Hey, I'm trying to rotate a line using 2d matrix rotation and have been getting some problems with negative values. The line appears to rotate by (the degree specified + 180) rather than purely the degree specified. I'm sure it's a pretty simple solution but I've been trying to work it out for about 2 hours now and am not anywhere nearer to a solution! If anyone could help me out it would be much appreciated!

Here is the code (it's in java, uses the acm library):





import acm.program.*;
import acm.graphics.*;
import java.lang.Math;

public class rotating_line extends GraphicsProgram {

GLine line;

private static final double degreesRotation = -5;

public void run() {

double difference = 300;

GPoint p1 = new GPoint(0, 0);
GPoint p2 = new GPoint(0, 500);


for (int i = 0; i < 19; i++) {

line = new GLine(p1.getX() + difference, p1.getY() + difference, p2.getX() + difference, p2.getY() + difference);
add(line);

GLabel xLabel = new GLabel("x: " + Integer.toString((int)p2.getX()), getWidth() - 150, (i * 10) + 20);
add(xLabel);
GLabel yLabel = new GLabel("y: " + Integer.toString((int)p2.getY()), getWidth() - 50, (i * 10) + 20);
add(yLabel);

p2 = rotateLeft(degreesRotation, p1, p2, i);

pause(20);

}
}

public GPoint rotateLeft(double degreesRotation, GPoint p1, GPoint p2, int i) {

double degree = Math.toRadians(degreesRotation);

double dx = p1.getX() - p2.getX();
double dy = p1.getY() - p2.getY();

double xPos = p1.getX();
double yPos = p1.getY();

double x1 = (dx * Math.cos(degree)) - (dy * Math.sin(degree));
double y1 = (dx * Math.sin(degree)) + (dy * Math.cos(degree));

GLabel lx1 = new GLabel("x1: " + (int)x1, getWidth() - 350, (i * 10) + 20);
add(lx1);
GLabel ly1 = new GLabel("y1: " + (int)y1, getWidth() - 250, (i * 10) + 20);
add(ly1);

GPoint temp = new GPoint(x1 + xPos, y1 + yPos);


return temp;


}

}




edit: actually writing this post made me realise that it was rotating 180 degrees more than it should, so i have now solved the problem merely by asking for help. (changed this line: double degree = Math.toRadians(180+degreesRotation)


post can be deleted!