1. blueX = redX + dist * cos(radians(ang));
2. blueY = redY + dist * sin(radians(ang));

where

redX, redY are the coordinates of the red dot
dist is the distance between the two dots (in this case half the rectangle width)
ang is the angle (in degrees) that the rectangle has been rotated (clockwise direction)

 3 O'clock = 0 degrees
 6 O'clock = 90 degrees
 9 O'clock = 180 degrees
12 O'clock = 270 degrees

In your picture ang ~= 225 degrees

Thank man for the response but I can't get I work, I think my problem is from my method of calculating the rotate angle

1. blueX = redX + dist * cos(ang);
2. blueY = redY + dist * sin(ang);

Try this you may already have ang measured in radians (it looks like it from your code)

Either.

Either.

Does that mean you have solved it or not?

Nop, I really can't find what is no correct.

I have your code - since I don't have your background image I need to know its size because you use this to size the sketch.

Do you want the cannon to point towards the mouse pointer?

If a point is being drawn there, surely screenX() and screenY() would work...

The image size is 800x600, and yes, the canon is supposed to point towards the mouse pointer.

The angle from the cannon centre to the mouse is given by the formula

1. ang = atan2(mouseY - cannonY, mouseX - cannonX);  // angle is in radians

The angle can be plugged into

1. blueX = redX + dist * cos(ang);
2. blueY = redY + dist * sin(ang);

If you are still having problems tomorrow then I will look at the code in more detail.

That's really better !

Since you are using a rectangle I suggest that you make the cannon width > height. That way at 0 degrees the cannon will be pointing east so you can use the angle from atan2 directly. Otherwise your connon it likely to be pointing 90 degrees to the mouse position.