Calculating Coordinates on Screen - coordinates

Question

I know there are a load of other questions on this topic, I think I've read most of them, along with Wikipedia and a bunch of other articles but I am missing (I think) some simple arithmetic to complete my coordinate calculations.
I have this code:
typedef struct {
double startX;
double startY;
double x2;
double y2;
double length;
double angle;
double lastAngle;
} LINE;
void lineCalc(LINE *lp) {
double radians = lp->angle * 3.141592653589793/180.0;
lp->x2 = lp->startX + (lp->length * cos(radians));
lp->y2 = lp->startY + (lp->length * sin(radians));
fprintf (stderr, "lineCalc:startX:%2.3f, startY:%2.3f, length:%2.3g, angle:%2.3f, cos(%2.3f):%2.3f, x2:%2.3f, y2:%2.3f\n", lp->startX, lp->startY, lp->length, lp->angle, lp->angle, cos(radians), lp->x2, lp->y2);
}
int main() {
// Initialise to origin of 250, 250. 0, 0 for initial end point. Length 150, first angle 60 (degrees), l.lastAngle currently not used
LINE l = {250, 250, 0, 0, 150, 60, 0};
lineCalc(&l);
//drawLine(&l);
l.startX = x2; l.startY = y2; // make last end point, new start point. Angle stays at 60 degrees
lineCalc(&l);
//drawLine(&l);
l.startX = x2; l.startY = y2;
lineCalc(&l);
//drawLine(&l);
}
Which calculates the end point of a line given its start point, length and angle. All fine and good, but what I want to be able to do is to draw a shape, a triangle would be a start.
At the moment the code will make the calculation, draw the line (in reality it is generating SVG), make the last end point the next origin, recalculate, draw the next line etc...
The crucial bit that I am missing is how to get angle to be relative to the last line drawn. At the moment, the moving of the origin works fine, but the angle stays the same, thus three lines with angles of 60 degrees will just draw a straight line because the angle is relative to the start rather than relative to the last line.
Just in case it is relevant, with SVG horizontal is zero degrees. Thus a line 50 units long, starting at y100, x100 at an angle of 90 degrees will have an end point of y150, x100.
Could someone point out the obvious thing that I a missing to make the angles correct relative to the last line please?

Answer 1

If you take the angle at which the first line is drawn at as theta:
theta + 180 deg OR theta - 180 deg will face you back down the line you just drew.
Then theta + 180 deg + 60 OR theta - 180 deg + 60 will face you at 60 degrees to the first line.
You need to choose whether to + or - the 180 based on the range of degrees that svg uses (does it go -180 to 180 or 0 to 360) and how big your starting theta is. Also you need to choose + or - 60 degrees based on the side of the first line that you want to draw the second line.
Once you've calculated the angle you're drawing the second line at (theta + 180 + 60 for instance) then you need to take that as your next theta to calculate the angle for the third line.