Text semigraphics
Encyclopedia
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
without having to implement the logic for a true "high resolution" mode.
Most often a number of special characters in the font of the video generator were used to support text semigraphics, normally a power of two like 4, or 8 of them. These characters are formed by creating a matrix of pixels, and assigning "ink" and "background" values to the elements of the matrix in a binary pattern, corresponding to the binary sequence of the position in the font table. when using a 2x3 matrix these six "pixels" are sometimes referred to as sixel
Sixel
Sixel is a bitmap graphics format supported by terminals and printers from DEC, most notably the VT320 and VT420. It encodes bitmaps into a character-based format that is easy to interpret, allowing it to be printed on a dot matrix printer with limited internal decoding.-Description:Sixel encodes...
s.
Sometimes the text semigraphic characters are simply incorporated into the systems font-set, sometimes special video hardware is used to directly convert the bit-pattern from video memory into the pixels.
TRS-80
TRS-80 was Tandy Corporation's desktop microcomputer model line, sold through Tandy's Radio Shack stores in the late 1970s and early 1980s. The first units, ordered unseen, were delivered in November 1977, and rolled out to the stores the third week of December. The line won popularity with...
where the only way to get discrete pixels on the screen that could be turned on and off individually (a feature called all points addressable
All Points Addressable
All Points Addressable , in the context of a video monitor, dot matrix or any display device consisting of a pixel array, refers to an arrangement bits or cells which can be individually manipulated, as opposed to rewriting the whole array every time a pixel changes.Generally, text modes are not...
) was by the use of a 2x3 pixel matrix of text semigraphics.
If the system also supported color, the color resolution of the resulting pixels was normally equal to the text resolution, often leading to attribute clash
Attribute clash
Attribute clash was a display artifact caused by limits in the graphics circuitry of a number of early color 8-bit home computers, most notably the Sinclair Research ZX Spectrum, where it meant that only two colors could be used in any area of 8×8 pixels...
as the color of a pixel could not be changed on a per pixel basis, but only to one "ink", and one "background" color for all pixels within a character position.
Sometimes the number of characters in the font, dedicated to text semigraphics, could be halved if the system also supported an "invert" attribute, as halve the characters in a full text semigraphics font set are the logical inverse of the other half of the font set. Other tricks that were used to decrease the number of needed characters, was to use a space for the "all bits are zero" character, and to use character 7F hex for the "all bits are on" character, as character 7F hex (decimal 128) was often defined as an "all pixels on" character (this because of the fact that when using papertape punching out al seven holes , created the 7F hex "byte" which was considered to be the "DEL" or "rubout" non-printable character). The assignment in ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
of the rubout character (to character #128) lead designers of VDU's to use "rubout" for an "all black" (or "all white") character, which was often used to represent the cursor.
Examples of use
- The ZX80 and ZX81 where a matrix of 2x2 pixels was used, with the space and invert attribute tricks to keep the number of characters in the ROM needed down.
- Other systems that used a 2x2 matrix were the Panasonic JR-200Panasonic JR-200The Panasonic JR-200U was a simple, relatively early , 8-bit home computer with a chiclet keyboard somewhat similar to the VTech Laser 200.Made of silver grey plastic it had a black matte area around the keyboard area...
and the Mattel AquariusMattel AquariusAquarius is a home computer designed by Radofin and released by Mattel in 1983. It features a Zilog Z80 microprocessor, a rubber chiclet keyboard, 4K of RAM, and a subset of Microsoft BASIC in ROM. It connects to a television set and uses a cassette tape recorder for secondary data storage...
. - The Commodore PETCommodore PETThe Commodore PET was a home/personal computer produced from 1977 by Commodore International...
also had a few characters to support 2x2 text semigraphics in its PETSCIIPETSCIIPETSCII , also known as CBM ASCII, is the variation of the ASCII character set used in Commodore Business Machines 's 8-bit home computers, starting with the PET from 1977 and including the VIC-20, C64, CBM-II, Plus/4, C16, C116 and C128...
character set, The PET was also one of the first systems to rely heavily on Semi graphical charactersSemi graphical charactersSemi graphical characters are characters in a font that are intended to give the impression that a system can support high resolution graphics, while in fact it cannot....
, to get any form of graphics on screen. - The TRS-80 Color ComputerTRS-80 Color ComputerThe Radio Shack TRS-80 Color Computer was a home computer launched in 1980. It was one of the earliest of the first generation of computers marketed for home use in English-speaking markets...
is a good example of a system using 2x2 matrix color semigraphics. But its Motorola 6847 VDCVideo Display ControllerA Video Display Controller or VDC is an integrated circuit which is the main component in a video signal generator, a device responsible for the production of a TV video signal in a computing or game system...
also supported a little used 6-block semigraphics mode - VideotexVideotexVideotex was one of the earliest implementations of an "end-user information system". From the late 1970s to mid-1980s, it was used to deliver information to a user in computer-like format, typically to be displayed on a television.In a strict definition, videotex refers to systems that provide...
and teletextTeletextTeletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
systems used a 2x3 matrix and one foreground ("ink"), and one background color, normally out of a set of eight colors (including black and white). Many systems used "Videotex graphics". One of the best know examples is the Acorn BBC MicroBBC MicroThe BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
's default graphics mode (mode 7) . Other systems which used "Videotex" like graphics are the: ABC 80, the Grundy NewBrainGrundy NewBrainThe Grundy NewBrain was a microcomputer sold in the early 1980s by Grundy Business Systems Ltd of Teddington and Cambridge, England.- Beginnings :...
, many of the Acorn Eurocard systems and the Philips P2000TPhilips P2000--The Philips P2000T home computer was Philips' first real entry in the home computer market, after the Philips Videopac G7000 game system which they already sold to compete with the Atari 2600 and similar game systems. There was also an P2000M version with an additional 80-column card for use...
which actually used a TeletextTeletextTeletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
chip designed for their TV's. - The PC-8001 managed to combine normal text with a full set of 256 text semigraphic "characters" in a 2x4 matrix, how the system managed to distinct the 8 bits used for "normal" fonts from the 8 bits needed for the text semigraphics is unclear.
see also
- list of home computers by video hardware
- text modeText modeText mode is a kind of computer display mode in which the content of the screen is internally represented in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a...
- Semi graphical charactersSemi graphical charactersSemi graphical characters are characters in a font that are intended to give the impression that a system can support high resolution graphics, while in fact it cannot....
- SixelSixelSixel is a bitmap graphics format supported by terminals and printers from DEC, most notably the VT320 and VT420. It encodes bitmaps into a character-based format that is easy to interpret, allowing it to be printed on a dot matrix printer with limited internal decoding.-Description:Sixel encodes...
- PETSCIIPETSCIIPETSCII , also known as CBM ASCII, is the variation of the ASCII character set used in Commodore Business Machines 's 8-bit home computers, starting with the PET from 1977 and including the VIC-20, C64, CBM-II, Plus/4, C16, C116 and C128...