How to calculate the center point of an arc? - math

Question

I have a CustomPieView which is made of several pie slices. I have to draw something in the middle of every pie section.
//Inside Activity's onCreate Method
public void onCreate(Bundle savedInstanceState){
int size = 400;
int bgColor = 0xffa11b1;
ViewPieChart piechart = (ViewPieChart) findViewById(R.id.pieChartView);
piechart.setCallBack(this);
piechart.setLayoutParams(new LayoutParams(size, size));
piechart.setGeometry(size, size, 2, 2, 2, 2, 2130837504);
piechart.setSkinparams(bgColor);
piechart.setData(piedata, maxCount);
piechart.invalidate();
}
//CustomPieView extends View
public void setGeometry(int width, int height, int gapleft, int gapright, int gaptop, int gapbottom, int overlayid) {
mWidth = width;
mHeight = height;
mGapleft = gapleft;
mGapright = gapright;
mGapBottm = gapbottom;
mGapTop = gaptop;
}
protected void onDraw(Canvas canvas){
canvas.drawColor(Color.DKGRAY);
mBagpaints.setAntiAlias(true);
mBagpaints.setStyle(Paint.Style.FILL);
mBagpaints.setColor(0x88FF0000);
mBagpaints.setStrokeWidth(0.0f);
mLinePaints.setAntiAlias(true);
mLinePaints.setColor(Color.WHITE);
mLinePaints.setStrokeWidth(3.0f);
mLinePaints.setStyle(Paint.Style.STROKE);
sLinePaint.setAntiAlias(true);
sLinePaint.setColor(Color.BLACK);
sLinePaint.setStrokeWidth(3.0f);
sLinePaint.setStyle(Paint.Style.STROKE);
RectF mOvals = new RectF(mGapleft, mGapTop, mWidth - mGapright, mHeight
- mGapBottm);
mStart = START_INC;
PieDetailsItem item;
for (int i = 0; i < mdataArray.size(); i++) {
item = (PieDetailsItem) mdataArray.get(i);
mBagpaints.setColor(item.color);
mSweep = (float) 360* ((float) item.count / (float) mMaxConnection);
canvas.drawArc(mOvals, mStart, mSweep, true, mBagpaints);
canvas.drawArc(mOvals, mStart, mSweep, true, mLinePaints);
// The function below is setting the global vars
cSegX, cSegY to the center of the segment. Not Working!!
calculateMidPointOfPieSegment(mSweep);
canvas.drawPoint(cSegX, cSegY, sLinePaint);
mStart = mStart + mSweep;
}
mState = IS_DRAW;
callBack.onDrawFinished(null);
}
private float calculateRadius(){
float width = mWidth/2;
float height = mHeight/2;
if(width < height){
return width;
}else{
return height;
}
}
private void calculateMidPointOfPieSegment(float sweepAngle){
cSegX = (float)((calculateRadius()/2)*Math.cos(Math.toRadians(sweepAngle/2))+(mWidth/2));
cSegY = (float)((calculateRadius()/2)*Math.sin(Math.toRadians(sweepAngle/2))+(mHeight/2));
}
Just see the dots in the image below, it has to be in the center of every arc.
Screenshot
Ref.
Complete Source

Answer 1

Let me know if this clears it up. This is the basic logic you want to implement.
//rimPoint = (rX, rY)
//centerPoint = (cX,cY)
//theta is the angle of the the midpoint measured anticlockwise from the x axis,
//or the average of the two angles making up the slice(measured anticlockwise from
//the x axis)
private void calculateMidPointOfPieSegment(cX,cY, theta)
{
float rX = cX + calculateRadius()*(cos(theta))
float rY = cY + calculateRadius()*(sin(theta))
cSegX = (rX+cX)/2;
cCegY = (rY+cY)/2;
}
If theta isn't in radians, sub it out for theta*Math.PI/180