Text semigraphics

Text semigraphics is a primitive method used in early video hardware to emulate per pixel addressable graphics 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 sixels.^[1] 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.

The seminal use of this technology was in the TRS-80 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) was by the use of a 2x3 pixel matrix of text semigraphics.^[2]

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 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 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

other examples of the use of text semigraphics are:

• 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.^[3]
• Other systems that used a 2x2 matrix were the Panasonic JR-200 and the Mattel Aquarius.
• The Commodore PET also had a few characters to support 2x2 text semigraphics in its PETSCII character set, The PET was also one of the first systems to rely heavily on Semi graphical characters, to get any form of graphics on screen.
• The TRS-80 Color Computer is a good example of a system using 2x2 matrix color semigraphics. But its Motorola 6847 VDC also supported a little used 6-block semigraphics mode ^[4]
• Videotex and teletext 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 Micro's default graphics mode (mode 7).^[5] Other systems which used "Videotex" like graphics are the: ABC 80, the Grundy NewBrain, many of the Acorn Eurocard systems and the Philips P2000T which actually used a Teletext 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.
• One of the last video systems where text semigraphics were a useful option was the Color Graphics Adapter (CGA). This graphics adapter for early IBM-PCs only supported four colors in its high-res graphics mode, but in text mode it could display 16 different colors. Although the CGA adapter's font ROM did not contain all of the otherwise typical text semigraphics characters, it was still possible to create a 160 x 100 CGA text semigraphics mode using a semi-documented trick.^[6] With this trick, much nicer games (like this one [1]) could be programmed on CGA equipped PCs.

See also

• list of home computers by video hardware
• text mode
• Semi graphical characters
• Sixel
• PETSCII

References