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ZX81 Block Graphics Characters

The Sinclair ZX81 did not have true high resolution graphics; its display was limited to text characters only. However by clever design of some characters it was possible to double the apparent resolution of the screen from 32 x 22 characters to 64 x 44 graphics squares.

The ZX81 character set included 22 graphics symbols as shown below: (For clarity the outline of the character cell is shown in grey.)

ZX81 Graphics Characters

The top sixteen characters were used by the PLOT statement to simulate a graphical display. As can be seen each character square is effectively divided into four smaller squares and every combination of none, 1, 2, 3 or 4 squares within the larger square is catered for.

When a PLOT command, using (x,y) values from (0,0) to (63,41), was executed the BASIC interpreter calculated which character square in the range (0,0) to (31,21) this corresponded to.
It then looked at which of the sixteen graphics characters was plotted there already (if any) and decided which one to change it to to give the effect of plotting the new quarter-square 'pixel'. The process was fairly slow but allowed a semi-graphics display to be produced with a low memory requirement.

The final six characters in the diagram above could be used to draw horizontal bar charts at double the normal resolution. One bar could be in solid black, the next one in the chequerboard pattern, the next one in black again, and so on.
Unfortunately there were no direct BASIC commands to handle this and it was awkward to program.

Some surprisingly effective displays could be produced on the ZX81, such as this one from J.K. Greye's '3D Monster Maze', which is believed to be the very first 3D-display, first-person-perspective game on a home computer:

The T-Rex approaches...

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