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    1Description
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    2Sample
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    3Notes
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    4See also
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    5References
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    6External links

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Sixel

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From Wikipedia, the free encyclopedia
Bitmap graphics format
This article is about the bitmap graphics format. For the South
African-Australian film editor, see Margaret Sixel.

         This article needs additional citations for verification.
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[220px-Sixel_Wikipedia_logo]
 
The Wikipedia logo converted to sixel format and rendered in an xterm
with VT340 emulation.

Sixel, short for "six pixels", is a bitmap graphics format supported
by terminals and printers from DEC. It consists of a pattern six
pixels high and one wide, resulting in 64 possible patterns. Each
possible pattern is assigned an ASCII character, making the sixels
easy to transmit on 7-bit serial links.

Sixel was first introduced as a way of sending bitmap graphics to DEC
dot matrix printers like the LA50. After being put into "sixel mode"
the following data was interpreted to directly control six of the
pins in the nine-pin print head. A string of sixel characters encodes
a single 6-pixel high row of the image.

The system was later re-used as a way to send bitmap data to the
VT200 series and VT320 terminals when defining custom character sets.
A series of sixels are used to transfer the bitmap for each
character. This feature is known as soft character sets or
dynamically redefinable character sets (DRCS). With the VT240, VT241,
VT330, and VT340, the terminals could decode a complete sixel image
to the screen, like those previously sent to printers.

Description[edit]

Sixel encodes images by breaking up the bitmap into a series of
6-pixel high horizontal strips. Each 1-pixel-wide vertical column in
a particular strip forms a single sixel. Each sixel's pixels are read
as binary and encoded into a single 6-bit number, with "on" pixels
encoded as a 1. This number, from 0 to 63 decimal, is then converted
into a single ASCII character, offset by 63 so that an all-black
sixel, 0 decimal, is encoded as ?. This ensures that the sixels
remain within the printable character range of the ASCII character
set. Carriage return (CR) is represented by $, and line feeds (LF)
with a -; both had to be sent in turn to return the cursor to the
start of the line, CRLF.

Sixel also includes a rudimentary form of compression, using
run-length encoding (RLE). This is accomplished with the ! character
followed by a decimal number of the times to repeat, and then a
single sixel character to be repeated. Since the ! and decimal digits
cannot be valid sixel data, lying outside the encoded range, the
encoding is easy to identify and expand back out in software.

"Sixel mode" is entered by sending the sequence ESC+Pp1;p2;p3;q. The
p1 through p3 were optional setup parameters, with p1 defining an
aspect ratio (deprecated in favor of p3), p2 how to interpret the
color of zeros, and p3 with simple grid size parameters. ESC+P is the
standard DEC "Device Control String", or DCS, which was used to turn
on or off a number of special features in DEC's equipment. The "q" is
the sixel identifier.^[a] Sixel data then followed the q. The "String
Terminator" sequence ESC+\ returned the device back to normal
character mode again.

For printing, sixels are sent to the printer, decoded back into
binary, and sent directly to six pins in the print head. The only
complexity involved expanding the RLEs into the internal print
buffer. Display on a terminal is somewhat more difficult. On
terminals supporting graphics, the ReGIS graphics system was used to
directly draw the sixel pattern into the screen's bitmap. This was
done at high speed by storing the bitmap patterns as a glyph and then
blitting them.

When used for defining custom character sets the format was almost
identical, although the escape codes changed. In terms of the data,
the only major difference is the replacement of the separate CR/LF
with a single /. In the VT300 series for instance, 80-column
character glyphs were 15 pixels wide by 12 high, meaning that a
character could be defined by sending a total of 30 sixels.

Color is also supported using the # character, followed by a number
referring to one of a number of color registers, which varied from
device to device. The colors in the registers are defined using
either RGB or HLS values in a peculiar DEC format. To create a color
image on a printer, a line of sixels is sent several times, each
representing a single bitplane from the register-based colors on the
terminals (normally 2 or 4 bits). Since the capabilities of the
hardware vary widely, a color sixel drawing can only be output to
targeted devices. Non-graphics terminals generally silently ignore
sixel escape sequences.

Sample[edit]

[Sixel_demo]
 
The sample Sixel code that says "HI" displayed in a xterm with VT340
emulation.

 <ESC>Pq
 #0;2;0;0;0#1;2;100;100;0#2;2;0;100;0
 #1~~@@vv@@~~@@~~$
 #2??}}GG}}??}}??-
 #1!14@
 <ESC>\

The example above enters sixel mode, sets up three color registers,
and then uses those registers to draw sixels in those colors. The #0;
2;0;0;0 is interpreted as "set color register 0, use mode 2 (RGB
mode), set R, G and B to 0%". This sets color 0 to black, and the
following commands set register 1 to yellow (100%, 100%, 0%) and 2 to
green (0%, 100%, 0%). Unlike most modern systems which assign an
8-bit value (0-255) to each color channel, RGB mode is based on
"intensity" of each channel from 0% to 100%.

The data lines following select a color, yellow for the first and
third and green for the middle, then draw sixels. The last line shows
the RLE in use, meaning "repeat @ (0b100000) 14 times". The final
output is 14 pixels by 7 pixels, so the last line simply repeats
0b100000 14 times to fill the last line. $ at the end of a line means
that the next line will overprint the current line, while - means
that the next line represents a new line of sixels.

Notes[edit]

 1. ^ Lower-case "p" was the ReGIS identifier.

See also[edit]

  * ReGIS - a means of displaying vector graphics on later DEC
    VT-series terminals
  * Semigraphics - text-based graphics, including "squot"
    sub-character square blocks

References[edit]

  * "VT330/VT340 Programmer Reference Manual, Volume 2: Graphics
    Programming", Digital, EK-VT3XX-GP-002, May 1988
  * Chris Chiesa, All About SIXELs, 29 September 1990

External links[edit]

  * "Sixel Graphic News"
  * "Sixel software library"
  * "VT320 Soft Character Sets"
  * "Sixel Viewer and sample Sixel images"
  * "BMP to Sixel converter"

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oldid=1120400928"
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