7. [Compressed+Expanded]/Rotated
      DECGRPM, 47

    [VT340 defaults are in square brackets.]

  • Level 2 prints are the only known way to get a proper 1:1 pixel aspect ratio. However, that requires the user to first go into the VT340 Set-Up menu to enable it as there appears to be no escape sequence to change from the default of Level 1. Phooey.

    [Set-Up] key -> Printer Set-Up -> Sixel Graphics Level = level 2

  • Level 2 graphics ignores the compressed/expanded option. Rotation does work.

  • Printout is shifted 50 pixels to the right by default, use the P[0,0] suboption to REGIS's H() command to undo that offset. Note that the order of nesting parentheses is incorrect in the VT3xx documentation:

    REGIS_H="S(H(P[0,0])[$X1,$Y1][$X2,$Y2])"	# This works

REGIS_H="S(H(P[0,0][$X1,$Y1][$X2,$Y2]))"	# This does not

  • It takes several minutes to send a full screen of sixel data, so you'll want to print just a cropped part if you're debugging this.

  • Before sending the sixel data, the VT340 sends ST (Esc \). However, ImageMagick gets confused by sixel images that start with the String Terminator (it thinks they have only one pixel). [Todo: I should report this bug to ImageMagick.]

  • VT340 defaults to printing in "mono". Must set to "color" using the DECGPCM (Esc [?44h) escape sequence.

  • ImageMagick cannot handle HLS sixel graphics. Must set to RGB by using DECGPCS (Esc [?45h).

SSU - Select Size Unit

The purpose of the introductory sequence Esc [ 2 Space I before the sixel data, is defined in ECMA-48 as setting the unit of measurement for all numeric parameters to the "Computer Decipoint" = 1/720th of an inch.

8.3.139 SSU - SELECT SIZE UNIT
Notation: (Ps)
Representation: CSI Ps 02/00 04/09
Parameter default value: Ps = 0

SSU is used to establish the unit in which the numeric parameters of certain control functions are expressed. The established unit remains in effect until the next occurrence of SSU in the data stream.

The parameter values are
0 CHARACTER - The dimensions of this unit are device-dependent
1 MILLIMETRE
2 COMPUTER DECIPOINT - 0,035 28 mm (1/720 of 25,4 mm)
3 DECIDIDOT - 0,037 59 mm (10/266 mm)
4 MIL - 0,025 4 mm (1/1 000 of 25,4 mm)
5 BASIC MEASURING UNIT (BMU) - 0,021 17 mm (1/1 200 of 25,4 mm)
6 MICROMETRE - 0,001 mm
7 PIXEL - The smallest increment that can be specified in a device
8 DECIPOINT - 0,035 14 mm (35/996 mm)

The VT340 sends SSU to ensure that the "horizontal grid" parameter in the sixel image is interpreted correctly and the image is printed at the correct size. The VT340's horizontal grid (space between pixels) on the screen is about .012 inches. For "rotated" printing, the VT340 sets the grid size in the sixel output to 9/720 = 0.0125", which makes hardcopy almost exactly the same size as the screen, 10" x 6". For "normal" printing, the VT340 sets the grid to 6/720 ≈ .00833, which shrinks the image to 6.66" x 4" so the width will fit on the 8.5" width of US Letter sized paper.

  • While the SSU data is currently thrown away by mediacopy.sh, the correct thing to do would be to embed the DPI information in the PNG output. (Since grid spacing is number of inches per dot, DPI is simply the inverse.)

  • Side note: Finding SSU was not easy. Reminder to self, when searching the control sequence standards look for "04/09" instead of simply the ASCII letter "I". ("04/09" == hexadecimal 0x49 == 'I'. Likewise, "02/00" == Space).

Eight-bit glitch

For some reason, in the middle of transmission, the VT340 will send a byte with the eighth bit high (which should never happen) and then pause for several minutes. ^Q does not wake it up. If I let it sit, it eventually finishes sending the entire image, but there will be a small glitch in the picture for every time the 8-bit bug happened.

  • The eighth-bit glitch is most apparent with large complex images, but that may simply because they take longer to send.

  • The glitch is not completely repeatable in that the exact value of the byte that gets mangled is not always the same.

Questions about 8-bit glitch:

  • Shabby serial line connection? Checked and seems okay.

  • Try second serial port. Checked and no different.

  • Try using printer port.

  • Are there certain pictures that trigger the glitch? It seems so as j4james's color_selection.sh was very difficult to get a media copy of until I switched the terminal to 132 column mode.

  • Why did 132 column mode help? Or did it just move the glitch to an unnoticeable area?

  • If it's a firmware bug, can I alter the ROM? (Currently Set-Up says I'm running "VT340 Version V3.7").

  • Did this glitch happen with Level 1 printing? I don't recall noticing it.

Level 1 printing does not work well and should be avoided

Design misfeatures and bugs make Level 1 Graphics not worth it for print outs from the VT340. Just use Level 2.

For more info, read level1.md.

What else doesn't work.

ReGIS is apparently required for sending graphics back to the host.

  • There is no escape sequence to simply print graphics. The normal VT340 "Media Copy" command, CSI i is intended for text, not graphics.

  • The Tektronix 4010/4014 hardcopy command, ESC ETB ($\e\x17), only works for Tek graphics and is, unless I'm mistaken, unusably buggy. As with ReGIS hardcopy, it does send data to the host, assuming I've sent Esc [ ? 2 i (media copy to host) before entering Tek mode.

    Unfortunately, the firmware has a serious glitch, cutting off the bottom of the image and sending it to the screen instead of to the host. (This seems a lot like the Sixel Level 1 errors I encountered before switching to Level 2).

    Even if it worked, it would be of limited use. Tektronix graphics are a completely separate system from ReGIS and the usual text modes. There is a special Tek hardcopy command, but that only works in Tek mode. Additionally, there is no way for Tek to send only a small crop of the screen. And, finally, redirecting printer graphics to the host must be done before the Tek drawing commences as it is not possible on the VT340 (as far as I know) to exit Tektronix mode and re-enter it without clearing the screen.