How to make Canvas draw area transparent in androi

问题:

I would like to make Canvas area transparent because I would like to add an Image behind it so that Canvas actions happen above the image.My code for the canvas is here

public class Panel extends SurfaceView implements SurfaceHolder.Callback {

private ViewThread mThread;
private ArrayList<Element> mElements = new ArrayList<Element>();

public Panel(Context context, AttributeSet attrs) { 
    super(context, attrs); 
    getHolder().addCallback(this); 
    mThread = new ViewThread(this); 
} 


public void doDraw(Canvas canvas) {
    canvas.drawColor(Color.TRANSPARENT);
    synchronized (mElements) {
        for (Element element : mElements) {
            element.doDraw(canvas);
        }
    }
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    // TODO Auto-generated method stub
}

@Override
public void surfaceCreated(SurfaceHolder holder) {
    if (!mThread.isAlive()) {
        mThread = new ViewThread(this);
        mThread.setRunning(true);
        mThread.start();
    }
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
    if (mThread.isAlive()) {
        mThread.setRunning(false);
    }
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    synchronized (mElements) {
        mElements.add(new Element(getResources(), (int) event.getX(), (int) event.getY()));
    }
    return super.onTouchEvent(event);
}

}

How to achieve it, any snippets on it will be very much helpful.Thanks

回答1:

I got the output by this

 public Panel(Context context, AttributeSet attrs) { 
    super(context, attrs); 
    this.setBackgroundColor(Color.TRANSPARENT);                 
    this.setZOrderOnTop(true); //necessary                
    getHolder().setFormat(PixelFormat.TRANSPARENT); 
    getHolder().addCallback(this); 
    mThread = new ViewThread(this); 

} 

回答2:

If you want to have a canvas with a transparent background you have to configure the background image for the

mCanvas = new Canvas(mBackgroundBitmap);

with Bitmap.Config.ARGB_4444

And use colors like 0x00000000 as transparent

    Bitmap mBackgroundImage = Bitmap.createBitmap(Size, Size,
                    Bitmap.Config.ARGB_4444);
    mCanvas = new Canvas(mBackgroundImage);

hope this helps! I have a pretty transparent canvas :D yay!

回答3:

canvas.drawColor(Color.argb(0, 255, 255, 255));

first attribute is alpha and rest are RGB colors.

or

canvas.drawColor(Color.TRANSPARENT);

回答4:

In your onDraw() method add this:

canvas.drawColor(0x00AAAAAA);

This will make your canvas transparent and background View will be visible.

回答5:

this make canvas tramsparent and it is work for me :)

canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.OVERLAY);

回答6:

You can create a Bitmap of the same size as the canvas. Erase all the colors of the bitmap using Color.TRANSPARENCY and set it as a canvas bitmap:

Bitmap transparentBitmap = Bitmap.createBitmap(canvas.getWidth(), canvas.getHeight(),
Bitmap.Config.ARGB_8888);
transparentBitmap.eraseColor(Color.TRANSPARENT);
canvas.setBitmap(transparentBitmap);

回答7:

canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)

Works fine for me with

override fun onFinishInflate() {
    super.onFinishInflate()
    setZOrderOnTop(true)
    holder.setFormat(PixelFormat.TRANSLUCENT)
    isFocusable = true
    holder.addCallback(surfaceCallback)
}

回答8:

well, I am not sure about drawing the transparent canvas, but in your case you can do a tweak, that draw the canvas using the background image iteself.

And then you can draw/ paint by finger on it.

Code example:

        BitmapDrawable bd = (BitmapDrawable)<YOUR_ACTIVITY>.this.getResources().getDrawable(R.drawable.<DRAWBLE_ID>);
        Bitmap b = bd.getBitmap();

        mBitmap = Bitmap.createBitmap(b,0,0,100,100); // This line is required only if you wanna some change in the bitmap you created
        mCanvas = new Canvas(mBitmap);

回答9:

Set canvas's view background color to #00000000