The following was extracted and converted to Markdown for vt340test from section 7.8 (Graphics Printing) of DEC STD 070. It describes the sixel graphics format from the point of view of printing from a terminal to an attached printer.
The DEC VT340 supports the Level 2 sixel protocol, so I suggest skipping all the confusing information about Level 1. The primary flaw in Level 1 is that it is unable to print 1:1 aspect ratio. Additionally, due to a bug in the VT340 firmware, at least on hackerb9's VT340+ (G2), Level 1 is unusable for MediaCopy. The only reason to use Level 1 sixels is if you are using an older terminal, such as a VT241.
VSRM - Printer Port Extension EL-00010-07
*** COMPANY CONF - DEC Internal Use Only 28-Apr-1987
This section describes graphics printing and is intended to apply to devices which support graphic displays and the Sixel Graphics Extension.
| Graphics Print Operations | Description |
|---|---|
| Graphics Print Screen | The complete graphics bit map is transferred to the printer. This can be invoked by the host through the ReGIS hardcopy command or by the user under local control. |
| Graphics Print Region | An image of a portion of the screen bit map is transferred to the printer. This can be invoked by the host through the ReGIS hardcopy command. Refer to the ReGIS chapter for determining how to define this region. |
A Sixel is a group of 6 vertical pixels represented by 6 bits within a character code. A one value for a bit indicates that a pixel-spot will be placed at the corresponding grid position, a zero will cause the corresponding grid position to be unchanged (not written).
Sixel printing consists of setting context and attributes for the pixels and then printing each Sixel in left-to-right, top-to-bottom order. Wherever possible, run-length encoding is used to reduce the number of characters transmitted for a line of Sixels.
See the Sixel Graphics Extension chapter for further details on the Sixel format.
Guideline: When a Sixel dump is initiated by the user under local control (Print Key), the terminal first transmits a CR to reset the ANSI text position on the printer to the left margin. If the Sixel dump is initiated by the host, the CR is not sent so the host can initialize the starting sixel position.
Guideline: The VT240 sends the following characters out the printer port (or host port) when transmitting a sixel dump of the screen.
ESC \ <CR> ESC P 1 q <sixel-data> ESC \
1/11 5/12 0/13 1/11 5/0 3/1 7/1 1/11 5/12
\________/ \_______/ \________/
ST DCS ST
On devices which support a color display, the user may select monochrome or color sixel printing. Monochrome Sixels are formed from the multiple plane image by a logical OR of all pixels which are selected for any color. If color printing is selected, the terminal will send out color specifiers at the beginning of each Sixel dump to reflect the contents of the terminal's color map.
If PRINT BACKGROUND is selected, all pixels with no bits set in any plane are printed in the color value selected for index 0.
Graphics expanded print mode determines whether the terminal generates a small (compressed) or large (expanded) graphics image. The small image fits on 8-1/2 inch wide paper and expanded images on 13 inch wide paper in portrait mode. This mode is selectable by the DEC-private mode DECGEPM or by the user under local control.
Graphics rotated print mode rotates the image by 90 degrees so that an expanded image can fit on a single 8-1/2 inch wide page (rotated images are always expanded). This mode is selectable by the DEC-private mode DECGRPM or by the user under local control.
Guideline: the VT240 rotates the image counter clockwise, so that the left side of the paper (as it comes out of a typical dot matrix printer) corresponds to the top of the image on the terminal screen. This scanning order was chosen to allow punching holes for a looseleaf notebook on the left side of the page as it comes out of the printer.
As the Sixel graphics protocol has become more general to accommodate a range of pixel aspect ratios and grid sizes, Terminals have begun supporting two levels of the Sixel graphics protocol to match the capabilities of the printer being used. Level 1 provides backward compatibility with older printers, while Level 2 allows the terminal to take full advantage of newer printer capabilities to improve print quality.
The Sixel Graphics Level determines how the terminal matches the printer's Sixel Aspect Ratio, Horizontal Grid Size, Background Printing, and Color Printing capabilities.
The Sixel Graphics Level is selectable by the user under local control.
Click to see information on Level 1 Sixel Devices
ASSUMPTIONS: Level 1 sixel devices do not support the Set Raster Attribute command, Background Select, Horizontal Grid Size, or Macro Parameter commands. The aspect ratio is fixed at 2:1, and the horizontal grid size is approximately 7.5 x .001 inches (800 pixels require 6 inches). Level 1 is the factory default.
