Ziv Kesten - 1 year ago 98

Android Question

I have an arc and i wish to draw scale marks at 0, 45, 90, 135, 180 degrees, can anyone help me with the math needed to achive the x,y of points 5 and 30 on this sketch?:

here is my code for drawing the 1 scale mark.

`private void drawScale(Canvas canvas) {`

//canvas.drawOval(scaleRect, scalePaint);

canvas.save();

Paint p = new Paint();

p.setColor(Color.WHITE);

p.setStrokeWidth(10f);

canvas.drawLine(rectF.left-getWidth()/20, rectF.height()/2, rectF.left, rectF.height()/2, p);

canvas.restore();

}

Answer Source

You can calculate its rotation using `sin`

and `cos`

. Lets assume that you have zero point `A`

and want to rotate it to point `B`

which is rotated for 30°.
Something like this:

Basically new point is at `(cx+x,cy+y)`

. In this particular case definition of `sin`

and `cos`

would be next:

```
sin = x/R
cos = y/R
```

It is not hard to get exact `x`

and `y`

. So to rotate point on particular angle in circle with know radius we need to calculate coordinates in next way:

```
x = cx + sin(angle) * R;
y = cy + cos(angle) * R;
```

Now lets get back to Android and Canvas!

```
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.save();
float cx = getWidth() / 2f;
float cy = getHeight() / 2f;
float scaleMarkSize = getResources().getDisplayMetrics().density * 16; // 16dp
float radius = Math.min(getWidth(), getHeight()) / 2;
for (int i = 0; i < 360; i += 45) {
float angle = (float) Math.toRadians(i); // Need to convert to radians first
float startX = (float) (cx + radius * Math.sin(angle));
float startY = (float) (cy - radius * Math.cos(angle));
float stopX = (float) (cx + (radius - scaleMarkSize) * Math.sin(angle));
float stopY = (float) (cy - (radius - scaleMarkSize) * Math.cos(angle));
canvas.drawLine(startX, startY, stopX, stopY, scalePaint);
}
canvas.restore();
}
```

Code will draw marks with step of 45°. Note you need to convert angle to radians and for Y axis I used minus cause on canvas it is flipped. Here is what I have got: