android 拍照人像對正框

項目中要繪製一個人像對正框，第一個想到的就是先繪製背景，然後利用PorterDuffXfermode(PorterDuff.Mode.XOR)將人像摳掉，繪製人像框，然而，調了許久才發現當預覽區域的背景是半透明的，PorterDuff.Mode.XOR不能將重疊區域摳掉，預覽區域是不透明時就可以摳掉，具體原因可能是PorterDuff.Mode對透明度有一些要求吧。後來想到ucrop庫在裁剪圖片時的對正框和這裏很類似，看了源碼後，才知道還有這種操作，主要是clipPath方法的第二個參數，話不多說，看圖上代碼。
設計稿：

實現：

實現方式：上方是一個貝塞爾曲線，下方是大半個橢圓

public class FaceAlignView extends View {
    private Context mContext;
    private RectF mBgRect;
    private Path mClipPath;
    private Paint mClipPaint;
    private float mVerticalMargin;
    private float mHorizontalMargin;
    private PathMeasure mPathMeasure;

    public FaceAlignView(Context context) {
        this(context, null);
    }

    public FaceAlignView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public FaceAlignView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mContext = context;
        initPaint();
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBgRect.left = 0;
        mBgRect.top = 0;
        mBgRect.right = w;
        mBgRect.bottom = h;
        mClipPath.reset();
        //畫下方的橢圓
        mClipPath.addArc(mHorizontalMargin, mVerticalMargin / 3f, w - mHorizontalMargin, h - mVerticalMargin, -35, 250);
        //取出橢圓的右上角的起始座標
        float[] pos = new float[2];
        float[] tan = new float[2];
        mPathMeasure.setPath(mClipPath, false);
        mPathMeasure.getPosTan(0, pos, tan);
        //畫上方的二階貝塞爾曲線
        mClipPath.moveTo(w - pos[0], pos[1]);
        mClipPath.cubicTo(w - pos[0] * 0.86f, pos[1] * 0.35f, pos[0] * 0.86f, pos[1] * 0.35f, pos[0], pos[1]);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.save();
        //截取人像透明區域 Region.Op.DIFFERENCE path外的區域才允許繪製
        canvas.clipPath(mClipPath, Region.Op.DIFFERENCE);
        canvas.drawColor(mContext.getResources().getColor(R.color.face_align_bg));
        canvas.restore();
        //繪製人像邊緣
        canvas.drawPath(mClipPath, mClipPaint);
    }

    private void initPaint() {
        mBgRect = new RectF();
        mClipPath = new Path();
        mPathMeasure = new PathMeasure();
        mClipPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mClipPaint.setColor(mContext.getResources().getColor(R.color.face_align_clip_path));
        mClipPaint.setStrokeWidth(DisplayUtils.dpToPx(4));
        mClipPaint.setStyle(Paint.Style.STROKE);
        mClipPaint.setStrokeCap(Paint.Cap.ROUND);
        mVerticalMargin = dpToPx(50);
        mHorizontalMargin = dpToPx(40);
    }

    public static int dpToPx(int dp) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, Resources.getSystem().getDisplayMetrics());
    }

}