Qt How can add an item to scene with an offset? - c++

Question

I add a QGraphicsItem(bullet) to the base of a line. The advance method moves the item through the item making it seem as if it comes from the tip of line. After adding collision this doesn't work. Is there a way to add an offset to the setPos(x,y) value to make it appear at the tip of the line instead of the base.
Also the line rotates in a 360 degree angle so it needs to translate to where ever the line is pointing.
//function that adds item to base of line created
qreal dirx = m_FireTarget1.x()+140;
qreal diry = m_FireTarget1.y()-195;
qreal length = sqrt(dirx*dirx+diry*diry);
if (length!=0)
{
// normalized direction vector
qreal invLength= 1.0/length;
dirx *= invLength;
diry *= invLength;
// creating an angle perturbation of +/- 3°
qreal alphaPerturbation = static_cast<qreal>(qrand()%6-3) * M_PI / 180.0;
qreal xPerturbation = cos(alphaPerturbation);
qreal yPerturbation = sin(alphaPerturbation);
dirx = dirx*xPerturbation - diry*yPerturbation;
diry = diry*xPerturbation + dirx*yPerturbation;
GraphicsCircle * circle = new GraphicsCircle(dirx, diry, -140, 195);
addItem(circle);
The -140, 195 is the base of where line is created. Seems like I already did what you're saying to do I believe.

Answer 1

Let's say your line has a certain degreeAngle and you want to move the bullet to a certain distance in that direction, you'll have to do:
// cos and sin functions get radians angle as argument so you must convert it
radiansAngle = degreeAngle * PI / 180;
offsetX = distance * cos(radiansAngle);
offsetY = distance * sin(radiansAngle);
In your case this translate to:
qreal radiansAngle = line.angle() * M_PI / 180;
qreal offsetX = line.length() * cos(radiansAngle);
qreal offsetY = line.length() * sin(radiansAngle);
So your new position is the old one plus the offset:
qreal newX = -140 + offsetX;
qreal newY = 195 + offsetY;
And then I'm sorry but I can't understand how you pass arguments to the GraphicsCircle constructor but if GraphicsCircle * circle = new GraphicsCircle(dirx, diry, -140, 195); place the circle to the coordinates (-140,195), then you should use...
GraphicsCircle * circle = new GraphicsCircle(dirx, diry, newX, newY);
addItem(circle);
...to place it to the new coordinates.