如何在不绘制透明部分的情况下用手指在View中绘制？

Question

假设我有 500 x 500 ImageView，一半 ImageView 是红色的，第二部分是透明的。我只想在 ImageVIew 的非透明部分用手指绘制，所以如果我在透明部分绘制，它将不可见，就像这个图像。可能吗？

我试过了，但这段代码不正确。

public class DrawingView extends ImageView {

        private boolean isTransparent;
        private float mX, mY;
        private static final float TOUCH_TOLERANCE = 4;
        private Bitmap mBitmap;
        private Canvas mCanvas;
        private Path mPath;
        private Paint mBitmapPaint;
        Context mContext;
        private Path circlePath;

        public DrawingView(Context c) {
            super(c);
            mContext = c;
            mPath = new Path();
            mBitmapPaint = new Paint(Paint.DITHER_FLAG);
            circlePath = new Path();
        }

        @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
            super.onSizeChanged(w, h, oldw, oldh);
            mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_4444);
            mCanvas = new Canvas(mBitmap);
        }

        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            if (mBitmap != null && canvas != null && !isTransparent) {
                canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
                canvas.drawPath(mPath, mPaint);
            }
        }

        private void touch_start(float x, float y) {
            mPath.reset();
            mPath.moveTo(x, y);
            mX = x;
            mY = y;
        }

        private void touch_move(float x, float y) {
            float dx = Math.abs(x - mX);
            float dy = Math.abs(y - mY);
            if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
                if(mBitmap.getPixel((int)x, (int)y) == Color.TRANSPARENT || mBitmap.getPixel((int)mY, (int)mY) == Color.TRANSPARENT) {
                    isTransparent = true;
                } else {
                    isTransparent = false;
                }

                mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
                mX = x;
                mY = y;

                circlePath.reset();
                circlePath.addCircle(mX, mY, 30, Path.Direction.CW);
            }
        }

        private void touch_up() {
            mPath.lineTo(mX, mY);
            circlePath.reset();
            mCanvas.drawPath(mPath, mPaint);
            mPath.reset();
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            float x = event.getX();
            float y = event.getY();

            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    touch_start(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_MOVE:
                    touch_move(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_UP:
                    touch_up();
                    invalidate();
                    break;
            }
            return true;
        }
    }

Answer 1

我不知道 mPaint 在哪里初始化，但由于 mPaint 用于绘制手指轨迹，因此将此行放入应该可以完成您想要的操作：

    mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_ATOP));

请参阅 this image 了解说明。

如何在不绘制透明部分的情况下用手指在View中绘制？

How to draw with finger in View without drawing in transparent parts?

android

uiimageview

draw

transparent