How to draw a diameter of a circle in Android

Question

I referred the question Draw a circle within circle at a distance of 10 which comes close to my requirement. I just needed two concentric circles one inside the other inner one with radius scaled to 300 mts and outer one with radius scaled to 500 mts.

I have been able to draw the two circles scaled in pixels on my screen using a transformation method that transformed distance in metres to corresponding pixel distances. The next step is to draw a plus inside the circle; the lines being the diameters. Hence they will pass through the center and contain two points on the circle.

1)I have the lat long and pixel details of the center of the circle.

2)I know that the angle between the center and the either points on the circle need to be 90 degrees.

3)I must use canvas.drawLine().

But what would be the best way to get these points on the circle so that a line can be drawn through these three points.

(Point on the circle at the top, center of the circle,Point on the circle at the bottom).

Greatly appreciate your help.

EDIT: I tried the following code after some searching

        //double degrees = 90.0;
        //double radians = Math.toRadians(degrees);
        //int x1 = (int) (500 * Math.cos(radians) + x);
        //int y1 = (int) (500 * Math.sin(radians) + y);      
        //canvas.drawLine(x, y, x1, y1, mSelectionBrush);


        canvas.drawLine(x, y-500, x, y+500, mSelectionBrush);
    canvas.drawLine(x-500, y, x+500, y, mSelectionBrush);

x,y are the coordinates of the center. 500 is the radius of the outer circle. The output I see is this. The line extends below. Am I going the right way?

enter image description here

Navjot Singh Navjot Singh · Accepted Answer · 2014-03-03T07:35:46

I think you're on the right track. Notice that only Y changes for the vertical line. Assuming center is given by C(a,b) So the end-points would be (a,b-r) and (a,b+r)

For the horizontal line only x changes: (a-r,b) and (a+r,b) would be the end-points.

r is the radius.

How to draw a diameter of a circle in Android

1 Answers