Syntax ARC [#ch][,x1,y1] TO x2,y2,angle *[[;xi,yi] TO xj,yj,anglej]*
Location QL ROM

ARC causes the two points at the co-ordinates (x1,y1) and (x2,y2) to be connected with an arc. The arc is defined as the sector of the circle formed by drawing two straight lines from the two given co-ordinates to the centre of the circle, where angle is the angle (in radians) between those two lines. Therefore, angle=0 is a straight line and angle=PI, half a circle.

It therefore follows that the greater ABS(angle), the more pronounced is the curve on the arc.

Multiple arcs can be draw with the same command by adding extra sets of parameters for each additional arc. For example:

ARC 100,10 TO 120,40,3 TO 80,70,3

will actually draw two arcs, one between the points (100,10) and (120,40) with angle=3 and the second between the points (120,40) and (80,70), also with angle=3.

When drawing multiple arcs, there is actually no need for the next arc in the series to begin at the end of the previous arc, provided that a semicolon ‘;’ is inserted between each set of parameters. For example:

ARC 100,10 TO 120,40,3;30,40 TO 50,60,3

Whether the arc is drawn clockwise or anti-clockwise depends upon two factors: If y1>y2 and angle>0, then the arc will be drawn anti-clockwise. Swapping the two co-ordinates or making the angle negative will force the arc to be drawn clockwise.

Co-ordinates refer to the window relative graphic co-ordinate system, which is relative to the graphic origin. The size and position of the arc also depend upon the SCALE of the window. If no first point is given then the current position of the graphic cursor is used. The graphic cursor is set to the last point of the arc on completion of the command.

Example 1

100 WINDOW 448,200,32,16: CLS: SCALE 4,-2,-2
110 FOR t=PI/16 TO 2*PI STEP PI/16
120   ARC SIN(t),COS(t) TO COS(t),SIN(t),PI*SIN(t/2)
130 END FOR t

Example 2

100 WINDOW 448,200,32,16: CLS: SCALE 100,0,0
110 FOR x=10 TO 90 STEP 10
120   FOR y=10 TO 90 STEP 10
130     ARC x,y TO y,x,PI/2
140   END FOR y
150 END FOR x

Example 3

100 POINT #2,150,50
110 FOR x=50 TO 150 STEP 20
120   ARC #2 TO x,50,PI/2
130 END FOR x


On non Minerva v1.89+ ROMs, ARC does not work properly - small angles produce rubbish, wrong co-ordinates are used and the last pixel of the arc is not always drawn. Even SMS does not cure these problems.


An angle of 2*PI would form a complete circle and cannot be drawn, therefore the maximum value for ABS(angle) is a value just less than 2*PI.


On some ROM versions, the command does not check that the TO separator is present - however, SMSQ/E (at least) does and therefore some programs may fail if used under SMSQ/E and they have used a comma instead of TO.


Some QDOS implementations of this command can corrupt the hard disk drive in some obscure circumstances. Get Minerva or SMSQ/E to be safe!!


ARC_R works in exactly the same way as ARC but uses a relative co-ordinate system, where the origin is the current position of the graphic cursor.

SCALE sets the graphic origin and also the size of the window.