Animating along an elliptical path in OpenGL - c++

Question

I am trying to make a red circle follow the path of a semi-circle using the DDA algorithm in OpenGL. I almost have it, though the circle is slightly offset on its X-axis, which increases as the angle of the semi-circle increases.
Any assistance would be greatly appreciated! Here's my code:
scrPt movecircle (scrPt p1, scrPt p2)
{
scrPt circlePos;
float angle, x = p1.x, y = p1.y, vectorX, vectorY;
// Get tahe x distance between the two points
int dx = p2.x - p1.x, steps;
// Get the y distance between the two points
int dy = p2.y - p1.y;
// Get the length between the points
float length = sqrt(dx*dx + dy*dy);
if (fabs (dx) > fabs (dy))
steps = fabs (dx);
else
steps = fabs (dy);
// calculate the direction
float xIncrement = float (dx) / float (steps);
float yIncrement = float (dy) / float (steps);
if (nextPos == 0)
{
for(int i = 0; i < steps; i++)
{
glClear(GL_COLOR_BUFFER_BIT);
angle = PI * i / steps;
vectorX = x + (length / 2) * cos(angle + theta);
vectorY = y + dy / 2 + (length / 2) * sin(angle + theta);
circlePos.x = round(vectorX - length / 2);
circlePos.y = round(vectorY);
drawCircle (circlePos.x, circlePos.y);
drawArch();
glFlush();
usleep(3000);
}
}
else
{
for (int i = 0; i < steps; i++)
{
glClear(GL_COLOR_BUFFER_BIT);
drawCircle (round(x),round(y));
glFlush();
usleep(3000);
x += xIncrement;
y += yIncrement;
}
}
return circlePos;
}

Answer 1

There were a couple of errors in the for-loop that were causing the issue. I needed to change
this:
vectorX = x + (length / 2) * cos(angle + theta);
to this:
vectorX = x + (dx / 2) + (length / 2) * cos(angle + theta);
and this:
circlePos.x = round(vectorX - (length / 2));
to this:
circlePos.x = round(vectorX);