在Android上屏蔽Drawable/Bitmap

Question

我目前正在寻找一种方法来使用黑白位图来掩盖另一个位图的alpha通道或Android上的Drawable.我很好奇最好的办法是什么.我当然对如何做到这一点有几点想法,但它们并不是最优的.

我需要能够经常对图像应用新的遮罩(黑白位图每隔几秒就会改变一次).

任何有关如何实现这一目标的反馈将不胜感激.

Answer 1

我的解决方案接近@ over_optimistic的解决方案,而不是一个saveLayer()调用.我使用Drawable蒙版而不是路径,在我的例子中它是一张光盘.

我将这些变量声明为字段(在onDraw方法之外分配内存是个好习惯):

private Paint maskingPaint = new Paint();
private Drawable mask = <insert your drawable here>

然后,在onDraw()之外的某处,设置对象:

// Xfermode won't work if hardware accelerated
setLayerType(View.LAYER_TYPE_SOFTWARE, null);

// Using destination shape as a mask
// For a good explanation of PorterDuff transfer modes : http://ssp.impulsetrain.com/porterduff.html
maskingPaint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));
maskingPaint.setAntiAlias(true);

// Position the mask
mask.setBounds(<insert your mask bounds here>);

最后,onDraw()方法应用掩码:

@Override
protected synchronized void onDraw(Canvas canvas)
{
    // Draw the mask first, making it the PorterDuff destination
    mask.draw(canvas);

    // Save the layer with the masking paint, that will be applied on restore()
    // Using CLIP_TO_LAYER_SAVE_FLAG to avoid any overflow of the masked image outside the mask bounds.
    Rect bounds = mask.getBounds();
    canvas.saveLayer(bounds.left, bounds.top, bounds.right, bounds.bottom, maskingPaint, 
            Canvas.CLIP_TO_LAYER_SAVE_FLAG);

    // Draw the shape offscreen, making it the PorterDuff source
    super.onDraw(canvas);

    // Apply the source to the destination, using SRC_IN transfer mode
    canvas.restore();
}

为了更好地理解传输模式,我参考了http://ssp.impulsetrain.com/porterduff.html.该页面非常有趣.在那之后,使用相同类型的代码,您将能够获得比简单的面具更多的功能!

Answer 2

我让它工作了，所以它是这样的

    // we first same the layer, rectF is the area of interest we plan on drawing
    // this will create an offscreen bitmap
    canvas.saveLayer(rectF, null, Canvas.MATRIX_SAVE_FLAG
            | Canvas.CLIP_SAVE_FLAG | Canvas.HAS_ALPHA_LAYER_SAVE_FLAG
            | Canvas.FULL_COLOR_LAYER_SAVE_FLAG
            | Canvas.CLIP_TO_LAYER_SAVE_FLAG);

    // draw our unmasked stuff
    super.draw(canvas);
    // We same a layer again but this time we pass a paint object to mask
    // the above layer
    maskPaint = new Paint()
    maskPaint.setColor(0xFFFFFFFF);
    maskPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.MULTIPLY));

    canvas.saveLayer(rectF, maskPaint,
            Canvas.MATRIX_SAVE_FLAG | Canvas.CLIP_SAVE_FLAG
                    | Canvas.HAS_ALPHA_LAYER_SAVE_FLAG
                    | Canvas.FULL_COLOR_LAYER_SAVE_FLAG
                    | Canvas.CLIP_TO_LAYER_SAVE_FLAG);
    // we draw the mask which is black and white. In my case
    // I have a path, and I use a blurPaint which blurs the mask slightly
    // You can do anti aliasing or whatever you which. Just black & white
    canvas.drawPath(path, blurPaint);
    // We restore twice, this merges the results upward
    // as each saveLayer() allocates a new drawing bitmap
    canvas.restore();
    canvas.restore();