Sixel Control Strings will be sent to the printer as follows:
ESC P 1 q S...S ESC \ (always 7-bit controls)
A terminal with a 1:1 pixel aspect ratio will combine each pair of horizontally adjacent pixels (from two successive scan lines) into a single pixel. The value of the combined pixel will be the LOGICAL OR of the values of the individual pixels.
1:1 Aspect Ratio Level 1 Printer
+---+ +---+
| a | | |
+---+ LOGICAL OR -> |a+b|
+---+ | |
| b | | |
+---+ +---+
This will produce images of the same size and aspect ratio as the VT240 (2:1 Aspect Ratio). ReGIS images will appear normal, but ANSI text may be distorted.
Terminals with pixel aspect ratios of 1:1 will transmit each Sixel twice (in immediate horizontal succession).
1:1 Aspect Ratio Level 1 Printer
+---+ +---+ +---+
| a | | | | |
+---+ PRINT TWICE -> | a | | a |
+---+ | | | |
| b | | | | |
+---+ +---+ +---+
. +---+ +---+
. | | | |
. | a | | a |
| | | |
| | | |
+---+ +---+
.
.
.
This will produce images of the same size and aspect ratio as the VT240 (2:1 Aspect Ratio), but with twice the vertical resolution.
Terminals with 1:1 pixel aspect ratios will transmit the selected portion of the display bitmap in vertical strips six pixels wide from top to bottom, and right to left.
display paper
+--------------+ +-----------+
| 2 1 | | 1-------> |
| | | | | 2-------> |
| | | | | |
| V V | | |
+--------------+ | |
| |
| |
+-----------+
Each Sixel is sent twice in immediate horizontal succession (with respect to the printer).
1:1 Aspect Ratio Level 1 Printer
+---+ +---+ +---+ +---+ +---+
. . . | | | b | | a | | | | | . . .
+---+ +---+ +---+ ROTATE | a | | a |
+---+ +---+ AND | | | |
| | | | PRINT | | | |
+---+ +---+ TWICE -> +---+ +---+
+---+ +---+ +---+
| | | | | |
+---+ | b | | b |
. | | | |
. | | | |
. +---+ +---+
.
.
.
This will produce images of the same size and aspect ratio as the VT240 (2:1 Aspect Ratio), but with twice the vertical resolution (with respect to the image displayed on the screen).
ASSUMPTIONS: Level 2 sixel devices support the Set Raster Attribute command, Background Select, Horizontal Grid Size and Macro Parameter commands.
Sixel control strings are sent as follows:
ESC P Ps1 ; Ps2 ; Pn3 q " Pn4 ; Pn5 ; Pn6 ; Pn7 ****** ESC \
\___/ \_______________/ \_____________________/ \____/ \___/
DCS Protocol Selector Raster Attributes Picture ST
data
\______________________/ \_____________________________/
DCS Introducer Sequence sixel data
Where:
-
Ps1 Is the Macro Parameter and is always ZERO.
-
Ps2 Background Select
- 1 if Background Printing is disabled in Set-Up
- 2 if Background Printing is enabled in Set-Up
-
Pn3 Horizontal Grid Size, given in units specified by ANSI SSU (default is decipoints, 1/720 inch). For default size units, the grid size should be 6 for COMPRESSED and 9 for EXPANDED or ROTATED print.
Since the host can change the printer between accesses, SSU should be sent once before each sixel dump.ESC [ 2 SP I (Set Size Unit 1/11 5/11 3/2 2/0 4/9 to Decipoints) -
Pn4 Pixel aspect ratio numerator, 1
-
Pn5 Pixel aspect ratio denominator, 1
-
Pn6 Horizontal extent (number of pixels in image horizontally)
-
Pn7 Vertical extent (number of pixels in image vertically)
When Level 2 sixel graphics is selected, the terminal will transmit the selected portion of the display bitmap as sixels from left to right, and top to bottom. No translation of screen pixels is required since the printer directly supports 1:1 sixels with the desired grid size. The horizontal grid size is set to 6 in the Set Raster Attributes command.
When Level 2 sixel graphics is selected, the terminal will transmit the selected portion of the display bitmap as sixels from left to right, and top to bottom. No translation of screen pixels is required since the printer directly supports 1:1 sixels with the desired grid size. The horizontal grid size is set to 9 in the Set Raster Attributes command.
When Level 2 sixel graphics is selected, the terminal will transmit the selected portion of the display bitmap in vertical strips six pixels wide from top to bottom, and right to left. Each sixel is sent once using the same sixel initiator as for expanded print above (horizontal grid size set to 9).