A pixel is specified by two numbers, its coordinates. The first, its x-coordinate, says how for it is across from the extreme left-hand column (remember, X is ACROSS), & the second, its y-coordinate, says how far if is up from the bottom. These coordinates are usually written as a pair in brackets, so (0,0), (63,0), (0,43) & (63,43) are the bottom left-, bottom right-, top left-, & top right-hand corners.
The statement
PLOT x-coordinate, y-coordinate
blacks in the pixel with these coordinates, while the statement
UNPLOT x-coordinate, y-coordinate
blanks it out.
Try this measles program:
10 PLOT INT (RND*64),INT (RND*44)
20 INPUT A$
30 GOTO 10
This plots a random point each time you press NEWLINE.
Here is a rather more useful program. It plots a graph of the function SIN (a sine wave) for values between 0 & 2
.10 FOR N=0 TO 63
20 PLOT N, 22+20*SIN (N/32*PI)
30 NEXT N
This next one plots a graph of SQR (part of a parabola) between 0 & 4:
10 FOR N=0 TO 63
20 PLOT N,20*SQR (N/16)
30 NEXT N
Notice that pixel coordinates are rather different from the line & column in an AT item. At the end of this chapter is a diagram which you may find useful in working out pixel coordinates & line & column numbers.
