112
tocalculate starting and ending points for a line (in our circle the “lines” are very short - sometimes the starting and ending points are the same). The coordinates of the starting point of the line are assigned to variables Xl and Y 1. The line ends at point
X2, Y2. When these coordinates have been calculated, a subroutine call is made to line 250. This subroutine calculates the coordinates of individual points along that line.
After these coordinates have been determined, the subroutine at line 400 is called. This routine turns “on” an individual dot in our array called BIT%. (Keep in mind that no printing has been done yet; the computer is still drawing the image on its “graph paper” in memory.) The way an individual dot is turned on is us- ing the logical OR function in line 470.
When all the points have been plotted in memory, printing begins at line 130. We first set the line spacing to 6/72 inch us- ing the (ESC)“A”CHR$( n ) command. This is so that there are no gaps between rows of dots. Then the loop from line 150 to line 220 prints the dot graphics image one line (which is six dots high) at a time. The variable A$ is used to build a string of all the columns of BIT% in a given row.
As you can see, by taking the program in small pieces and
analyzing it, graphics programming does not have to be dif- ficult. If you want to try some other plots, try these (replace lines after 600 with the lines below). The printouts from each program are shown below the listing.
600 '
610' Subroutine to plo,t a star
6201
630RAD=9
640FOR ANG%=O TO 360 STEP 45
650RANG=ANG%*3.14159/180
660RANG2=(ANG%+l35)*3.14159/180
670Xl=RAD*COS(RANG)tlO
680Yl=RAD*SIN(RANG)+lO 6go X2=RAD*COS(RANG2)+10
700Y2=RAD*SIN(RANG2)+10
710GOSUB 250
