波特达夫和路径

Question

在我的项目中，我有一个填充整个屏幕的位图。在这个位图上，我用

画了一条路径

android.graphics.Canvas.drawPath(Path path, Paint paint)

设置绘制是为了描边和填充路径的内容。我要实现的是擦除与路径相交的位放大器部分。我已经设法在另一个位图而不是路径上获得相同的行为，并使用 porter duff 规则。有没有机会对路径做同样的事情？

    mPaintPath.setARGB(100, 100, 100, 100);// (100, 100, 100, 100)
    mPaintPath.setStyle(Paint.Style.FILL_AND_STROKE);
    mPaintPath.setAntiAlias(true);
    mPath.moveTo(x0, y0));
    mPath.lineTo(x1, y1);
    mPath.lineTo(x2, y2);
    mPath.lineTo(x3, y3);
    mPath.lineTo(x0, y0);
    mPath.close();
    c.drawPath(mPath, mPaintPath);

Answer 1

当然，只需将路径绘制到屏幕外缓冲区，以便在绘制位图时将其用作掩码，如下所示：

// Create an offscreen buffer
int layer = c.saveLayer(0, 0, width, height, null,
        Canvas.HAS_ALPHA_LAYER_SAVE_FLAG | Canvas.FULL_COLOR_LAYER_SAVE_FLAG);

// Setup a paint object for the path
mPaintPath.setARGB(255, 255, 255, 255);
mPaintPath.setStyle(Paint.Style.FILL_AND_STROKE);
mPaintPath.setAntiAlias(true);

// Draw the path onto the offscreen buffer
mPath.moveTo(x0, y0);
mPath.lineTo(x1, y1);
mPath.lineTo(x2, y2);
mPath.lineTo(x3, y3);
mPath.lineTo(x0, y0);
mPath.close();
c.drawPath(mPath, mPaintPath);

// Draw a bitmap on the offscreen buffer and use the path that's already
// there as a mask
mBitmapPaint.setXfermode(new PorterDuffXfermode(Mode.SRC_OUT));
c.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

// Composit the offscreen buffer (a masked bitmap) to the canvas
c.restoreToCount(layer);

如果你能承受锯齿，有一个更简单的方法：只需设置一个剪辑路径（注意使用 Region.Op.DIFFERENCE 会导致路径内部被剪掉，而不是剪掉路径外的所有内容）：

// Setup a clip path
mPath.moveTo(x0, y0);
mPath.lineTo(x1, y1);
mPath.lineTo(x2, y2);
mPath.lineTo(x3, y3);
mPath.lineTo(x0, y0);
mPath.close();
c.clipPath(mPath, Op.DIFFERENCE);

// Draw the bitmap using the path clip
c.drawBitmap(mBitmap, 0, 0, mBitmapPaint);