Semigraphics

Text-based semigraphics, pseudographics, or character graphics is a primitive method used in early text mode video hardware to emulate raster graphics without having to implement the logic for such a display mode.

There are two different ways to accomplish the emulation of raster graphics. The first one is to create a low-resolution all points addressable mode using a set of special characters with all binary combinations of a certain subdivision matrix of the text mode character size; this method is referred to as block graphics, or sometimes mosaic graphics.

The second one is to use special shapes instead of glyphs (letters and figures) that appear as if drawn in raster graphics mode, sometimes referred to as semi- or pseudo-graphics; an important example of this is box-drawing characters.

Semigraphical characters (including some block elements) are still incorporated into the BIOS of any VGA compatible video card, so any PC can display these characters from the moment it is turned on, even when no operating system is yet loaded. Single and double lines are still often drawn with this method when the system uses text mode; for example when running the BIOS setup program.

Many of these historical ideas have been adopted into Unicode, in, for example in the Symbols for Legacy Computing, Block Elements, Box Drawing and Geometric Shapes Unicode blocks.

For characters consisting of 8 vertical and 8 horizontal pixels (an 8x8 character) you would need 2^64 or 10^19 characters to contain every possible combination of the 64 pixels. Instead, if you divide the 8×8 character in 2×2 "pixels" called quadrants you only need 16 characters for every combination. This was, for example, used in the Sinclair ZX81).

Another example divided an 8×12 pixel character vertically in two halves and horizontally in three parts, and then 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 of a 2×3 block mosaic matrix of so-called squots (square dots) or sextants. Less often used versions use a 1×6 "matrix", in which case these six "pixels" are sometimes referred to as sixels.

Sometimes the text semigraphical 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. In rare cases a character matrix was not dividable vertically by three, for example in an 8×8 matrix the mosaic is sometimes divided so that it uses a 3:2:3 scan line scheme. The Galaksija's graphics mode is an example, although with a 4:5:4 scheme the distortion effect was minimal.