如何在android上繪制橢圓的弧線？

Question

我想在我的 android 應用程序中繪制一條橢圓弧。 我使用了這個代碼：

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.save();
        angle=45; //angle of ellipse major axis with X-axis.
        startAngle=0; //start angle of arc
        sweepAngle=90; //sweep angle of arc
        a = 200; //major axis of ellipse
        b = 100; //minor axis of ellipse
        
        canvas.rotate(angle, center.x, center.y);
        //draw the arc
        canvas.drawArc(rect, startAngle - angle, sweepAngle, true, paintLine);

        paintLine.setPathEffect(new DashPathEffect(new float[] { 5, 5 }, 0));
        canvas.drawOval(rect, paintLine);

        canvas.restore();
        paintLine.setPathEffect(null);
    }

我收到這個形狀：

我需要的弧應該在此圖像的紅點處開始和結束：

請告訴我我犯了什么錯誤。

謝謝。

Answer 1

當 android 繪制橢圓弧時，就好像它首先在提供的掃掠角上繪制圓弧一樣。 然后它縮放這個圓弧，使其適合指定的橢圓。 橢圓的起始角和掃描角將與指定的起始角和掃描角不同。 要獲得想要的角度，必須將實際角度指定為 tan(actual-angle) = (a/b)*tan(wanted-angle)

public class Main3Activity extends Activity {
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    View view = new View(this) {
        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            canvas.save();
            float angle=45; //angle of ellipse major axis with X-axis.
            // These are the wanted angles
            float startAngle_ = (float)Math.toRadians(315); //start angle of arc
            float endAngle_ = (float)Math.toRadians(45); //end angle of arc
            float a = 200; //major axis of ellipse
            float b = 100; //minor axis of ellipse
            float xc = getWidth()/2f;
            float yc = getHeight()/2f;

            // These are the angles to use
            float startAngle = (float)Math.toDegrees(
                            Math.atan2(a*Math.sin(startAngle_),b*Math.cos(startAngle_)));
            float endAngle = (float)Math.toDegrees(
                    Math.atan2(a*Math.sin(endAngle_),b*Math.cos(endAngle_)));
            float sweepAngle = endAngle - startAngle;

            RectF rect = new RectF(xc-a,yc-b,xc+a,yc+b);
            Paint paintLine = new Paint(Paint.ANTI_ALIAS_FLAG);
            paintLine.setStyle(Paint.Style.STROKE);

            canvas.rotate(angle,xc,yc);
            //draw the arc
            canvas.drawArc(rect, startAngle, sweepAngle, true, paintLine);

            paintLine.setPathEffect(new DashPathEffect(new float[] { 5, 5 }, 0));
            canvas.drawOval(rect, paintLine);

            canvas.restore();
            paintLine.setPathEffect(null);
        }
    };

    FrameLayout.LayoutParams layout = new FrameLayout.LayoutParams(
            ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);

    addContentView(view,layout);
}

這個問題已經快七年了！ 是時候更新文檔了？