Question

I know this isn’t a hard trig issue, but sadly I am math retarded.

I need to draw a line of 50 pixels from a known starting point along a known angle to an unknown ending point. The angle is derived from a starting point (400,400) and a mouse click; the line needs to be drawn towards the mouse click, but only 50 pixels towards the click.

I’ve google’d extensively and found a number of solutions, but it’s just not coming together for me.

Here is how I’m getting the angle.:

float angle = (float) Math.toDegrees(Math.atan2(400 - event.getY(), 400 - event.getX()));
float angleInDegrees = (angle + 270) % 360;

“event” is a mouse click.

float endX = 250 + 50 * (float)Math.cos(angleInDegrees);
float endY 250 + 50 * (float)Math.sin(angleInDegrees);

line.setStartX(400);
line.setStartY(400);
line.setEndX(endX);
line.setEndY(endY);

Everything I’ve found revolved around Math.cos and Math.sin but I’m still not getting it. I think the issue is related to mapping radians to scene coordinates, but I’m not sure. So people, in what way am I stupid? I’d appreciate any help.

Answer 1

You don’t even have to deal with radians/degrees. Go back to the geometrical definition of sine and cosine: sine is opposite/hypotenuse, cosine is adjacent/hypotenuse. (“Opposite” and “adjacent” mean the legs of the right triangle respectively opposite and adjacent to the angle you’re taking the sine or cosine of).

So:

float opposite = event.getY() - 400;
float adjacent = event.getX() - 400;
float hypotenuse = Math.sqrt(opposite*opposite + adjacent*adjacent);

float cosine = adjacent/hypotenuse;
float sine = opposite/hypotenuse;

float endX = 400 + 50 * cosine;
float endY = 400 + 50 * sine;